a deep ultraviolet laser light source by frequency

A deep ultraviolet laser light sourceby frequency doubling of

GaN based external cavity diode laser radiation

vorgelegt vonM. Sc.

Norman Ruhnke

an der Fakultät IV - Elektrotechnik und Informatikder Technischen Universität Berlin

zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften- Dr. rer. nat. -

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr. Wolfgang HeinrichGutachter: Prof. Dr. Günther TränkleGutachter: Priv.-Doz. Dr. Bernd SumpfGutachter: Prof. Dr. Paul Michael Petersen

Tag der wissenschaftlichen Aussprache: 24. August 2020

Berlin 2020

ZusammenfassungEine kompakte und portable Laserlichtquelle im Wellenlängenbereich zwischen 210 nmund 230 nm würde zahlreiche Anwendungen im alltäglichen Kontext ermöglichen, wie zumBeispiel die Sterilisation und Desinfektion von medizinischem Equipment und Wasserdesin-fektion, die Gasanalyse mittels Absorptionsspektroskopie, oder die Identifikation undQuantifizierung von Proteinen und Biomolekülen mittels laserinduzierter Fluoreszenz- oderRaman-Spektroskopie. Leuchtdioden sind zwar besonders kompakt, emittieren jedochzu breitbandiges Licht für einige dieser Anwendungen. Auf der anderen Seite stehenetablierte Lasersysteme in diesem Wellenlängenbereich zur Verfügung, die zwar ausreichendschmalbandig emittieren, jedoch komplexe Laborsysteme mit hohem Stromverbrauch undgroßen Abmaßen darstellen, wodurch diese für Feldanwendungen außerhalb des Laborsoftmals ungeeignet sind.

In dieser Arbeit wird daher ein neuartiges Konzept entwickelt und untersucht, um mittelsFrequenzkonversion Diodenlaser-basierter Laserstrahlung eine besonders kompakte undportable im tiefen ultravioletten Spektralbereich emittierende Laserlichtquelle mit geringemStromverbrauch zu realisieren, die diese Anwendungslücke schließen kann. Das Konzeptbasiert auf einer single-pass Frequenzverdopplung des blauen Lichtes einer kommerziellerhältlichen Hochleistungs-GaN-Laserdiode, die in dieser Form in dieser Arbeit nach bestemWissen des Autors zum ersten Mal demonstriert wird.

Als Pumpquelle für die Frequenzverdopplung in einem BBO-Kristall wird aufgrund dergeringen Konversionseffizienzen in diesem Wellenlängenbereich von ca. 10−4 W−1 eineLaserdiode mit hoher Ausgangsleistung über 1 W mit schmalbandiger Emission im Be-reich der Phasenanpassungs-Akzeptanzbandbreite des verwendeten BBO-Kristalls benötigt,um eine für die genannten Anwendungen ausreichende Ausgangsleistung von mindestens100 µW zu erreichen. Da GaN-basierte Hochleistungslaserdioden für gewöhnlich ein brei-tes Emissionsspektrum mit einer vollen Halbwertsbreite von ∆λ = 1...2 nm aufweisen,wird in dieser Arbeit zum ersten Mal die Wellenlängenstabilisierung und Verringerungder spektralen Bandbreite einer solchen GaN-Hochleistungslaserdiode mithilfe externerwellenlängenselektiver optischer Elemente gezeigt und untersucht.

Um ein besseres Verständnis davon zu erlangen wie sich die verwendete Laserdiodeunter optischer Rückkopplung verhält, wird zunächst in einer Machbarkeitsstudie einmakroskopischer Aufbau mit einem reflektierenden Oberflächengitter als externes optischesElement realisiert. Mithilfe des Aufbaus wird analysiert welche spektrale Bandbreite deroptischen Rückkopplung und welche Rückkopplungsstärke zu einer für die nachfolgendeFrequenzverdopplung ausreichend schmalen Emissionsbandbreite bei gleichzeitig hoher Aus-gansleistung führt. Es wird gezeigt, dass ein Rückkopplungsanteil von 15% ausreichend fürdie Wellenlängenstabilisierung der Laserdiode ist und die resultierende Emissionsbandbreitein etwa der spektralen Breite der optischen Rückkopplung des Oberflächengitters entspricht.

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Mit diesem makroskopischen Aufbau wird eine schmale Emissionsbandbreite von∆λ ≤ 20 pm bis zu einer optischen Ausgangsleistung von 470 mW, und von ∆λ ≤ 70 pmbis zu einer Ausgangsleistung von 680 mW mit einer maximalen Unterdrückung der longi-tudinalen Moden der Laserdiode von 42 dB (bei 545 mW Ausgansleistung) bei einerEmissionswellenlänge von 445 nm erreicht. Außerdem lässt sich die Emissionswellenlängeüber einen Bereich von 4 nm schmalbandig durchstimmen.

Der makroskopische, wellenlängenstabilisierte Diodenlaser wird als Pumpquelle für diesingle-pass-Frequenzverdopplung zu 222.5 nm in einem BBO-Kristall getestet, um zu ver-stehen welchen Einfluss die Strahlformung auf die Konversionseffizienz und auch auf diePhasenanpassungstoleranzen hat. Es zeigt sich, dass die Phasenanpassungsbandbreitenim BBO-Kristall bei einem fokussierten Pumpstrahl aufgrund der verkürzten Wechsel-wirkungslänge im Kristall deutlich breiter sind als die mit der Näherung der ebenenWellen kalkulierten Phasenanpassungsbandbreiten und darüber hinaus von der Stärke derFokussierung abhängen. In diesem Setup wird mit einer Pumpleistung von 680 mW eineschmalbandige ultraviolette Ausgangsleistung von 16 µW erzeugt.

Basierend auf den erlangten Erkenntnissen wird ein miniaturisiertes Diodenlasermodulauf einem passiv gekühlten Kupferblock mit einer Grundfläche von 25 mm x 25 mmentwickelt, bei dem ein holographisches Volumen-Bragg-Gitter als wellenlängenselektivesexternes Element eingesetzt wird. Dieses miniaturisierte Lasermodul hat keine beweglichenKomponenten mehr und wird an einem Mikromontage-Messplatz mit hoher Präzisionaufgebaut. Dadurch zeigt es verglichen mit dem makroskopischen Aufbau verbessertespektrale Eigenschaften mit einer spektralen Halbwertsbreite von ∆λ ≤ 50 pm bis zu eineroptischen Ausgangsleistung von 1.4 W bei einer Emissionswellenlänge von 445 nm und beigleichzeitiger hoher Unterdrückung der longitudinalen Moden der Laserdiode von bis zu53 dB über den gesamten Arbeitsbereich.

Das miniaturisierte Lasermodul wird dann als Pumpquelle für die Frequenzverdopplungeingesetzt, um eine ultraviolette Laserlichtquelle mit möglichst kleinen Abmaßen zu real-isieren. Basierend auf den Erkenntnissen mit dem makroskopischen Aufbau wird die Strahl-fokussierung für die Frequenzverdopplung weiter optimiert. Aus der Analyse verschiedenerStrahlformungen ergibt sich, dass für den asymmetrischen und nicht-beugungsbegrenztenPumpstrahl der Laserdiode grössere Strahltaillenradien im Bereich von 20 µm bis 30 µm imBBO-Kristall zu höheren Konversionseffizienzen führen als der in der Theorie von Boyd undKleinman für sphärische Gauss-Strahlen empfohlene optimale Strahltaillenradius von 15 µm.Mit der verbesserten Strahlformung und der höheren Pumpleistung des miniaturisiertenLasermoduls kann schließlich eine maximale optische Ausgangsleistung von 160 µW beieiner Wellenlänge von 222.5 nm generiert werden.

Durch das in dieser Arbeit gewonnene Verständnis wird mit dem entwickelten neuartigenKonzept schließlich eine ultraviolette Laserlichtquelle mit einer kompakten Grundflächevon ungefähr 5 cm x 30 cm realisiert, die aufgrund der unbeweglichen Komponentenäußerst robust ist und eine geringe Leistungsaufnahme von unter 10 W aufweist. Mit derdemonstrierten Ausgangsleistung von über 100 µW eröffnen sich somit zahlreiche Anwen-dungsmöglichkeiten außerhalb von Laborumgebungen im alltäglichen Kontext und in derIndustrie, für die bisherige Lasersysteme zu komplex und zu kosten- und energieinstensivsind.

AbstractA compact and portable laser light source emitting in the wavelength range between210 nm and 230 nm would enable numerous applications especially outside the laboratoryenvironment such as sterilization and disinfection of medical equipment, water purification,gas and air analysis by means of absorption spectroscopy, or the identification and quan-tification of proteins and biomolecules by means of laser induced fluorescence or Ramanspectroscopy. Light emitting diodes are especially compact but their spectrally broandbandemission is not suitable for some of the mentioned applications. On the other hand, thereare established laser systems in this wavelength region available that show sufficientlynarrow emission bandwidths but are too complex laboratory systems with high powerconsumptions and large footprints making them not suitable for field applications outsideof laboratory environments.

This thesis therefore developes and investigates a novel concept to realize an especiallycompact and portable light source with low power consumption emitting around 222 nmthat is based on frequency conversion of laser diode emission and able to close this appli-cational gap. The concept is based on single-pass frequency doubling of a commerciallyavailable high-power GaN laser diode emitting in the blue spectral range and, to the bestof the authors knowledge, will be presented for the first time in this work.

Due to the low power conversion efficiencies of about 10−4 W−1 in this wavelength range,a laser diode with high optical output power above 1 W with narrowband emission in therange of the acceptance bandwidth of the applied nonlinear BBO crystal is required aspump source to achieve an ultraviolet output power sufficient for all of the mentionedapplications of at least 100 µW. Since GaN based high-power laser diodes typically exhibita broad emission spectrum of ∆λ = 1...2 nm, wavelength stabilization and narrowing ofsuch GaN based high-power laser diode emission by the use of external wavelength selectiveelements will be presented and investigated for the first time in this work.

To gain a better understanding of the laser diodes behavior under optical feedback, anexternal cavity diode laser (ECDL) system with a surface diffraction grating as externalelement is realized as a proof-of-concept study. With this setup it is analyzed whichspectral bandwidth of the grating feedback and which feedback strength leads to a sufficientnarrowband emission with simultaneous high optical output power for the subsequentsecond harmonic generation. It will be shown that feeding as much as 15% of the laserdiode radiation back into the laser is sufficient for the wavelength stabilization and thatthe resulting ECDL emission bandwidth is in the range of the spectral bandwidth of theoptical feedback from the grating.

The proof-of-concept ECDL setup exhibits a narrow emission bandwidth of ∆λ ≤ 20 pm(FWHM) up to an output power of about 470 mW, and ∆λ ≤ 70 pm (FWHM) up to anoutput power of about 680 mW with a maximum suppression of the longitudinal laser

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diode modes of 42 dB (at 545 mW output power), at an emission wavelength of 445 nm.Furthermore, the narrowband emission can be coarsely tuned over 4 nm.

The ECDL system is then tested as pump source for the single-pass second harmonicgeneration of laser light at 222.5 nm using a BBO crystal as nonlinear material to un-derstand what influence the beam shaping has on the conversion efficiency and also onthe phase matching tolerances. It is shown that due to the decreased interaction lengthinside the crystal the phase matching acceptance bandwidths of BBO are much broaderfor focused beams than the simulated phase matching acceptance bandwidths derived fromthe plane-wave approximation and also depend on the focusing strength. With the ECDL,narrowband DUV laser light with a continuous wave output power of 16 µW is generatedwith a pump power of 680 mW.

Based on the previous findings, a micro-integrated ECDL (µECDL) module assembledon a conduction cooled copper package with a footprint of 25 mm x 25 mm is developed.Here, a holographic volume Bragg grating serves as external wavelength selective element.This µECDL module has no moveable parts and is built using a mirco-assembly setup withhigh precision. Therefore, it shows an improved performance compared to the macroscopicEDCL setup having a narrow emission bandwidth of ∆λ ≤ 50 pm up to an output powerof 1.4 W at an emission wavelength of 445 nm, and a mode suppression ratio as high as53 dB over the whole operating range.

To realize an ultraviolet laser light source that is as compact as possible the µECDLmodule is then applied as pump source for the frequency doubling. Based on the findingswith the macroscopic ECDL, the beam focusing for the frequency conversion is furtheroptimized. From the analysis of different focusing conditions it is found that for theasymmetric and non-diffraction limited laser diode output, a larger beam waist radiusin the range of 20 µm to 30 µm results in the highest conversion efficiency, in contrastto the Boyd-Kleinman theory for focused Gaussian beams that recommends an optimumbeam waist radius inside the BBO crystal of 15 µm. With the improved beam shaping andthe higher pump power, an optical output power of PDUV = 160 µW at a wavelength ofλDUV = 222.5 nm is generated.

With the understanding for the novel concept gained in this work, a compact ultra-violet laser light source with a small footprint of approximately 5 cm x 30 cm is realized,that due to its immoveable components is exceptionally robust and has a low powerconsumption of less than 10 W. This light source with a demonstrated output powerabove 100 µW enables numerous applications outside of the laboratory environment in theeveryday context and in the industry for which previous laser systems are too complex andtoo cost- and energy-intensive.

List of publicationsParts of this work were published in peer-reviewed journals and presented on national andinternational conferences.

Peer-reviewed journal publications1. N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, and G. Tränkle,

400 mW external cavity diode laser with narrowband emission at 445 nm, OpticsLetters, 39(13), 3794-3797 (2014).

2. N. Ruhnke, A. Müller, B. Eppich, R. Güther, M. Maiwald, B. Sumpf, G. Erbert,and G. Tränkle, Single-pass UV generation at 222.5 nm based on high-power GaNexternal cavity diode laser, Optics Letters, 40(9), 2127-2129 (2015).

3. N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, and G. Tränkle,Micro-Integrated External Cavity Diode Laser With 1.4-W Narrowband Emission at445 nm, IEEE Photonics Technology Letters, 28(24), 2791-2794 (2016).

4. N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, and G. Tränkle,Compact Deep UV System at 222.5 nm Based on Frequency Doubling of GaN LaserDiode Emission, IEEE Photonics Technology Letters, 30(3), 289-292 (2018).

Conference Contributions1. N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, and G. Tränkle,

400 mW output power at 445 nm with narrowband emission from an external cavitydiode laser system, Proc. SPIE 9382, Photonics West, San Francisco, USA, Feb.07-12, 93820P (2015).

2. N. Ruhnke, A. Müller, B. Eppich, R. Güther, M. Maiwald, B. Sumpf, G. Erbert,and G. Tränkle, Narrowband GaN external cavity diode laser with 400 mW outputpower at 445 nm for deep ultraviolet frequency doubling, Conf. on Lasers and Electro-Optics/Europe and European Quantum Electronics Conf. (CLEO/Europe-EQEC2015), Jun. 21-25, Munich, Germany, ISBN: 978-1-4673-7475-0, paper CB-P-6 (2015).

3. N. Ruhnke, A. Müller, B. Eppich, R. Güther, M. Maiwald, B. Sumpf, G. Erbert,G. Tränkle, Compact deep UV laser system at 222.5 nm by single-pass frequencydoubling of high-power GaN diode laser emission, Proc. SPIE 9731, Photonics West,San Francisco, USA, Feb. 13-18, 97310A (2016).

4. N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, G. Tränkle,Compact deep UV laser system at 222.5 nm by frequency doubling wavelength-stabilizedemission of a micro-integrated high-power GaN diode laser module, Proc. SPIE 10516,Photonics West, San Francisco, USA, Jan. 27 - Feb. 1, 10516-5 (2018).

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5. B. Sumpf, N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, G. Erbert, G. Tränkle,Deep UV light source at 222 nm based on second harmonic generation of GaN highpower diode lasers, ICULTA (International Conference on UV LED Technologies &Applications), Berlin, Germany, April 22-25 (2018), (invited).

6. B. Sumpf, N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, G. Erbert, G. Tränkle,Deep UV laser systems at 222.5 nm by single-pass frequency doubling of wavelengthstabilized high-power GaN diode lasers, 2nd International UV WORKshop (hosted byLASER COMPONENTS), Olching, Germany, November 26-27 (2019), (invited).

DanksagungZunächst gilt mein besonderer Dank Herrn Professor Günther Tränkle für die Möglichkeit,meine Doktorarbeit am Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztech-nik (FBH), anfertigen zu dürfen. Ohne unsere regelmäßigen Besprechungen, in denenmir immer wieder neue Perspektiven auf meine Arbeit aufgezeigt wurden, und seine auchüber inhaltliche Fragen hinausgehenden Ratschläge wäre diese Arbeit nicht möglich gewesen.

I would also like to express my sincere gratitude to Professor Paul Michael Petersenfor his willingness to act as the external reviewer for my thesis.Einen besonderen Dank möchte ich auch an Herrn Götz Erbert aussprechen. Gerade inder Anfangszeit war er maßgeblich für die Ausrichtung und Zielsetzung meiner Arbeitmitverantwortlich und hat mit seiner langjährigen Erfahrung und seinem Wissen auf demGebiet der Laserdioden immer wieder für die richtigen Impulse gesorgt.

Bernd Sumpf hat als Gruppenleiter des Laser Sensors Lab und als mein direkter Be-treuer ebenso großen Anteil am Gelingen dieser Arbeit. Sein Enthusiasmus und seinewertvollen Tipps zu experimentellen Fragestellungen waren ein ständiger Antrieb. Undseine zahlreichen kleinen Anekdoten und Scherze haben die Arbeit am FBH ein ganzesStück unterhaltsamer gemacht.

Auch meinen Kollegen Martin Maiwald, André Müller, Christof Zink und Bernd Eppichmöchte ich für die angenehme Zusammenarbeit und die tagtägliche Unterstützung beiinhaltlichen Fragestellungen und im Labor danken.

Außerdem danke ich den Mitarbeitern der Werkstatt Sebastian Deutscher, Detlef Grimpeund Thomas Roos für die Anfertigung zahlreicher Bauteile und Ihre Unterstützung beivielen kleinen und großen technischen Problemen. Neben allen anderen FBH-Mitarbeiternmöchte ich speziell Manuela Münzelfeld danken, die ohne Übertreibung als die gute Seeleder Optoelektronik-Abteilung des FBH bezeichnet werden kann.

Meinen Doktoranden-Kollegen Marcel Braune, Martin Winterfeldt, Mahmoud Tawfieq,Lara-Sophie Theurer, Jonathan Decker, Carlo Frevert, Matthias Karow, Thorben Kaul,Daniel Jedrzejczyk und Juliane Rieprich möchte ich ebenso meinen großen Dank aussprechen.Ihr habt die Zeit am FBH zu einem unvergesslichen Erlebnis gemacht und einige von Euchsind zu guten Freunden geworden.

Alex, Andreas, Eric, Florian, Jakob, Johannes, Josi, Karl, Linda, Lisa, Lukas, Matthias,Michelle, Ryan, Sascha und Tillmann. Danke für Eure aufmunternden Worte, gelegentlicheAblenkung, und Eure ständige Unterstützung.Gleiches und noch mehr gilt für meine Eltern und Oma Dora. Ohne Euch wäre ich nichtbis hierher gekommen, danke für alles.

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Contents1 Introduction 1

2 Fundamentals for frequency doubling with GaN based laser diodes 72.1 Nonlinear optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.1 Second harmonic generation . . . . . . . . . . . . . . . . . . . . . . . 92.2 Nonlinear materials for deep ultraviolet light generation . . . . . . . . . . . 122.3 Birefringent phase matching in BBO . . . . . . . . . . . . . . . . . . . . . . 14

2.3.1 Angle tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.3.2 Phase matching tolerances . . . . . . . . . . . . . . . . . . . . . . . . 18

2.4 Second harmonic generation with focused Gaussian beams . . . . . . . . . . 20

3 Characterization of the applied laser diode 233.1 Working principle of laser diodes and vertical layer structure . . . . . . . . 243.2 Longitudinal modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.3 Optical gain and threshold condition . . . . . . . . . . . . . . . . . . . . . . 273.4 Electro-optical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 283.5 Spectral emission characteristics . . . . . . . . . . . . . . . . . . . . . . . . 293.6 Spatial emission characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 313.7 Implications for the development of a deep ultraviolet laser light source . . 35

4 External cavity diode lasers as pump sources for DUV generation 374.1 Wavelength stabilization by external optical feedback . . . . . . . . . . . . . 37

4.1.1 ECDLs with surface diffraction gratings . . . . . . . . . . . . . . . . 404.2 Macroscopic external cavity diode laser in Littrow configuration . . . . . . . 44

4.2.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.2.2 Electro-optical characteristics . . . . . . . . . . . . . . . . . . . . . . 484.2.3 Spectral emission characteristics . . . . . . . . . . . . . . . . . . . . 514.2.4 Wavelength tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.2.5 Spatial emission characteristics . . . . . . . . . . . . . . . . . . . . . 55

4.3 Volume-Bragg-Grating stabilized external cavity diode laser module (µECDL) 564.3.1 Working principle of volume Bragg gratings . . . . . . . . . . . . . . 574.3.2 Concept and development of the µECDL module . . . . . . . . . . . 604.3.3 Electro-optical characteristics . . . . . . . . . . . . . . . . . . . . . . 644.3.4 Spectral emission characteristics . . . . . . . . . . . . . . . . . . . . 654.3.5 Temporal stability of the µECDL emission . . . . . . . . . . . . . . . 684.3.6 Spatial emission characteristics . . . . . . . . . . . . . . . . . . . . . 69

4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

5 Compact deep ultraviolet laser light source 715.1 Proof-of-concept setup with macroscopic ECDL as pump source . . . . . . . 72

5.1.1 Detection of deep ultraviolet light . . . . . . . . . . . . . . . . . . . 745.1.2 Investigation of conversion efficiency and phase matching tolerances 76

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5.2 Micro-integrated ECDL module as pump source . . . . . . . . . . . . . . . . 855.2.1 Compact deep ultraviolet laser light source . . . . . . . . . . . . . . 855.2.2 Optimization of the focusing conditions . . . . . . . . . . . . . . . . 87

6 Conclusion and Outlook 93

A Datasheet information for laser diode PL TB450B (Osram Opto Semi-conductors GmbH) 97

Bibliography 99

List of Figures 111

List of Tables 117

1 Introduction

MotivationThe development of the laser diode with its high efficiency, compactness, robustness andlong lifetime has enabled the construction of compact and non-sophisticated laser systemsthat have accessed many new applications outside of the laboratory environment. Nowa-days, many fields of application such as information and communication technology, datastorage, consumer electronics, materials processing, spectroscopy, biophotonics, and lifescience are addressed with laser diodes.

A spectral region, that is particularly interesting for numerous applications, is the wave-length range below 300 nm, hereinafter called deep ultraviolet (DUV). DUV light between200 nm and 300 nm can sterilise bacteria, viruses and other pathogens by disrupting thestructure of their DNA and can therefore be used for chemical-free disinfection of medicalequipment [1–3], air conditioning systems and for water purification [4, 5].

It also has a great potential for sensing applications as many gases, that are majoratmospheric pollutants and are primarily produced by fossil fuel combustion such as NO,NO2, SO2, and NH3, exhibit strong electronic transitions in the wavelength region between210 nm and 230 nm [6–8]. DUV absorption spectroscopy can be used to monitor thepollutant concentrations in ambient air. Typically, a spectral linewidth of about 5-10 cm−1

is usually sufficient for absorption spectroscopy of liquids and solids and in many casesalso of gases [9].Biomolecules and proteins such as tryptophan, NADH, tyrosine, DNA, RNA, and manyothers exhibit strong electronic transitions in the wavelength range below 250 nm as welland can be identified and quantified by DUV laser induced fluorescence (LIF) spectroscopyand related techniques [10–12]. In general, these techniques do not require a particularlynarrow excitation linewidth and an excitation power of a few µW is sufficient [13].

Another prominent example for the wide range of application possibilities of DUV laserlight is DUV Raman spectroscopy that can also be used for the identification of proteinsand biomolecules [14, 15], explosives [16], and many other substances [17]. As the scatteringcross section of the Raman signal scales with ω4 and due to the possible excitation ofresonant effects, the Raman signal can be enhanced by many orders of magnitude underDUV excitation compared to excitation with visible or NIR laser light [9]. Addition-ally, for excitation wavelengths below 260 nm, the Raman signal is spectrally separatedfrom the usually much stronger and therefore disturbing fluorescence spectrum of mostmolecules [18]. Figure 1.1 further illustrates this situation with an exemplary Ramanspectrum of the so-called fingerprint region of polystyrene exhibiting a spectral width of∆ν̃ = 1600 cm−1. The hatched curve with the maximum at 600 nm represents a typicalfluorescence background. For simplicity, all spectra are normalized to 1. Depending on theexcitation wavelength λex, the same Raman spectrum has a different spectral width on

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2 1 Introduction

fluorescence

200 400 600 800 1000 1200 14000.0

0.5

1.0

10 nm 50 nm 130 nm

ex =

225

nm

ex =

488

nm

ex =

785

nm

Nor

mal

ized

inte

nsity

Wavelength nm

ex =

106

4 nm

250 nm

Figure 1.1: Illustration of the spectral separation of Raman signal and fluorescence back-ground for DUV excitation using the example of the fingerprint region of the Raman spec-trum of polystyrene.

the wavelength scale. For 225 nm excitation, it has a spectral width of only 10 nm and isspectrally separated from the fluorescence background. The spectral separation of Ramanand fluorescence signal can also be used to obtain both signals in a single measurementwhich might be advantageous in some cases.To resolve the investigated Raman spectra, the spectral emission bandwidth of the excita-tion laser light has to be in the range of the spectral width of individual Raman lines. Forliquid and solid samples, these lines typically have a width of ∆ν̃ = 10 cm−1 [19]. For anexcitation wavelength of λex = 225 nm, this translates into a required spectral width of∆λ = 50 pm on the wavelength scale.Although higher optical output power is always desired, it was demonstrated that a DUVoutput power around 100 µW or even lower is already sufficient for absorption spectrocscopyof gases [20], for LIF detection [13], and DUV Raman spectroscopy [21, 22]. In a laboratoryenvironment, an excitation power of a few nW was shown to be sufficient for absorptionspectroscopy [7]. Especially when biological samples are under investigation too highoutput powers can lead to the destruction of the molecules as absorption in the DUVwavelength range is usually strong [22].Table 1.1 summarizes the required wavelength range λ, average optical output powerPopt, and spectral linewidth ∆ν̃ in wavenumbers for a laser light source targeting theseapplications, which all have a great potential to be used outside of laboratory environments.This requires a compact and robust, if possible handheld and battery driven DUV laserlight source. Within the frame of this work, the development and characterization ofsuch a DUV light source based on laser diode emission is pursued that meets the physicalrequirements summarized in table 1.1 and utilizes the advantages of laser diodes in termsof robustness, small footprint and low power consumption.

Established DUV laser light sources Established laser light sources emitting in theDUV wavelength range are gas lasers and frequency-quadrupled solid state lasers. Table 1.2gives an overview of the emission wavelength λ, average optical output power Popt, andpower consumption Pcon of these laser systems.

3

Application Technique λ Popt ∆ν̃

Sterilisation/disinfectionof med. equipment and

water purification

DNA disruption by absorption 200 - 300 nm – –

Gas/air analysis Absorption spectroscopy 210 - 230 nm ≥ nW < 10 cm−1

Identification andquantification ofproteins andbiomolecules

Laser Induced Fluorescence(LIF) spectroscopy

210 - 230 nm ≥ µW –

Raman spectroscopy < 260 nm ≥ µW ≤ 10 cm−1

Table 1.1: Possible applications for deep ultraviolet laser light, corresponding techniques,wavelength ranges, and required specifications.

Gas lasers like the KrF excimer laser (248 nm) [23] or the HeAg (224.3 nm) and NeCu(248.6 nm) hollow cathode lasers [24] directly emit in the wavelength range below 250 nm.The frequency doubled Ar+ laser emitting at 244 nm [25, 26] is a well established DUVlaser light source, too.Despite offering high average output powers of more than 100 mW [23, 25, 26], KrF andAr+ lasers have a large footprint, a high power consumption of more than 1 kW [23, 25, 26],require frequent maintenance, and entail considerable production costs.HeAg and NeCu hollow cathode lasers have a lower power consumption of less than 100 W,but deliver lower average output powers of less than 1 mW [24]. They also require frequentmaintenance and have a relatively large footprint.Another commonly applied solution is the generation of the fourth harmonic of Nd:YLFor Nd:YAG solid state lasers, that directly emit at infrared wavelengths of 1047 nm or1064 nm, respectively. These systems deliver output powers of more than 10 mW, havesmaller footprints and a moderate power consumption of around 100 W [27, 28]. Unfor-tunately, they only emit on fixed wavelengths of 262 nm or 266 nm, whereas an emissionwavelength well below 250 nm is more suitable for the aforementioned applications.DUV laser systems based on the generation of higher harmonics of the infrared emissionfrom Ti:sapphire lasers offer tunable output in a wavelength range from 193 nm to 270 nm

Laser type λ mode average Popt Pcon

HeAg 224 nm CW < 1 mW < 100 WAr+ (2nd harmonic) 244 nm pulsed > 100 mW > 1 kWKrF excimer 248 nm pulsed > 100 mW > 1 kWNeCu 249 nm CW < 1 mW < 100 WNd:YLF (4th harmonic) 262 nm CW > 10 mW ≈ 100 WNd:YAG (4th harmonic) 266 nm CW > 10 mW ≈ 100 WTi:Sa (up to 4th harmonic) 193 - 270 nm pulsed 5 - 50 mW > 1 kW

Table 1.2: Established DUV laser light sources and their emission wavelength λ, typicalaverage optical output power Popt, and power consumption Pcon.

4 1 Introduction

[29] together with excellent spectral and spatial beam properties. However, their highpower consumption of more than 1 kW, their complexity and large footprint make themmore suitable as scientific tools in the laboratory environment.

Each of the established light sources offers sufficient optical properties in terms of outputpower and emission linedwidth. However, they all have large footprints, high power con-sumptions and are rather complex, which usually restricts experiments to the laboratoryenvironment. Furthermore, except for the Ti:sapphire based laser systems they all emit onfixed wavelengths only. To utilize DUV lasers in fields of application outside the laboratoryenvironment as a portable in situ analysing and monitoring tool, a more compact andreliable DUV laser light source with minimal power consumption ideally suitable for batteryoperation is necessary. In this sense, a promising approach is frequency conversion of laserdiode emission into the DUV wavelength range.

Diode laser based DUV light sources GaAs based laser diodes cover a wavelengthrange between 600 nm and 1200 nm with optical output powers in the watt range andreach wall-plug efficiencies of more than 70% [30] making them to the most efficient devicesin converting electrical into optical energy [31]. Since the first demonstration of continuouswave emission from an InGaN based laser diode by Nakamura et al. in 1996 [32], alsothe green, blue, and parts of the ultraviolet spectral region can be addressed by directemission from quantum well diode lasers. However, the shortest wavelength emitted byan electrically pumped AlGaN based laser diode so far is 336 nm [33] and direct DUVemission from laser diodes is still out of reach until now.

Therefore, a diode laser based DUV light source can only be realized by frequency conver-sion. A selection of relevant works on DUV frequency conversion of laser diode radiationis summarized in table 1.3. The concept of fourth harmonic generation of the infraredradiation of GaAs based laser diodes was already demonstrated in the 90s. It was realizedby using either two successive single-pass (SP) [20, 34] or resonant cavity-enhanced (CE)[35, 36] frequency doubling (second harmonic generation, SHG) configurations. Goldberg[34] and Koplow [20] used very similar single-pass setups with a GaAlAs tapered amplifier(TA) laser diode emitting at 860 nm as pump source and a BBO crystal for DUV generation.Goldberg achieved a DUV optical output power of 15 µW at 215 nm. In the work of Kliner,the focus was on a minimized DUV emission bandwidth for absorption spectroscopy andan output power of 240 nW at 215 nm was generated.Zimmermann et al. demonstrated a scheme with a GaAs master oscillator power amplifier(MOPA) laser diode (LD) emitting at 972 nm as pump source and two successive cavity-enhanced frequency doubling stages [35]. In the second stage, a BBO crystal was againused for the DUV generation of 2.1 mW at a wavelength of 243 nm. Schwedes et al. useda seeded GaAs tapered amplifier laser diode emitting at 922 nm to generate 1 mW of laserlight at 231 nm [36] with a similar setup. The latter concept leads to higher DUV outputpowers than the single-pass arrangements, but can also become increasingly complex. Acommercially available system from TOPTICA Photonics AG based on successive cavity-enhanced frequency doubling nowadays offers DUV output powers of 10 mW at 213 nmand even 300 mW at 266 nm [37, 42]. This system has a footprint of 9 cm x 41 cm x 69 cmand a power consumption of typically 100 W [42].

5

Author Laser type Method λDUV Popt

2x SHG Goldberg (1995) [34] GaAlAs TA SP 215 nm 15 µWKoplow (1998) [20] GaAlAs TA SP 215 nm 240 nW

Zimmermann (1995) [35] GaAs MOPA CE 243 nm 2.1 mWSchwedes (2003) [36] GaAs TA CE 231 nm 1 mWToptica (2019) [37] GaAs TA CE 213 nm 10 mW

266 nm 300 mW

SFG Alnis (2000) [38] GaN ECDL + GaAs LD SP 254 nm 1 nWCarruthers (2005) [39] GaN ECDL + GaAs LD SP 254 nm 50 nWAnderson (2005) [40] GaN ECDL + GaAs LD SP 254 nm 4 nW

SHG Nishimura (2003) [41] GaN ECDL CE 209 nm 9 µW

Table 1.3: Overview of diode laser based DUV light sources. SHG: second harmonic gen-eration, SFG: sum frequency generation, TA: tapered amplifier, MOPA: master oscillatorpower amplifier, SP: single-pass, CE: cavity-enhanced.

Another approach is based on sum frequency generation (SFG) of the emission froma blue and a red emitting laser diode in a single-pass arrangement with BBO as nonlinearcrystal [38–40]. In these works, only relatively low output powers in the nanowatt rangewere achieved and the concept has the inherent disadvantage of needing two laser diodesin operation.A concept that leads to a DUV laser light source with further reduced footprint, powerconsumption, complexity, and overall production cost is direct second harmonic generationof GaN based laser diode radiation in the blue spectral range. This was already demon-strated by Nishimura et al. [41] using a low-power GaN external cavity diode laser (ECDL)as pump source. To achieve sufficient DUV optical output power, a BBO nonlinear crystalwas integrated in an enhancement cavity. With this setup, a continuous wave (CW) outputpower of 9 µW at 209 nm was generated from 26 mW pump power at 418 nm. Besidesthis work, no further studies of this concept have been published so far.

Goal of this workThe goal of this work is to develop and characterize a novel diode laser based DUV lightsource with a smaller footprint and lower power consumption than previous light sources.The emission wavelength should be in the range between 210 nm and 230 nm with acontinuous wave output power of about 100 µW and an emission bandwidth < 10 cm−1 or< 50 pm. Its specifications are defined by the applications referred to above and are listedin table 1.4.The concept of direct second harmonic generation of GaN based laser diode radiation inthe blue spectral range promises to result in the most compact and inexpensive DUV laserlight source with minimal power consumption. Conversion efficiencies for DUV generationare quite low (typically 10−4 W−1) usually making cavity-enhanced frequency doublingstages necessary [41]. However, with the recent development of commercially available

6 1 Introduction

wavelength λ 210-230 nmoutput power P ≈ 100 µWlinewidth ∆ν̃ < 10 cm−1

∆λ < 50 pmpower consumption Pcon < 10 W

Table 1.4: Targeted specifications for the diode laser based DUV light source.

high-power GaN laser diodes emitting around 450 nm with output powers beyond 1 W[43, 44], direct single-pass frequency doubling of such laser diode radiation with DUVoutput powers around 100 µW has become feasible.This work is intended to serve as proof-of-concept study, i.e. to demonstrate the feasibilityof this concept and to investigate the physical challenges, that need to be considered.

The thesis is organized as follows: In chapter 2, the theoretical background that needs tobe considered for DUV frequency doubling using a high-power GaN based laser diode, ispresented. This includes fundamentals of nonlinear optics with an emphasis on secondharmonic generation (SHG), an overview of crystals suitable for DUV generation, andestimations regarding the expected phase matching tolerances and nonlinear conversionefficiencies.The applied laser diode (PL TB450B, OSRAM Opto Semiconductors) and its character-istics are presented in chapter 3. At the time of this work, the laser diode provided thehighest optical output power of P = 1.6 W from a commercially available GaN based laserdiode. Its broad spectral emission of ∆λ ≈ 1...2 nm full width at half maximum (FWHM)limits its applicability for efficient frequency conversion and does not meet the applicationrequirements on the DUV light source listed in table 1.4. Hence, the emission bandwidthis reduced by implementing an external cavity diode laser (ECDL) setup. As this is achallenging and also crucial step for the development of the DUV light source, the conceptis tested in a proof-of-principle ECDL system using surface diffraction gratings in Littrowconfiguration.Chapter 4 starts with a brief description of wavelength stabilization of laser diodes byexternal optical feedback and a short literature review on GaN ECDLs with surface grat-ings. In section 4.2 and the following, the proof-of-principle ECDL system in Littrowconfiguration and its performance is analyzed. A miniaturized ECDL module (µECDL)with a volume Bragg grating as wavelength selective optical element is presented anddiscussed in section 4.3.Both ECDL systems are demonstrated and evaluated in single-pass frequency doublingsetups with BBO as nonlinear optical crystal in chapter 5. In section 5.1, the whole conceptis again tested in a proof-of-principle setup by using the macroscopic ECDL in Littrowconfiguration as pump source. Here, challenges and difficulties of the single-pass conceptregarding optimal focusing conditions and phase matching tolerances are discussed. Themore compact DUV laser light source with the micro-integrated µECDL as pump sourceis presented in section 5.2 and the optimization of the focusing conditions is discussed insection 5.2.2.Chapter 6 concludes this work, outlines different ideas on possible improvements of thecompact DUV system and summarizes remaining challenges of the examined concept.

2 Fundamentals for frequency doublingwith GaN based laser diodes

This chapter gives an overview of the relevant physical background for the realization ofa DUV laser light source based on second harmonic generation (SHG) of blue GaN laserdiode emission. A brief theoretical insight into nonlinear optics with a special emphasison second order phenomena and second harmonic generation is given in section 2.1. Thederivations of the necessary equations are thereby taken from the textbooks of William P.Risk et al. [45], Robert W. Boyd [46], and Richard L. Sutherland [47]. A full descriptionof nonlinear phenomena can also be found in standard textbooks on nonlinear optics [46–48].

Section 2.2 gives an overview of nonlinear crystals suitable for frequency conversion intothe ultraviolet wavelength range in general and discusses the advantages and disadvantagesof different crystals with respect to second harmonic generation into the wavelength rangebelow 250 nm. It will be shown that the nonlinear material of choice for the purpose ofthis work is β-BaB2O (β-barium borate, BBO).Using BBO for DUV generation by collinear second harmonic generation requires birefrin-gent phase matching, which is explained in section 2.3. Here, the wavelength, temperatureand angle tolerances for type I critical phase matching in BBO are calculated for thewavelength addressed in this work.A brief summary of the Boyd-Kleinman theory that predicts the optimum focusing condi-tions inside the nonlinear crystal for circular Gaussian beams is given in section 2.4.

7

8 2 Fundamentals for frequency doubling with GaN based laser diodes

2.1 Nonlinear opticsThe interaction of dielectric materials with the electric field E(z,t) of an incident light wavepropagating in z-direction is described by the polarization P (z,t). For moderate intensitiesof the incident light, the response of the material is linearly dependent upon the strengthof the electric field. However, for high intensities of the optical field as provided by lasers,the response of the material can be nonlinearly dependent upon the optical field strength.The first discovery of a nonlinear phenomenon was demonstrated by Franken et al. in1961 [49]. They observed the second harmonic at a wavelength of 347 nm generated byirradiating a quartz crystal with a ruby laser emitting at a wavelength of 694 nm.

In nonlinear optics, the response of the material is usually generalized by expressingP (z,t) as a power series in the field strength E(z,t):

P (z,t) = ε0(χ(1)E(z,t) + χ(2)E2(z,t) + χ(3)E3(z,t) + ...

), (2.1)

where ε0 is the vacuum permittivity, and χ(1), χ(2), and χ(3) are the first, second, andthird-order susceptibilities, respectively. The first term describes linear phenomena like theindex of refraction. The second term with the square of the electric field leads to secondorder phenomena like second harmonic generation (SHG), sum frequency generation (SFG),difference frequency generation (DFG), parametric fluorescence or optical rectification.The third term with the cube of the electric field treats third order phenomena like third-harmonic generation, the intensity-dependent refractive index, or Brillouin scattering ([45],p. 23) .

For the description of SHG, only the second-order polarization P (2)(z,t) is consideredin the following:

P (2)(z,t) = ε0χ(2)E2(z,t) (2.2)

The strength of the induced polarization depends on the second-order susceptibility χ(2)

which is taken to be constant in equation (2.2). This assumption is justified, if all thefrequencies taking part in the interaction are lying far below the lowest resonance frequencyof the nonlinear material. For SHG, this condition is usually fulfilled and the nonlinearsusceptibility is independent of the frequency ([46], p. 37).In general, ~P (~r,t) and ~E(~r,t) are 3-dimensional vectors with components in x-, y-, andz-direction. The self-convolution of ~E(~r,t) in equation (2.2) leads to 3 x 3 possible com-ponents for each of the three components of the second-order polarization. This means,that χ(2) is actually a third-rank tensor written as χ(2)

ijk, where the indices i, j, k canindependently take on the values x, y, and z leading to 27 different components [46]. Somesymmetries fortunately reduce the number of independent components [46]. For instance,in the case of SHG, the indices j and k become interchangeable and can therefore bereplaced by a new index l according to the following scheme ([46], p. 38):

jk xx yy zz yz,zy xz,zx xy,yxl 1 2 3 4 5 6

The second-order susceptibility is then expressed by the contracted notation of the nonlinear

2.1 Nonlinear optics 9

coefficient dil, that is a 3 x 6 matrix with 18 components ([45], p. 28):

dil = 12χ

(2)ijk (2.3)

Due to other symmetries including spatial crystal symmetries, the number of independentcomponents in dil is further reduced for most nonlinear crystals. A detailed discussionon the symmetries influencing the nonlinear susceptibility can be found in reference [46](p. 32 ff.). The second-order polarization can now be re-written in the following form ([46],p. 38):

P(2)x

P(2)y

P(2)z

= 2ε0

d11 d12 d13 d14 d15 d16d21 d22 d23 d24 d25 d26d31 d32 d33 d34 d35 d36

︸︷︷︸

dil

ExExEyEyEzEz2EyEz2ExEz2ExEy

(2.4)

For known polarization and propagation directions of the participating waves, an effectivenonlinear coefficient deff can be calculated and equation (2.2) can be expressed with scalarvalues (see [46], p. 39):

P (2)(t) = 2ε0deffE2(t) (2.5)

2.1.1 Second harmonic generation

The electric field of a plane wave propagating in z-direction and oscillating with frequenyω is written as:

E1(z,t) = A1 cos(k1z − ωt+ φ1) (2.6)

where A1 is the amplitude of the electric field, k1 is the wave vector, and φ1 is the phase.

Inserting equation (2.6) into (2.5) gives the second-order polarization induced in a nonlinearmaterial by the fundamental electric field E1(z,t):

P (2)(z,t) = 2ε0deff [A1 cos(k1z − ωt+ φ1)]2 (2.7)

After using a simple trigonometric identity, an expression for the second-order polarizationconsisting of two terms is obtained:

P (2)(z,t) = ε0deffA21 [1 + cos(2k1z − 2ωt+ 2φ1)] (2.8)

The first term is at zero frequency giving rise to the process of optical rectification in whicha static electric field is created within the nonlinear material. The second term in equation(2.8) oscillates at the second harmonic frequency 2ω. According to the inhomogeneouswave equation

∂2

∂z2E2(z,t)− n22c2

0

∂2

∂t2E2(z,t) = 1

ε0c20

∂2

∂t2P (2)(z,t), (2.9)

the second-order polarization can be a source of a second electric field E2(z,t) oscillatingwith the frequency 2ω:

E2(z,t) = A2 cos(k2z − 2ωt+ φ2) (2.10)


w

w

2w

w

w

2wc(2)

a) b)

Figure 2.1: a) Schematic sketch of a second harmonic generation process inside a nonlinearcrystal. b) SHG process depicted in an energy level diagram.

In this process, called second harmonic generation (SHG), two photons oscillating atfrequency ω are transformed into one photon oscillating at the second harmonic frequency2ω. Figure 2.1 illustrates the SHG process geometrically (a) and in an energy level diagram(b). By substituting equation (2.10) into (2.9) and applying the slowly varying amplitudeapproximation, one obtains a coupled-amplitude equation for the amplitude of the generatedsecond harmonic wave (see [47], p. 57):

dA2dz = i

2ωdeffn2ωc

A21 · ei∆kz (2.11)

deff is the effective nonlinear coefficient, n2ω the refractive index for the generated wave,and ∆k is the wavevector mismatch:

∆k = 2k1 − k2. (2.12)

The amplitude of the generated second harmonic wave can be derived by integratingequation (2.11) over the length Lcr of a nonlinear crystal:

A2(Lcr) = i2ωdeffn2ωc

A21

Lcr∫0

ei∆kz dz

= i2ωdeffn2ωc

A21

(ei∆kLcr − 1

i∆k

) (2.13)

Substituting the amplitude for its intensity

I = 2ε0nc|A|2, (2.14)

gives with the use of (see [46], p.75)∣∣∣∣∣ei∆kLcr − 1∆k

∣∣∣∣∣2

= L2cr sinc2(∆kLcr/2) and (2.15)

and ω = 2πc/λω, (2.16)

an expression for the intensity of the generated second harmonic light:

I2 = 8π2d2eff

ε0cn2ωn2ωλ

2ω

I21 · L2

cr · sinc2(∆kLcr

2

)(2.17)

2.1 Nonlinear optics 11

This equation will be important for the calculation of the conversion efficiency in a nonlinearcrystal in dependence of the phase matching angle and temperature presented in section2.3. The second harmonic intensity strongly depends on the effective nonlinear coefficientdeff, the intensity of the fundamental wave I1, and the crystal length Lcr. The intensityI2 has its maximum for a vanishing wavevector mismatch ∆k = 0. This so-called phasematching condition can be written as:

∆k = 2k1 − k2 = 4πλω

(n(ω)− n(2ω)) = 0 (2.18)

→ n(ω) = n(2ω) (2.19)

This means, that the refractive indices for fundamental and second harmonic wave haveto be equal, i.e. the phase velocities of both waves have to be equal, so that constructiveinterference can occur and the SHG intensity can build up inside the nonlinear crystal.Due to dispersion, this condition is usually not fulfilled (n(ω) 6= n(2ω)).

However, there are methods to overcome phase-mismatch like quasi-phase matching in peri-odically poled nonlinear crystals or critical (angle tuning) and non-critical phase matching(temperature tuning) in birefringent nonlinear crystals.The phase matching technique applied in the course of this work is critical birefringenttype I phase matching and will be explained in detail in section 2.3.


2.2 Nonlinear materials for deep ultraviolet light generationEstablished nonlinear crystals used for deep ultraviolet frequency conversion below 300 nmare summarized in table 2.1. A more comprehensive overview of nonlinear crystals includingnewly developed and rarely used crystal can be found in [50].λcut-off is the UV wavelength at which the crystals exhibit close to "0" transmittance.Potassium fluoroboratoberyllate (KBe2BO3F2, KBBF) has the lowest cut-off wavelengthof 147 nm, followed by lithium triborate (LiB3O5, LBO), potassium dihydrogen phosphate(KH2PO4, KDP), cesium lithium borate (CsLiB6O10, CLBO), and beta-barium borate(β-BaB2O4, BBO) with 155 nm, 176 nm, 180 nm, and 189 nm, respectively. Frequencyconversion almost down to the respective transmission cut-off wavelength of each crystalhas been demonstrated by means of sum frequency generation (KBBF: 163 nm [51], CLBO:175 nm [52], LBO: 187.7 nm [53], KDP: 190 nm [54], BBO: 190.8 nm [55]).

The aim of this work however, is to convert GaN laser diode emission between 440 nm and460 nm by direct SHG, which leads to the necessity of applying type I phase matching (seesection 2.3). For LBO, KDP, and CLBO, the minimum SHG wavelength that can be phasematched with type I phase matching is 277 nm, 256 nm, and 240 nm, respectively, whichis not small enough for the targeted wavelength range in this work of 210 nm to 230 nm.A relatively new crystal that was first reported in 1996 [56] and shows promising nonlinearoptical properties is KBBF. Based on the Sellmeier equations from [57], phase matchingdown to 165 nm for type I SHG could in principle be achieved [58] with this crystalmaterial. Togashi et al. experimentally demonstrated direct SHG down to 172.5 nm [51].However, KBBF possesses a plate-like form and the growth of KBBF crystals thickerthan a millimeter is challenging. This makes it difficult to cut the crystal along the phasematching direction for the generation of deep UV light. To circumvent this problem,a prism-coupling technique was suggested that is nonetheless inconvenient for deep UVSHG. Recently, KBBF crystals with a length of up to 8 mm were grown by a new growthtechnique. Unfortunately, due to their worse optical quality these longer crystals exhibitSHG conversion efficiences up to two orders of magnitude smaller than the plate-likecrystals until now [64].

The most established and often used nonlinear crystal for frequency conversion below300 nm, is beta-barium borate (β-BaB2O4, BBO), that was discovered by Chen et al.

Crystal λcut-off [50] λSHG-cut-off (Type I) deff

KBBF 147 nm 165 nm [58] 0.38 pm/V (λSHG = 223 nm) [59]

LBO 155 nm 277 nm [60] 0.16 pm/V (λSHG = 280 nm) [59]

KDP 176 nm 256 nm [61] 0.48 pm/V (λSHG = 260 nm) [59]

CLBO 180 nm 240 nm [62] 0.92 pm/V (λSHG = 240 nm) [59]

BBO 189 nm 205 nm [63] 1.26 pm/V (λSHG = 223 nm) [59]

Table 2.1: Transmission cut-off wavelength λcut-off, minimum SHG wavelength at whichphase matching with type I SHG can be achieved at room temperature (T ≈ 293 K), andeffective nonlinear coefficient deff for selected crystals.

2.2 Nonlinear materials for deep ultraviolet light generation 13

in 1985 [65]. It has a significantly higher effective nonlinear coefficient deff and also ahigher laser induced damage threshold (10 GW/cm2 for 100 ps pulse-width at 1064 nm[50]) compared to the other crystals listed in table 2.1 [59, 66]. The SHG conversionefficiency depends on the square of deff. A higher deff therefore means a higher intensity ofthe generated second harmonic light. Additionally, due to the strong birefringence in BBO,type I phase matching down to 205 nm is theoretically possible and was also demonstratedby Kato et al. in 1986 [63].BBO is a negative uniaxial crystal that belongs to the point group symmetry class 3m andits nonlinear coefficient dil is [47]:

dil =

0 0 0 0 d15 −d22−d22 d22 0 d15 0 0d31 d31 d33 0 0 0

(2.20)

A general derivation for calculating deff for each of the crystal classes was presented byMidwinter and Warner [67] (see [46], pp. 39). They showed, that the effective nonlinearcoefficient deff for SHG with type I phase matching in a negative uniaxial crystal of crystalclass 3m is given by the expression [67]:

deff = d31 sin(θ + ρ)− d22 cos(θ + ρ) sin(3φ) (2.21)

where θ is the phase matching angle, ρ the walk-off angle, and φ the azimutal angle. Inthe literature, different absolute values for the coefficients d31 and d22 can be found. Shojiet al. recommend absolute values of d22 = 2.6 pm/V, and d31 = 0.04 pm/V [68]. However,for better comparison with the effective nonlinear coefficient of the other crystals, all defffrom table 2.1 are derived using the software SNLO (Version 68) by Arlee Smith [59].For BBO, the software uses the values d22 = 2.2 pm/V, and d31 = 0.08 pm/V and calcu-lates the effective nonlinear coefficient for type I SHG from 445 nm to 222.5 nm in BBO(θ = 65◦, ρ = 4◦, φ = π/2) to be deff ≈ 1.26 pm/V.

In summary, BBO is the best suitable nonlinear crystal for direct SHG to wavelengthsbetween 210 nm and 230 nm and will therefore be applied in this work. The next sectiondescribes phase matching in BBO. As the refractive indices in BBO posses a relativelyweak dependence on temperature [69], phase matching is usually achieved by angle tuning,which is described in the next section as well. Here, it should be noted that one drawbackof BBO compared to other crystals is its high walk-off angle when critical phase matchingis applied, which leads to a highly elliptical second harmonic beam. A way to circumventthe disadvantage of the heavy walk-off in BBO, would be the use of periodically polednonlinear crystals. Conventional used nonlinear materials like PPLN and PPLT howevershow strong absorption in the wavelength range below 300 nm and the lowest generatedsecond harmonic wavelength with these crystals is 325 nm generated in PPLT [70]. Thereis ongoing research towards periodically poled nonlinear crystals suitable for the DUVwavelength range. Recently, Hirohashi et al. demonstrated the generation of laser radiationat 266 nm in a periodically poled LaBGeO5 crystal [71]. In principle, this material canalso be used to generate laser light in the wavelength range around 222 nm. However, thishas not been achieved up to now.


2.3 Birefringent phase matching in BBOIt was shown in section 2.1 that for efficient second harmonic generation the phase matchingcondition ∆k = 0 needs to be fulfilled, which means that the refractive indices for thefundamental and the second harmonic wave have to be equal:

n(ω) = n(2ω) (2.22)

Unfortunately, this is prevented by the normal dispersion occuring in most materials forwhich the refractive index increases with increasing frequency. A phenomenon that canbe used to compensate for the normal dispersion and to achieve phase matching is thebirefringence of optically anisotropic materials. In such materials, the refractive indexdepends on the polarization and the direction of the light propagating through it.

The nonlinear crystal BBO which is applied throughout this work is an optically anisotropicmaterial exhibiting birefringence, more precisely it is a negative uniaxial crystal. Uniaxialcrystals are characterized by a particular direction, which is called the optical axis (caxis). A light wave polarized in the plane containing its own propagation vector k andthe optical axis experiences the extraordinary refractive index ne(ω,θ), that depends onthe angle θ between k and the optical axis. If the polarization of the light is polarizedperpendicular to the plane containing k and the optical axis, the light wave experiences theordinary refractive index no(ω). In a negative uniaxial crystal like BBO, the extraordinaryrefractive index is smaller than the ordinary refractive index (ne < no), which is illustratedin figure 2.2.One can see that there is no pair of refractive indices in BBO that fulfills the phasematching condition for the targeted wavelength range so that no(ω) = ne(2ω). For example,for a fundamental wave at 445 nm, no(445 nm) = 1.68397 and ne(222.5 nm) = 1.65749.The birefringence in BBO can be used to compensate for the normal dispersion, if anonlinear interaction is chosen that involves an ordinary polarized fundamental beam andan extraordinary polarized second harmonic beam. The combination for which two ordi-

Figure 2.2: Dispersion of the ordinary and extra-ordinary refractive index in BBO for acrystal temperature of TBBO = 50◦C according to the Sellmeier equations from [72].

2.3 Birefringent phase matching in BBO 15

nary polarized waves generate one extraordinary polarized wave is known as type I-phasematching (neω2 = noω1 + noω1, ω2 = 2ω1) and will be applied in this work. The phasematching can then be realized by either temperature or angle tuning.In order to include a temperature dependence for the following phase matching calculationsthe Sellmeier equations determined by Zhang et al. [72] at T0 = 293 K (19.85◦C) are usedand expanded by the known temperature derivatives of the refractive indices [69]. Thetemperature dependencies of the ordinary and the extraordinary refractive index of BBOare [69]

dnodT

= −16.6 · 10−6 K−1 (2.23)

anddnedT

= −9.3 · 10−6 K−1. (2.24)

The temperature dependent Sellmeier equations for BBO can then be written in the formnT = nT0 + (T − T0) · dn/dT [73]:

n2o = 2.7359 + 0.01878

λ2 − 0.01822 − 0.01471λ2 + 0.0006081λ4

− 0.0000674λ6 − 16.6 · 10−6(T − 19.85◦C)(2.25)

n2e = 2.3753 + 0.01224

λ2 − 0.01667 − 0.01627λ2 + 0,00005716λ4

− 0.00006305λ6 − 9.3 · 10−6(T − 19.85◦C)(2.26)

With these equations one can also calculate the phase matching temperature acceptance ofthe BBO crystal as will be shown in section 2.3.2.


2.3.1 Angle tuning

The method of angle tuning utilizes the strongly angle-dependent nature of the extraordinaryrefractive index ne(2ω, θ), which behaves according to the following equation:

1n2e(θ)

= sin2(θ)n2e

+ cos2(θ)n2o

(2.27)

By precisely adjusting the angle θ between the optical axis of the crystal and the wavevector k of the propagating light wave, the extraordinary refractive index that is experiencedby the second harmonic wave can be matched with the ordinary refractive index no of thefundamental wave. The geometry of this method for SHG is illustrated in figure 2.3.

k

c

q2ww

extraordinaryordinary

nonlinear crystal

Figure 2.3: Schematic illustration of phase matching via angle tuning for second harmonicgeneration, top view (adapted from [46], p. 98).

Figure 2.4 shows the ordinary refractive index for a fundamental wavelength of 445 nmand the extraordinary refractive index for a second harmonic wavelength of 222.5 nm asa function of the angle θ in BBO at a crystal temperature of 50◦C. For θ = 0◦, ne(2ω,θ)conincides with the principal value no, and for θ = 90◦ it coincides with the principal valuene. For a phase matching angle of θPM = 64.97◦, the refractive indices for the fundamentaland second harmonic waves are equal and the phase matching condition (2.22) is satisfied.It can be rewritten as:

no(ω) = ne(2ω,θ) = 1.68397 (2.28)

Figure 2.4: Ordinary refractive index of the fundamental beam and extraordinary refrac-tive index of the second harmonic beam as a function of the angle θ between the optical axisof the crystal and the wave vector k of the extraordinary beam in BBO (TBBO = 50◦C).


The drawback of the angle tuning method is, that for a phase matching angle θ betweenangles of 0◦ and 90◦, the Poyting vector S for the extraordinary wave of the secondharmonic is not parallel to the wave vector k, as illustrated in figure 2.5.

k

c

q

nonlinear crystal

Sr

Figure 2.5: Schematic illustration of the spatial walk-off occuring with critical phasematching (top view).

Therefore, the energy flow of the second harmonic propagates with a walk-off angle ρ to thewave vector k of the fundamental, which significantly reduces the spatial overlap betweenboth waves leading to a reduced conversion efficiency. The walk-off angle can be calculatedaccording to [45]:

tan(ρ) =tan(θ)

(n2on2e− 1

)1 + n2

on2e

tan2(θ)(2.29)

For instance, phase matching in BBO for an SHG process from 445 nm to 222.5 nm ata crystal temperature of 50◦C with the phase matching angle of θ = 64.97◦ leads to awalk-off angle of ρ = 4◦ for the extraordinary second harmonic wave.

A method to circumvent the drawback of walk-off is non-critical phase matching alsoreferred to as temperature tuning, for which the angle θ is either exactly or close to 0◦ or90◦. However, this method is only applicable for limited wavelength ranges and in the caseof BBO this method cannot be applied for the DUV wavelength around 223 nm targetedin this work. The term non-critical refers to the fact, that the phase matching tolerancesregarding the emission bandwidth of the fundamental, the phase matching angle θ, andthe crystal temperature are much less critical compared to critical phase matching (angletuning), which is applied in this work.


2.3.2 Phase matching tolerances

The dependence of the SHG intensity upon the emission wavelength of the fundamentalwave, the temperature of the BBO crystal, and the phase matching angle θ for a fixedfundamental wavelength and crystal temperature can all be simulated using equation (2.17)with the temperature dependent Sellmeier equations (2.25) and (2.26). It should be noted,that all calculations are carried out by assuming infinite plane waves. In real experiments,the propagating waves are not infinite plane waves but focused beams, which eventuallyresults in some deviations from the theoretically calculated values especially for stronglyfocused beams. This circumstance will be discussed in the results section and further an-alyzed with respect to the Boyd-Kleinman theory for focused Gaussian beams in section 2.4.

The following tolerances are calculated for the BBO crystal used throughout this workwith a length of 7.5 mm, and for a phase-matched fundamental wavelength of 445 nm.Figure 2.6 shows the normalized SHG intensity as a function of the fundamental wave-length for a BBO crystal temperature of 50◦C. For a spectral deviation of ±8 pm to thephase matching wavelength, the SHG efficiency already decreases to 90% of its maximum.The full width at half maximum of the wavelength acceptance in this particular case is∆λPM = 42 pm (FWHM).The normalilzed SHG intensity as a function of the BBO crystal temperature is presentedin figure 2.7. Here, the SHG intensity decreases to 90% of its maximum for a deviation of±0.8 K from the phase matching temperature of 50◦C. The full-width of half maximum ofthe temperature acceptance curve is ∆TPM = 4.2 K.As a consequence, precise control of the crystal temperature and a fundamental emissionbandwidth in the range of approximately ∆λ ≈ 40 pm (FWHM) is required for efficientSHG.The simulated normalized SHG intensity as a function of the phase matching angle θin BBO (TBBO = 50◦) is shown in figure 2.8. It can be seen, that the estimated phasematching angle acceptance bandwidth is only ∆θPM = 0.013◦ (FWHM) requiring precisecontrol of the phase matching angle.

Figure 2.6: Normalized SHG intensity as a function of the pump wavelength in BBO at acrystal temperature of T = 50◦ (LBBO = 7.5 mm).


Figure 2.7: Normalized SHG intensity as a function of the crystal temperature in BBO fora phase matching temperature of T = 50◦ (LBBO = 7.5 mm, λ = 445 nm).

Figure 2.8: Normalized SHG intensity as a function of the phase matching angle for afixed fundamental wavelength of 445 nm (LBBO = 7.5 mm).

As mentioned earlier, the phase matching tolerances are only valid for the approximation ofinfinite plane waves. In the single-pass SHG experiment presented in chapter 5, a focusedlaser beam is used. It will be seen, that the strong focusing reduces the effective interactionlength in the BBO crystal and thereby leads to wider phase matching tolerances. Theplane wave approximation however can be seen as a limiting case. The weaker the focusing,the closer are the real phase matching tolerances to the tolerances in the plane waveapproximation.It is worth noting, that for waveguide structured cystals, the plane wave approximation isalways valid, because the light is confined to a narrow stripe over the whole crystal lengthand therefore behaves like a plane wave inside the crystal.


2.4 Second harmonic generation with focused Gaussianbeams

The intensity of the second harmonic wave in equation (2.17) increases with the square ofthe fundamental intensity and the square of the crystal length. This result is derived byassuming infinite plane waves taking part in the nonlinear interaction. With real opticalbeams, high intensities are achieved by realizing spatial confinement of the light throughfocusing. Stronger focusing results in higher intensities in the beam waist. However, thehigher divergence of a strongly focused beam reduces its effective interaction length insidethe nonlinear crystal, as the intensities away from the beam waist decrease more rapidly.On the other hand, too weak focusing results in relatively low intensities over the wholecrystal length which leads to an unefficient nonlinear interaction as well. Both limitingcases are illustrated in figure 2.9.Another critcial parameter that has to be considered is the walk-off in the phase matchingplane in case of critical phase matching which reduces the overlap between fundamental andsecond harmonic wave. Therefore, a trade-off between strong focusing and a maximizedinteraction length has to be found to optimize the nonlinear conversion efficiency for agiven crystal length Lcr.Boyd and Kleinman [74] have treated this problem analytically for the case of non-depletedSHG with focused Gaussian beams in a negative uniaxial crystal in which critical type I-phase matching is applied.Under the assumption of a circular Gaussian beam with a radial distribution e−r2/w2

0 thatis characterized by its confocal parameter

b = 2πnωw20/λω (2.30)

(with the beam waist radius w0) and is focused into a crystal with negligible loss, theyderived the generated SHG power according to

P2 = 16π2d2eff

ε0cnωn2ωλ3ω

P 21 · Lcr · h(σ,β,κ,ξ,µ). (2.31)

P1 is the fundamental power, and h(σ,β,κ,ξ,µ) is the Boyd-Kleinman (BK) function, thatcan be maximized by optimizing the focusing conditions.The parameters in the argument of the function are σ representing the wave mismatch,β representing the walk-off effect, κ describing losses at the fundamental and the second

Figure 2.9: Geometry of a focused beam inside a nonlinear crystal of length Lcr. The solidline indicates a weakly focused and the dashed line a strongly focused beam.

2.4 Second harmonic generation with focused Gaussian beams 21

harmonic wavelength, ξ representing the focusing conditions, and µ the beam waist positionin the crystal.Assuming a lossless material (κ = αb/2 = 0) with the focus at the center of the crystal(µ = 0) the BK function is reduced to three parameters in its argument h(σ, β, ξ) with

σ = b∆k

2 , (2.32)

β = ρ

θ0(2.33)

andξ = Lcr

b. (2.34)

Here, ∆k is the wave vector mismatch, ρ the walk-off angle, and θ0 the half beam divergenceangle. To maximimize the Boyd-Kleinman function and thereby the nonlinear conversionefficiency ∆k needs to be optimized with regard to the focusing condition expressed byξ. Figure 2.10a) shows the BK function as a function of the beam waist radius w0 on alogarithmic scale in the case without walk-off and with walk-off for a 7.5 mm long BBOcrystal at a crystal temperature of 50◦C and for a pump wavelength of 445 nm. In case ofno walk-off (ρ = 0), the maximum value of the Boyd-Kleinman function derived from theBK analysis is hmax = 1.07 at a beam waist radius of w0 = 10.5 µm, resulting in a confocalparameter b = Lcr/ξ = Lcr/2.84.In the case of critical phase matching and spatial walk-off additionally depicted on alinear scale in figure 2.10b), the Boyd-Kleinman function, i.e. the conversion efficiency, issignificantly reduced. As explained in section 2.3.1, the second harmonic beam experiencesa walk-off angle of about 4° in this case. Here, the maximum value of the BK function ishmax = 0.048 for a beam waist radius of w0 = 15 µm, leading to a confocal parameter ofb = Lcr/ξ = Lcr/1.4. ξ is a measure for the focusing strength and therefore also takes the

Figure 2.10: a) Boyd-Kleinman function in dependence of the beam waist radius w0 forSHG in a BBO crystal without walk-off (dashed line) and with walk-off (solid red line) on alogarithmic scale. b) Boyd-Kleinman function in dependence of the beam waist radius w0 incase of walk-off on a linear scale. (TBBO = 50◦C, LBBO = 7.5 mm, λpump = 445 nm).


Figure 2.11: Simulated SHG power as function of the pump power according to theBoyd-Kleinman theory for a BBO crystal with TBBO = 50◦C, LBBO = 7.5 mm andλpump = 445 nm.

strength of the beam divergence generated by the focusing into account. b can be seenas the distance over which the beams cross sectional area is relatively constant. For astrongly focused beam, b is small and consequently ξ is large. The smaller value for ξ andthe higher optimum beam waist radius w0 compared to the no walk-off case indicates, thatweaker focusing is favorable in case of strong walk-off as observed in BBO. The strongdecrease of the BK factor by a factor of 20 demonstrates the negative effect of the spatialwalk-off on the nonlinear conversion efficiency.Figure 2.11 shows the expected SHG power PSHG as a function of the pump power Ppump.It follows the quadratic behavior for SHG without pump depletion according to equa-tion (2.31). For TBBO = 50◦C, LBBO = 7.5 mm and λpump = 445 nm and with thefocusing conditions optimized according to the BK analysis with w0 = 15 µm, the expectednormalized conversion efficiency is η = 13.8 · 10−5 W−1, assuming an effective nonlinearcoefficient of deff = 1.26 pm/V (see section 2.2). This means, that an SHG power of 138µW at 222.5 nm can be expected for a pump power of 1 W at 445 nm from an optimallyfocused Gaussian beam.

It should be noted, that the optimum BK factor in the Boyd-Kleinman theory is ob-tained at ∆k > 0, resulting from a phase shift in the focus of a Gaussian beam [75, 76].

In some expansions of the Boyd-Kleinman theory also elliptical focusing is considered. Itwas shown that weaker focusing in the phase matching (PM) plane reducing the walk-offeffect, and strong focusing perpendicular to the PM plane can increase the efficiency inlarge walk-off crystals like BBO [77–79]. Furthermore, it was demonstrated in periodicallypoled crystals that for non-diffraction limited pump beams an increased spot size withrespect to the BK theory is favorable [80].

3 Characterization of the applied laserdiode

In this chapter, the working principle of laser diodes is introduced briefly. Laser parametersare explained based on the electro-optical, spectral and spatial emission characteristics ofthe laser diode used in this work and only to an extent that is relevant for the followingexperiments.The laser diode applied throughout this work is a commercially available InGaN basedlaser diode from OSRAM Optosemiconductors GmbH (model PL TB450B). The laserdiode is intentionally developed for high-quality projectors in the professional field or forstage and decoration illumination and even medical applications [81]. It is packaged in astandardized TO56 package, that can be seen in figure 3.1.

Figure 3.1: Photography of the applied laser diode packaged in a TO56 can [43].

One of the challenges of applying such a commercially available laser diode is the lackof information about the exact vertical quantum well structure and the strength of thefront and back facet reflectivities. Only the experimentally accessible parameters andthe information divulged by OSRAM Optosemiconductors GmbH in some publicationsand the datasheet (see appendix A) for the laser diode can be used for the design andsimulation of the experiments. However, the electro-optical, spectral and spatial emissioncharacteristics can be measured and are presented in this section. Wherever possible,measured parameters are compared with published results and the datasheet.Another challenge is the limited accessability of the laser diode due to its packaging inthe TO56 package. This package has a window made of D 263® T eco glass (Schott AG,refractive index at 445 nm: n = 1.5342) with a thickness of 250 µm, through which thelaser diode emission is coupled out. The window has a distance of about 0.8 mm to thefront facet of the laser diode, which prevents the application of micro-lenses with shorterfocal lengths.A first visual inspection of the laser diode is carried out with a simple microscopicmeasurement through the protective glass of the TO56 package. Figure 3.2 shows threemicroscope images with different magnification factors. In figure 3.2.a), the whole laserdiode package is shown with fivefold magnification. On the left side of the image, an ESDprotection diode protecting the laser diode from overvoltage can be seen. In the middle isthe laser diode chip itself. Figure 3.2.b) is an image of the laser diode chip with tenfold

23

24 3 Characterization of the applied laser diode

15 mm

5x 10x

50x

a) b) c)

100 mm200 mm

Figure 3.2: Optical microscope images of the applied laser diode with different magnifica-tion factors (5x (a), 10x (b), 50x (c)).

magnification. The chip width is measured to be 200 µm and its height to be 100 µm. Theimage is taken with a minimal applied voltage so that blue spontaneous emission from thelaser diode is identifiable. The noticeable brighter spot originates from the laser diodesactive region, that can be seen magnified fifty times in figure 3.2.c). The lateral width ofthe active region is measured to be 15 µm.Considering the time of the publication and comparing the optical characteristics, one canassume that the laser diode used throughout this work is essentially comparable with theR&D laser device presented in a paper by Vierheilig et al. in 2012 [82]. In this paper, aridge waveguide laser diode with a ridgewidth of 15 µm and a resonator length of 1.2 mmis shown. The laser diode is reported to have a peak conversion efficiency of 29% and athermal roll-over at 2.5 W at room temperature. Its slope efficiency is specified to be about1.7 W/A. These values are also in accordance with the characterization measurementspresented in this chapter.

3.1 Working principle of laser diodes and vertical layerstructure

In principle, laser operation is achieved by amplification of stimulated emission inside of anactive medium that provides optical gain. This active medium is placed into a resonator,that creates optical feedback into the active medium. Figure 3.3 shows a simple schematicillustration of a laser diode, in which the optical feedback is provided by the cleaved rearand front facets, that serve as plane mirrors with reflectivities R1 and R2, and herebyform a Fabry-Perot (FP) resonator of length L. In a simple semiconductor quantumwell-based diode laser, the active medium is based on a heterostructure consisting of anundoped semiconductor layer with a direct band gap embedded between p- and n-dopedsemiconductor layers with a larger band gap energy than the undoped layer.By applying a forward bias, holes from the p-side and electrons from n-side are injectedand recombine in the undoped active region, which leads to the emission of photons withenergies slightly larger than the energy band gap. In figure 3.3, the emission direction is thez direction, in which a high photon density in the active region required for constant lasingis realized by the optical feedback. For efficient operation, photons and charge carriersneed to be confined in the active region in both transverse directions (x and y direction)as well.In the laser diode applied in this work, the lateral confinement is realized by the alreadymentioned ridge waveguide. For the vertical direction, charge carrier and photon confine-

3.1 Working principle of laser diodes and vertical layer structure 25

x

z

y

L R1 R2

active region p-doped

n-doped

Figure 3.3: Schematic illustration of a Fabry-Perot laser diode resonator of length L withmirror reflectivities R1 and R2 of the rear and front facet, respectively.

ment is realized by the design of the vertical layer structure. As presented in the paper byVierheilig, the diode is grown on a c-plane GaN substrate by metal organic vapor phaseepitaxy (MOVPE). The active region consists of InGaN multiple quantum well (MQW)structures instead of a simple intrinsic undoped semiconductor layer. It is embeddedbetween n-type and p-type AlGaN cladding and GaN waveguide layers [82]. From anotherpaper by Hempel et al., that investigates the kinetics of catastrophic optical damage inthese GaN based diode lasers, one can find that the chip is mounted by hard solder in ap-side up configuration [83].Figure 3.4.a) shows the vertical layer structure without knowledge of the number ofquamtum wells and the individual layer thicknesses. Such a structure is called separateconfinement heterostructure (SCH) as the optical confinement of the propagating wave isseparated from the confinement of the charge carriers in the vertical (x) direction. Theenergy band structure for such a laser diode is exemplary illustrated in figure 3.4.b). TheMQWs confine the charge carriers in the active region, where they recombine and emitphotons with energies sligtly larger than the band gap energy Eg. The AlGaN layers withthe higher band gap energies also exhibit a lower refractive index than the GaN waveguidelayers, which leads to an optical confinement of the generated wave. The advantage of theseparate confinement is that both charge carrier and optical confinement can be optimizedseparately.

n-AlGaN

n-GaN

GaN substrate

p-GaN

p-AlGaN

a)

x

y GaN/InGaN MQW

EC

EV

Eg

b)

y

x

MQW

optical waveguide

p-A

lGaN

n-A

lGaN

Figure 3.4: a) Exemplary transverse layer structure of a multiple quantum well separateconfinement heterostructure GaN based laser diode. b) Exemplary transverse energy bandstructure for a multiple quantum well separate confinement heterostructure laser diode. Egis the band gap energy of the quantum wells.


3.2 Longitudinal modesIn an FP resonator, only longitudinal modes fulfilling the following relation can oscillate:

L = mλ0

2ng,eff(3.1)

m is the order number of allowed modes (m = 1,2,3,..), λ0 is the vacuum wavelength, andng,eff is the effective group refractive index of the active region.The spectral distance between the allowed modes is called free spectral range (FSR) and isdefined as

∆λFSR = λ20

2ng,effL, (3.2)

where 2ng,effL is the distance travelled by the light in one roundtrip around the cavity. Forthe laser diode applied in this work, the FSR can be determined by measuring an emissionspectrum below the threshold current, where many longitudinal modes can be observed.Figure 3.5 shows such a spectrum at an injection current of I = 140 mA and T = 20◦Cheatsink temperature for the laser diode applied in this work. It is measured with a doubleechelle monochromator (Demon, LTB Lasertechnik Berlin GmbH) with a resolution of6 pm around 445 nm. A spectral distance of ∆λFSR = 27 pm between the longitudinalmodes occuring around 442 nm is observed. By knowing the cavity length of L = 1.2 mm[82], an effective group refractive index of ng,eff = 3.02 can be calculated from equation (3.2)using λ0 = 442 nm.This value is in good agreement with the value of ng = 3.0297 derived for the groupindex of GaN at λ0 = 442 nm using the dispersion formula given on the webpagewww.refractiveindex.info [84], that is taken from the paper by Barker and Ilegems [85]:

n2 − 1 = 2.60 + 1.75λ2

λ2 − 0.2562 + 4.1λ2

λ2 − 17.862 (3.3)

and solving ng = n− λ0dnλ0.

Figure 3.5: Emission spectrum of the laser diode applied in this work below threshold at acurrent of I = 140 mA and at T = 20◦C heatsink temperature.

3.3 Optical gain and threshold condition 27

3.3 Optical gain and threshold conditionThis part follows the description in the textbook of R. Diehl ([86] p. 10 ff.).When a planar optical wave propagates through an absorbing material in z direction, itsintensity I exponentially decreases:

I = I0e−αz, (3.4)

where I0 is the initial intensity and α the intensity absorption coefficient. In a laser activematerial, the wave is amplified by stimulated emission leading to an exponential intensityincrease, that can be described with a negative value of α, which is called the optical gaing of the semiconductor material itself. In a laser resonator, only a part of the intensitypattern of the optical mode overlaps with the active region of the optical waveguide. Theratio of this overlap is described by the confinement factor Γ and the gain of the opticalmode is then described by the modal gain gmodal = Γg, that is significantly lower than thematerial gain g.Lasing can only occur when the gain provided by the optical mode Γg compensates all thelosses in the laser resonator, which leads to the threshold condition [86]:

Γgth = αi + αmirror = αi + 12L ln

( 1R1R2

)(3.5)

The modal gain depends on the density of the injected charge carriers. The minimum gainat which laser operation starts and the losses are compensated is called threshold gain gth.αi describes the intrinsic absorption losses in the active material, and αmirror representsthe combined mirror losses expressed through the reflectivities of the rear and front facetafter one roundtrip around the cavity.In figure 3.6, the allowed longitudinal modes in a FP resonator with the spectral distance∆λFSR to each other are illustrated together with the modal gain Γg, that is representedby a Gaussian distribution for the sake of simplicity. When the peak of the modal gain atλp reaches the threshold value, the longitudinal mode in the closest vicinity to λp starts tooscillate. As the modal gain is usually much broader than the free spectral range of theFP resonator, FP diode lasers exhibit several simultaneously lasing modes referred to aslongitudinal multi-mode emission. To achieve longitudinal single-mode emission, furthertechniques have to be applied. One of them is to use wavelength selective optical feedbackfrom external gratings, which will be discussed in chapter 4.

Figure 3.6: Spectrum of the modal gain and the longitudinal FP modes of a laser diode atthreshold. The modal gain takes its maximum Γgth at λp.


3.4 Electro-optical characteristicsBy applying the rate equations for steady-state operation together with the thresholdcondition (3.5), an expression for the optical output power of a FP laser diode can bederived ([86], p. 41 ff.):

Popt = ηiαi

αi + αmirror

hν

q(I − Ith) = ηd

hν

q(I − Ith) (3.6)

The internal efficiency ηi represents the fraction of the current injected current into thelaser diode that generates carriers in the active region ([86], p. 38). Above the thresholdcurrent Ith, the optical output power linearly increases with the injection current I. ηd isthe differential efficieny and is defined as ([86], p. 44) :

ηd = q

hν

dPoptdI

(3.7)

It describes the differential increase in emitted photons divided by the differential increasein injected electrons per time above the lasing threshold. dPopt/dI is the slope efficiency Sin W/A from the linear part of the power-current characteristics.By simultaneously measuring the applied voltage U , one can derive the electro-opticalconversion efficieny ηc (also referred to as the wall-plug efficiency) from the power-currentcharacteristic of a laser diode as the ratio of the total electrical power Pel injected into thelaser diode to the emitted optical output power Popt:

ηc = PelPopt

= U · IPopt

(3.8)

Figure 3.7 shows the electro-optical characteristics of the laser diode applied in this workfor a heatsink temperature of T = 20◦C.

0.0 0.2 0.4 0.6 0.8 1.0 1.20

1

2

3

4

5

S = dPopt / dI

Volta

ge U

/ V

Injection current I / A

Ith

0.0

0.4

0.8

1.2

1.6

2.0

Opt

ical

out

put p

ower

Pop

t / W

0.0

0.2

0.4

0.6

0.8

1.0

Con

vers

ion

effic

ienc

yc

T = 20 °C

Figure 3.7: Voltage U , optical output power P and electro-optical conversion efficiency ηcas a function of the injection current I for the laser diode applied in this work at a heatsinktemperature of T = 20◦C.

3.5 Spectral emission characteristics 29

The laser diode reaches a maximum optical output of P = 1.6 W at an injection current ofI = 1.2 A. The power current characteristic exhibits a slope efficieny of S = 1.55 W/A.The threshold current is Ith = 145 mA. The maximum operating voltage is U = 4.5 V. Forthe maximum injection current of 1.2 A, an electro-optical conversion efficieny of ηc = 30%is measured. These values are in good accordance with the paper from Vierheilig et al. [82]and the datasheet published by OSRAM Opto Semiconductors GmbH (see table A.1 inthe appendix).

3.5 Spectral emission characteristicsThe laser diode applied in this work exhibits longitudinal multi-mode emission with aspectral emission bandwidth of ∆λ = 1...2 nm (FWHM) over the entire operating range.Figure 3.8 shows an exemplary emission spectrum at an injection current of I = 1.2 A andT = 20◦C heatsink temperature, measured with a double echelle monochromator (Demon,LTB Lasertechnik Berlin GmbH) with a resolution of 6 pm around 445 nm. The emissionbandwidth is ∆λ = 1.4 nm (FWHM).The emission wavelength of a diode laser is temperature dependent. Here, one has todistinguish between the shift of the longitudinal modes and the shift of the spectral gaincurve with increasing temperature. The further is mainly caused by an increase of therefractive index with increasing temperature, which leads according to equation (3.1) to anincrease of the longitudinal wavelength. Additionally, the length of the FP cavity increaseswith temperature, which also leads to a higher emission wavelength. The stronger effect isthe shift of the gain curve, which is mainly caused by the decrease of the band-gap energywith temperature.

Figure 3.9.a) shows the wavelength shift with increasing temperature for the laser diodeapplied in this work at a constant injection current of I = 1.2 A. A linear increase of thecentral emission wavelength of ∆λC/∆T = 52 pm/K is measured for a temperature increasefrom 15◦C to 45◦C heatsink temperature. This value is also in good accordance with

440 442 444 446 448 4500.0

0.5

1.0

FWHM

Norm

aliz

ed in

tens

ity

Wavelength / nm

I = 1.2 AT = 20°C

= 1.4 nm

Figure 3.8: Emission spectrum for an injection current of I = 1.2 A at a heatsink tempera-ture of T = 20◦C.


15 20 25 30 35 40 45444

445

446

447

448

200 400 600 800 1000 1200441

442

443

444

445

446T = 20°CI = 1.2 A

Cen

ter w

avel

engt

h C /

nm

Heatsink temperature T / °C

b)

C/ T= 52 pm/K C/ I = 3.5 nm/A

Cen

ter w

avel

engt

h C /

nmInjection current I / mA

a)

Figure 3.9: a) Shift of the central emission wavelength λC versus the heatsink temperatureat an injection current of I = 1.2 A. b) Shift of the central emission wavelength λC versusthe injection current at T = 20◦C heatsink temperature.

the reported value of ∆λC/∆T = 50 pm/K for this laser diode [82]. Figure 3.9.b) showsthe shift of the central emission wavelength of the laser diode with increasing injectioncurrent of ∆λC/∆I = 3.5 nm/A for T = 20◦C heatsink temperature. This effect is causedby Joule heating with increasing injection current leading to an increased temperaturein the active region, even though the heatsink temperature is set to 20◦C. Considering∆λC/∆T = 52 pm/K, means that the temperature increases with increasing injectioncurrent with ∆T/∆I = 67 K/A.

3.6 Spatial emission characteristics 31

3.6 Spatial emission characteristicsTo couple the emission of a laser diode into an optical system or a lens it is necessaryto have knowledge of the spatial emission characteristics of the laser diode. When thelaser diode emission is coupled out through the front facet it experiences diffraction to adifferent extent in the two transverse directions. The smaller vertical dimension of theoptical waveguide leads to a quickly diverging beam in the vertical direction, which istherefore referred to as the fast axis. The larger dimension of the optical waveguide in thelateral direction leads to a slowly diverging beam and this direction is referred to as theslow axis. Consequently, the resulting spatial beam profile has an elliptical shape. Thebeam divergence angles, also referred to as the far-field angles, for the laser diode appliedthroughout this work are measured to be θlat = 8◦ (FHWM) and θvert = 23◦ (FWHM) inthe lateral and vertical direction, respectively [87]. These values are in accordance withthe values from the datasheet provided by OSRAM (see table A.1 in the appendix).

Theoretically, a diffraction-limited Gaussian beam is ideal for practical applications due toits minimal divergence. Real laser beams however, often exhibit deviations from the idealGaussian profile, which results in a higher divergence and a larger beam waist diameterafter focusing by a lens.

When a Gaussian beam propagating in z direction, as illustrated in figure 3.10, is focusedby a lens, the beam diameter d(z) increases with increasing distance to the beam waist,where the beam has its smallest diameter d0 = 2w0 at the position z = 0, according to [88]:

d(z) = d0

√1 +

(z

zR

)2(3.9)

The Rayleigh length zR = πd20/4λ is defined as the distance to z0, at which the beam

diameter is√

2 times larger than its minimum value d0. In the far-field for z � zR, thediameter increases linearly with z and the full beam divergence angle θ is defined to [88]:

θ = d0zR

= 4λπd0

(3.10)

An important parameter, that describes the spatial beam quality and thereby the focusingcapability of a laser beam, is the beam parameter product.

z

b

zR

w0 w02

Figure 3.10: Geometry of a Gaussian beam propagating in z direction.


For a diffraction-limited Gaussian beam, the beam parameter product is defined to [88]:

d0θ

4 = λ

π(3.11)

If higher order transverse modes oscillate, the beam waist diameter d0 and the beamdivergence angle θ are both increased by a factor M , referred to as the beam propagationfactor, and the beam parameter product is written as [88]:

d0θ

4 = M2λ

π(3.12)

As a result, an M2 > 1 in each transverse direction indicates to what extent a realbeam deviates from an idealized diffraction-limited Gaussian beam (M2 = 1) and canbe seen as a measure of the beam quality of a laser beam with reference to a Gaussian beam.

For the spatial characterization of the laser diode applied in this work, the beam propa-gation factor M2 in both transverse directions is determined according to the ISO 11146standard [88]:The collimated beam is focused by a spherical lens and thereby a beam waist is created.The beam profile is then measured with a CCD camera at at least ten positions along thepropagation direction. Half of the measurement points should lay in between the Rayleighlength zR and the other half should lay at positions with a distance to the beam waistgreater than 2zR.

A CCD image of the intensity profile of the collimated beam, that can be interpreted asthe far-field of the laser diode applied in this work, is presented in figure 3.11. The beamis collimated by an aspheric lens with a focal length of f = 4.02 mm (NA = 0.6, ThorlabsC671TME-A) with an AR-coating from 350 nm to 700 nm. It can be seen that in both

0 1 2 3 4 5 6 7Lateral position / mm

0

1

2

3

4

5

Verti

cal p

ositi

on /

mm

Figure 3.11: Intensity profile of the collimated beam from the laser diode applied in thiswork.

3.6 Spatial emission characteristics 33

transverse directions the intensity profiles are asymmetric. In the lateral axis, a multi-lobedlateral far-field pattern can be observed, that is charactertic for broad ridge (Al,In)GaNlaser diodes [89]. And on one side of the beam, low peripheral intensities appear. Thislateral far-field pattern probably occurs from an interplay of the photon density with thelateral refractive index profile, the local carrier density and thermal effects, that lead toself focusing of the optical laser mode, which is known as filamentation [89].

For such a laser beam clearly exhibiting a beam propagation factor M2 > 1, it is anythingbut trivial to choose an appropriate criterion for the determination of the beam diameterfrom a measured intensity profile, as different criteria might result in significantly differentbeam diameters. Furthermore, the appropriate criterion may also depend upon the intendedapplication for the laser beam under investigation.The definition for the beam diameter that is maybe the most accurate with respect to thereal beam is the variance definition, that weights low peripheral intensities quadratically. Ituses the second moment of the beam intensity profile I(x,y) across the transverse coordinatex (or alternatively across the y coordinate) according to [90]:

σ2x =

∫∞−∞(x− x0)2I(x,y)dxdy∫∞

−∞ I(x,y)dxdy , (3.13)

where x0 is the focal point of the beam. For this method, the peripheral intensities have tobe included accurately as they are quadratically weighted. The square root of the varianceis known as the standard deviation σ and the beam waist radius is defined to be twice thestandard deviation. The beam waist diameter in both transverse directions is then fourtimes the standard deviation:

dx,y = 4σx,y (3.14)

The hypberbolic fit from equation (3.9) can then be rewritten for the variance definition ofthe beam diameter as:

d4σ(z) = d0,4σ

√√√√1 +(M2

4σ4λ(z − z0)πd2

0,4σ

)2

. (3.15)

Another possibility to define the beam diameter in both transverse directions is to usethe knife-edge method. In this method, the total intensity of the beam is used as thereference. An imaginary knife-edge is then translated across the beam profile and thetransmitted power is measured as a function of the knife-edge position. The criterion, thatis used in this work, is to define the diameter as the spatial distance between the points atwhich the transmitted power is 95% and 5% of the total power in each transverse direction,respectively. This means, that on both sides of the intensity profile, 5% of the peripheralintensities are neglected and the beam diameter in each transverse direction contains 90%of the total intensity. The hypberbolic fit from equation (3.15) can then be expressed forthe 90%-knife-edge definition of the beam diameter as:

d90%(z) = d0,90%

√√√√1 +(M2

90%4λ(z − z0)πd2

90%

)2

. (3.16)


The knife-edge method results in smaller beam diameters and beam divergences andtherefore in smaller M2 values than the variance method. However, the 90%-knife-edgemethod seems to be an appropriate criterion considering second harmonic generation asthe intended application of the beam. Since the generated SHG intensity depends on thesquare of the fundamental intensity, very low intensities have a small contribution to theconversion process and can be neglected. In this sense, the peripheral intensities of thelaser beam shown in figure 3.11 would be over-interpreted by the variance method.

Figure 3.12 shows the caustic measurements according to ISO 11146 for the varianceand the knife-edge method in the fast (left) and slow (right) axis of the laser diode emissionfor the maximum injection current of 1.2 A.In the fast axis, a second moment beam propagation factor of M2

4σ = 2.6 and a 90%-knifeedge value of M2

90% = 1.1 is measured. This means, that the beam quality in verticaldirection is relatively good and as can be seen from figure 3.11 the beam shape in verticaldirection is close to a Gaussian shape. In the slow axis, the second moment beam propaga-tion factor is determined to be M2

4σ = 5.9 and the 90%-knife edge value is M290% = 4.5,

which correponds to the lateral multi-lobed far-field pattern.As explained above, the 90%-knife edge value seems to be more appropriate consideringthis particular beam profile and second harmonic generation as application. Therefore,these values will be considered when the focusing conditions for efficient second harmonicgeneration are discussed in chapter 5.

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

I = 1.2 A

M290% = 1.1

M24 = 2.6

Beam

wai

st d

iam

eter

d /

mm

Position z / mm

Variance (4 ) Knife edge (90%) caustic fit

fast axis I = 1.2 A

M290% = 4.5

M24 = 5.9


slow axis

Figure 3.12: Caustic measurements of the laser diode in fast (left) and slow (right) axisaccording to the variance and the 90%-knife-edge diameters at T = 20◦C heatsink tempera-ture and I = 1.2 A.

3.7 Implications for the development of a deep ultraviolet laser light source 35

3.7 Implications for the development of a deep ultravioletlaser light source

After having discussed several applications, that can be addressed with deep ultravioletlaser light, it has become clear, that single-pass frequency doubling of blue GaN based laserdiode radiation is a novel and promising concept for a compact and portable DUV laserlight source. A desired emission wavelength below 250 nm is most suitable, and althoughhigher optical output power is always desired, it has been shown, that an output poweraround 100 µW or even lower is already sufficient.In chapter 2, an introduction to nonlinear optics related to second harmonic generationwas given. It was shown, that BBO is the most suitable nonlinear crystal material with thehighest expected conversion efficiency. The simulation of the birefringent phase matchingin BBO and the simulation of the expected conversion efficiency using the Boyd-Kleinmantheory revealed, that a pump source with high optical output power in the watt-rangetogether with narrowband emission in the range of the acceptance bandwidth of BBO ofabout 40 pm is needed to achieve a DUV output power around 100 µW with the single-passconcept.Therefore, a commercial GaN based laser diode from OSRAM Opto Semiconductorsproviding the - at the time of this work - highest available optical output power of 1.6 Waround 450 nm was chosen to serve as pump source and its electro-optical, spectral andspatial characteristics were presented in this chapter. It was shown, that the laser diode hasa spectral width ∆λ ≈ 1 nm, which is too broad for efficient second harmonic generation aswell as for the intended applications. Therefore, the spectral emission needs to be reducedby means of wavelength selective optical feedback, which will be presented in the followingchapter 4.

4 External cavity diode lasers as pumpsources for DUV generation

Due to their broad gain spectrum diode lasers usually exhibit an emission bandwidthof a few nanometers making them unsuitable for nonlinear frequency conversion. Twomain approaches to reduce the bandwidth of laser diodes are most commonly applied.Either wavelength selective sections like distributed feedback (DFB) gratings or distributedBragg reflectors (DBR) are integrated on-chip or external cavities are realized by externalwavelength selective elements in an external cavity diode laser (ECDL) setup.The implementation of DFB [91] and DBR [92] structures has been realized for GaAs basedlaser diodes [93–95] and DFB and DBR diode lasers are already commercially availablefor this material system [42]. In contrast, the manufacturing of such structures in GaNbased laser diodes faces some technological challenges and remains an objective of intensiveresearch until now [96–99]. Therefore, the realization of an ECDL system is currently amore feasible approach to achieve narrowband emission from a GaN based laser diode.

4.1 Wavelength stabilization by external optical feedbackThe basic principle of an ECDL system consists in feeding a narrow bandwidth portion ofthe light back into the laser diode cavity via an external optical element. If a dispersive,i.e. wavelength selective element is used, a reduced loss over a narrow spectral range isinduced forcing the laser diode to emit in this desired wavelength range with a significantlyreduced bandwidth.In general, optical feedback can cause variations in the lasing threshold, the output power,and the emission bandwidth and wavelength. These manifold effects have been studiedextensively [100–109]. Hereby, the variations of the spectral characteristics under differentstrenghts of optical feedback also from dispersive external cavity elements have beeninvestigated with special emphasis. The interaction between the internal field from thelaser diode and the external field is most importantly dependent on the length of theexternal cavity, i.e. the distance between the reflective element and the coupling facet, thestrength of the optical feedback and the reflectivity of the coupling facet. Depending onthese parameters, the description of ECDL properties can become rather complex, whichis beyond the scope of this work. Instead, the reader may be referred to the works alreadyreferenced above and to some more detailed reviews on that topic [110, 111].

Instead, a three-mirror-cavity model [112] as depicted in figure 4.1 helps to illustrate thesituation in an ECDL setup with a wavelength selective element and to explain the mostimportant parameters that have an influence on the spectral response of the ECDL in arather simple way. The internal laser diode cavity is formed by the rear and the front facetof the laser diode interpreted as mirrors with intensity reflectivities R1 and R2, respectively.The laser diode emission is coupled out via the front facet and collimated by a lens. Inthe case described here, a wavelength selective and reflective filter with reflectivity R3 is

37

38 4 External cavity diode lasers as pump sources for DUV generation

R1 R2 R3

d LECDL

laser diode cavity

laseroutput

lens

Figure 4.1: Schematic illustration of an external cavity diode laser as a three-mirror cavitylaser. R1 and R2 are the reflectivities of the back and front facet and R3 is the reflectivityof the external dispersive element.

placed in front of the laser diode in a distance LECDL to the front facet. Depending on thedispersion strength and the reflectivity of the dispersive element, a defined portion within adefined spectral bandwidth of the emitted light is fed back into the laser diode via its frontfacet. With stray or absorption losses at the dispersive element being neglected, the mainportion of the light T = 1− R3 is coupled out via the dispersive element serving as thelaser output of the ECDL. In principle, three different Fabry-Perot resonators - betweenthe front and rear facet, the rear facet and the dispersive element and between the frontfacet and the dispersive element - are formed and influence the emission spectrum of anECDL.Saliba et al. give an intuitive and didactic description of the main wavelength dependentgain and loss factors, that influence the emission wavelength of an ECDL with a surfacegrating as dispersive element [113]. These factors are the transmission functions of the threeresonators, the semiconductor gain profile g and the dispersion of the surface grating D.The product of these factors gives the spectral response of the ECDL:

TECDL = T12 · T13 · T23 · g ·D (4.1)

T12, T13 and T23 are the transmission functions of the resonators between R1 and R2, R1and R3, and R2 and R3, respectively, that are described by the Airy-function [114]:

T = 11 + F sin2(∆φ/2) . (4.2)

F = 4√R1R2/(1 −

√R1R2)2 is the finesse coefficient with, in case of the laser diode

cavity, the intensity reflectivities of the rear and front facets R1 and R2, respectively.∆φ = 4πng,effLc/λ is the phase difference after one round trip with ng,eff being the effectivegroup refractive index and Lc the respective cavity length.The dispersion of the surface grating can be described as a diffracted intensity D, approx-imated by assuming a square-slit profile with the slit width equal to half of the gratingperiod dG of the surface grating [114]:

D =[ sin(kNdG sinαL)N sin(kdG sinαL)

]2sinc2

(kdG sinαL

2

), (4.3)

where k = 2π/λ is the wave vector, N = 2r/dG the number of illuminated grooves for agiven beam radius r and grating constant dG (distance between grating grooves), and αLis the Littrow angle, that is introduced in section 4.1.1.

4.1 Wavelength stabilization by external optical feedback 39

Figure 4.2.a) illustrates a simulation of the relative dispersive factors on the basis of apractical example of an ECDL setup, that is very similar to the ECDL systems realizedin this work. The black curve shows the transmission function of the laser diode cavity,whereas the green curve shows the combined transmission functions of the cavities formedbetween mirror R1 and R3 and between R2 and R3. The assumed values for the intensityreflectivities in this example are R1 = 0.96, R2 = 0.05, and R3 = 0.15. Even thoughthe exact laser diode facet reflectivities are unknown, the assumed values for R1 and R2represent typical values for a high-power InGaN based laser diode (see [115], p.2). Thevalue for R3 corresponds to the reflectivity of the surface grating and the volume Bragggrating used in this work. The cavity length of the laser diode is L = 1.2 mm. The grouprefractive index was calculated to be ng,eff = 3.02 at 442 nm. This gives a free spectralrange of ∆λFSR = 27 pm. As can be seen, these values correspond to the characterizationof the applied laser diode in the previous chapter. The distance between the front facet andthe dispersive element is set to be L = 2 cm, which is approximately the distance betweenthe volume Bragg grating and the front facet of the laser diode in the micro-integratedECDL setup realized in this work (see section 4.3). The resulting free spectral range of thecombined external cavities is about ∆λFSR = 4 pm.The red line illustrates the grating feedback dispersion for a beam radius r = 1.4 mm,assuming a grating with a groove density of 3600 lines/mm resulting in a grating constantof dG = 278 nm which gives N = 10080. The center wavelength of the grating feedbackis at λ = 445 nm for a Littrow angle of αL = 53.23◦. These values correspond to theexperimental setup of the macroscopic ECDL in Littrow configuration, that will be presentedin section 4.2. In this case, the optical feedback from the grating has a spectral bandwidthof ∆λ = 38 pm (FWHM).The gain profile g (blue line) is very broad compared to the other dispersive factors andtherefore appears to be constant in figure 4.2.a), even though it is approximated with aGaussian distribution. The resulting spectral response of the ECDL system as the productof all dispersive factors is depicted in figure 4.2.b) (black line). The grating dispersion

Figure 4.2: a) Calculated transmission functions in a three-mirror ECDL: The green lineis the combined transmission function of the the cavities formed between mirror R1 andR3 and between R2 and R3. The black line is the transmission function of the laser diodecavity. The red line indicates the dispersion D of the surface grating and the blue line thesemiconductor gain. b) Spectral response of the ECDL system as the product of all disper-sive factors (black line) and grating dispersion D (grey line) for comparison.


indicated as grey line is the dominant mechanism here and one can see several externalcavity modes within the spectral window of the grating feedback. Even though thesemodes have a very similar combined gain, it is possible to achieve single-mode operationunder such conditions [116–118]. However, figure 4.2.b) shows, that without further activecontrol of the setup, several external cavity modes will oscillate at the same time and theECDL emission bandwidth is expected to be similar to or slightly smaller than the spectralbandwidth of the external feedback.

4.1.1 ECDLs with surface diffraction gratings

ECDLs can be realized with different optical elements as wavelength selective filter. Inthe most common approach, diffraction gratings are used to direct a narrow bandwidthportion of the incident light back into the laser diode cavity. These gratings can be surfacediffraction gratings or volume Bragg gratings. The former are typically aligned in eitherthe Littrow [116, 117, 119] or the Littman-Metcalf [120, 121] configuration as illustratedin figure 4.3.In the Littrow configuration (see figure 4.3.a)), the first diffraction order from the gratingis directly sent back into the laser diode through the front facet of the laser diode chipand the light reflected from the grating, e.g. the zeroth order diffraction, is used as outputbeam. This is realized by aligning the grating in the so-called Littrow angle for whichthe incident beam and the diffracted first order beam are anti-parallel to each other. TheLittrow angle can be calculated from the grating equation mλ = dG(sinα+sin β), where mis the diffraction order, λ the wavelength of the incident light, and α and β are the anglesbetween the incident beam and the grating surface normal and between the diffractedbeam and the grating surface normal, respectively. As α equals β for the first diffractionorder, the Littrow angle αL is calculated by:

αL = arcsin(

λ

2dG

). (4.4)

Wavelength tuning is realized by tilting the grating and thereby changing the angle of theincident light with respect to the grating normal, e.g. the Littrow angle. The drawback ofthe Littrow configuration is the change of the output beam direction during wavelengthtuning. This lateral beam displacement can be compensated by using an additional mirrorparallel to the grating [122] or a triangular prism [123].In the Littman-Metcalf, also called grazing-incidence, configuration (see figure 4.3.b)), thefirst diffraction order from the grating is directed onto a mirror, which reflects the light

0 orderth

1 orderst

0 orderth

1 orderst

laserdiode

gratinggrating

mirror

lenslens

a) b)

aL

laserdiode

Figure 4.3: Illustration of an external cavity diode laser in a) Littrow and b) Littman-Metcalf configuration.


back onto the grating from where it is diffracted back to the front facet of the laser diodechip. The zeroth order reflection serves as output beam.Wavelength tuning in the Littman-Metcalf configuration is achieved by a rotation of themirror, that reflects the first order diffraction back onto the grating, that is kept in afixed position. The advantage of the Littman-Metcalf configuration is a fixed output beamduring wavelength tuning, which is more practical in experimental setups. Additionally,the bandwidth of the optical feedback is smaller than in the Littrow configuration, as thelight is diffracted twice on the surface grating before it is directed back into the laser diodecavity. The drawback of this configuration is a lower output power than in the Littrowconfiguration, because the reflection from the first diffraction order beam on the gratingcannot be used (dashed arrow in figure 4.3.b)).In practice, the emission bandwidth of an ECDL system is influenced by electronic andacoustic noise, and mechanical vibrations and therefore the simpler and more robust Littrowconfiguration often shows smaller emission bandwidths.

To achieve a high suppression of the laser diode cavity modes, the outcoupling facetof the laser diode is often anti-reflection (AR) coated resulting in a very low reflectivity R2in the order of 10−4. When a high AR-coating is applied, the laser diode only serves asthe active medium and does not show laser operation itself. In this case, laser operation isonly possible with the additional external optical feedback and the external cavity modesoriginating from the cavity formed by the rear facet (R1) and the external dispersive ele-ment (R3) dominate the mode spectrum relatively independent of the operating conditions.With such ECDL systems, a high stability of the lasing frequency and extremely narrowlinewidths down to the Hz regime have been achieved at low output powers using narrowstripe GaAs based laser diodes [124].

In the following, publications on GaN based ECDL setups with surface diffraction gratingsare briefly presented. Since the Littrow configuration promises to result in higher opticaloutput powers and also in a more compact and robust laser system, only setups in thisconfiguration will be presented.


Low-power GaN based ECDLs in Littrow configuration

Low-power lateral single-mode GaN based laser diodes emitting in the violet and blue spec-tral range have already been integrated into external cavity setups with surface gratings inLittrow configuration [125–130]. The electro-optical ECDL parameters from a few selectedpublications are summarized in table 4.1.In some works, the laser diodes were treated with an AR coating: Lonsdale et al. demon-strated a coarse tuning range of 6.3 nm around a wavelength of 398 nm with an emissionbandwidth of 11 MHz (6 fm) using a GaN based laser diode with a maximum continuouswave (CW) output power of 5 mW [126]. They showed, that an AR coating significantlyincreases the tuning range compared to the same device without AR coating.Hildebrandt et al. demonstrated an ECDL output power of 20 mW (CW) at 410 nm withan emission bandwidth of 0.8 MHz (0.45 fm) using an AR coated GaN based laser diode[127]. They also showed the tuning enhancement effect of an AR coating and evaluatedthe effect of optical feedback on the lasing threshold and the photon density of the FPlaser diode. Using a laser diode emitting around 405 nm, that had a front facet reflecitivitybelow 1%, Tanaka et al. presented a Littrow-ECDL with a single-mode output power of80 mW (CW) and a linewidth of ∆ν = 21 MHz (12 fm) [130].

Ref. AR λ PLD PECDL ∆ν ∆λtun,contin ∆λtun,coarse

[126] (2002) y 398 nm 5 mW N/A 11 MHz 0.4 pm 6.3 nm[127] (2003) y 410 nm 22 mW 20 mW 0.8 MHz 28 pm 4 nm[130] (2007) y 405 nm N/A 80 mW 21 MHz N/A N/A

[125] (2000) n 392 nm 5 mW 3.5 mW 5 MHz 3 pm 2.7 nm[128] (2004) n 450 nm 5 mW 1 mW 8 MHz 71 pm N/A[129] (2005) n 410 nm 30 mW 1 mW N/A 50 pm N/A

Table 4.1: ECDL parameters from a selection of publications on low-power GaN basedECDLs in Littrow configuration. PLD: nominal maximum laser diode output power,PECDL: maximum ECDL output power, ∆ν: ECDL emission bandwidth, ∆λtun,contin: mode-hop-free or continuous tuning range, ∆λtun,coarse: manual or coarse tuning range.

An easier and more inexpensive approach is to use a commercially available diode laser,that may be housed in a standardized TO package and that may not have an AR coatedfacet. Conroy et al. demonstrated an output power of 3.5 mW (CW) at 392 nm with anemission bandwidth of ∆ν = 5 MHz and a coarse tuning range of 2.7 nm using a laserdiode without AR-coating [125].Burns et al. and Hult et al. used GaN based laser diodes without AR coatings emitting at410 nm and 450 nm in ECDL setups delivering a single-mode CW output power around1 mW with emission bandwidths of 8 MHz (5 fm) [128, 129]. By modulating the diodeinjection current, and thus modulating the effective laser diode cavity length, in synchro-nization with the external cavity tuning, they matched the diode FP and external cavitymodes and thereby achieved a mode-hop-free tuning range of 50 pm (90 GHz) and 71 pm(105 GHz) at 410 nm and 450 nm, respectively.In order to obtain higher output powers, a GaN diode laser-based master-oscillator power-amplifier was presented by Shimada et al. and 110 mW (CW) of single-mode output powerat 461 nm was achieved [131].


High-Power ECDLs

Wavelength stabilization of the emission of broad-area [132] and tapered diode lasers[133, 134] and even diode-array bars [135, 136], that are based on GaAs, was realizedwith surface diffraction gratings in Littrow configuration. In these works, output powersin excess of 1 W with emission bandwidths in the GHz [132, 134, 135] and even MHz[133, 136] range were achieved.At the time of this work, no publications on high-power GaN based ECDLs with surfacegratings could be found in the literature.

The works cited above used lateral single-mode laser diodes in optical feedback setups witha focus on a very narrow linewidth in the kHz and MHz regime and high tunability of theemission wavelength rather than achieving high optical output powers. As stated earlier,the object of this work is to realize a pump source for frequency conversion with an outputpower in the watt range and narrowband emission in the order of the wavelength acceptancebandwidth of the BBO crystal of ∆λ = 42 pm (∆ν = 64 GHz at 445 nm) (FWHM).Only recently, GaN based laser diodes have reached output powers beyond 1 W and wave-length stabilization of high-power GaN based laser diodes in an external cavity setup wasnot realized until now. Therefore, the first objective of this work was to test the feasibilityof using a commercially available high-power GaN based laser diode as the active mediumin a non-sophisticated external cavity diode laser setup, that uses a surface diffractiongrating in Littrow configuration as wavelength selective element.The laser diode used throughout this work and that was characterized in the previouschapter (PL TB450B, OSRAM Opto Semiconductors) was chosen, because it provided thehighest optical output power of P = 1.6 W from a commercially available GaN based laserdiode at the time of this work.In the next section, the macroscopic ECDL in Littrow configuration will be presented and,for the first time, wavelength stabilization of a high-power GaN based laser diode will bedemonstrated.


4.2 Macroscopic external cavity diode laser in Littrowconfiguration

To understand the performance of the external cavity system, it is necessary to evaluate afew basic properties of the free-running laser diode, such as the peak position of its gaincurve below threshold, the center wavelength of the emission above threshold, and itswavelength shift with increasing injection current. These values are important, becausethe ECDL performance strongly depends on the spectral position of the optical feedbackin comparison to the center wavelength of the laser diode emission, e.g. the peak positionof the gain curve. For the macroscopic ECDL, another laser diode of the same batchwith the same specifications as the one presented in section 3 is used. The electro-opticalparameters might therefore slightly differ.Figure 4.4 shows a measurement of the amplified spontaneous emission (ASE) followingthe gain curve of the laser diode far below threshold at an injection current of I = 0.1 A.The spectrum is measured with a high dynamic range optical spectrum analyzer (OSA)(Yokogawa, AQ6373) with a dynamic range of 60 dB and a spectral resolution of 50 pm.The ASE takes its maximum at a wavelength of λC = 444.96 nm and shows an emissionbandwidth of around 8 nm (FWHM, - 3 dB).Three emission spectra of the free-running laser diode at an injection current of I = 0.2 A,I = 0.7 A, and I = 1.2 A, again measured with the optical spectrum analyzer, are depictedin figure 4.5. The center wavelength of the emission shifts with ∆λ/∆I = 3.9 nm/A fromλC = 444.01 nm at I = 0.2 A to λC = 447.87 nm at I = 1.2 A. This shift is mainly causedby Joule heating of the laser diode chip for higher injection currents and is sligtly higherhere than for the laser diode presented in section 3 due to differences in the packaging ormounting that influence the thermal management of the chip.

430 435 440 445 450 455 460

-10

-5

0C = 444.96 nm

Inte

nsity

I / d

B

Wavelength / nm

I = 0.1 AT = 20°C

Figure 4.4: ASE emission spectrum ofthe laser diode at an injection current ofI = 0.1 A.

Figure 4.5: Emission spectra of the laserdiode at an injection current of I = 0.2 A,0.7 A, and 1.2 A.

4.2 Macroscopic external cavity diode laser in Littrow configuration 45

4.2.1 Experimental Setup

The setup of the external cavity diode laser system, that is built up as a proof-of-conceptof using a commercially available high-power GaN laser diode in an ECDL system, isshown in figure 4.6. The FP laser diode (1), that was characterized in section 3, is usedas gain medium. Based on the measured spatial emission characteristics (section 3.6)of the laser diode and optical beam shaping simulations according to ISO 11146 usingthe program WinABCD1, an aspheric lens with a focal length of f = 4.02 mm (NA= 0.6, Thorlabs C671TME-A) with an AR-coating from 350 nm to 700 nm is used forcollimation (2). The collimated beam has an elliptic beam shape with beam diametersof dlat = 1.1 mm and dvert = 2.8 mm in lateral and vertical direction, respectively. Inthe course of this work, holographic surface diffraction gratings (3) with different groovedensities of 1800, 2400, and 3600 grooves/mm were tested in the same setup. The gratingson a 12.5 mm x 12.5 mm x 6 mm substrate (Richardson Gratings) are mounted on afour-axis diffraction grating mount (Newport, DGM-1) with the Pivot point located onthe surface of the grating. The distance between the laser diode and the surface grating isapproximately 15 cm. For the characterization measurements of the ECDL system, theoutput beam is redirected by a mirror (4).

aL

LFP

Linner

Louter

Pivot point

1 2

3

4

Figure 4.6: Schematic view of the ECDL system in Littrow configuration: (1) FP laserdiode, (2) collimating lens, (3) surface grating, (4) mirror.

The diffraction efficiency of the gratings in the first diffraction order depends on theorientation of the laser diode polarization with respect to the grooves of the grating.Additionally, the spectral bandwidth of the grating dispersion depends on the number ofilluminated grating grooves. Figure 4.7 illustrates two possible configurations of the ECDLsetup in Littrow configuration. In configuration a), the laser diodes fast axis is alignedparallel to the grating grooves, and, as the laser diode is TE-polarized (TE:TM = 100:1),the polarization direction E is perpendicular to the grating grooves. Here, the lateralplane (slow axis) of the laser diode undergoes dispersion. The number of illuminatedgrating grooves for a grating with 3600 grooves/mm (groove density dG = 278 nm) isN = 2 · rlat./dG = 2 · 0.55 mm/278 nm = 3960 in this configuration.In configuration b), the laser diode is aligned with its fast axis perpendicular to the gratinggrooves and therefore its polarization is in the p-plane of the grating.

1 WinABCD is a ray transfer matrix analysis software developed at the Ferdinand-Braun-Institut by Dr.Bernd Eppich


E

fastaxis

LD

grating

E

fastaxis

LD

grating

a) b)

Figure 4.7: Illustration of two ECDL configurations: a) The laser diode (LD) polarizationis perpendicular to the grating grooves, the grating dispersion occurs in the lateral LDplane. b) The LD polarization is parallel to the grating grooves, the grating dispersionoccurs in the vertical LD plane.

Here, the vertical plane of the laser diode undergoes the grating dispersion. Due to thelarger beam diameter in vertical direction of 2.8 mm, more grating grooves (N = 10080)are illuminated in this configuration, which leads to a higher grating resolution than inconfiguration a), i.e. a smaller spectral bandwidth of the grating feedback. On the otherhand, the vertical aperture of a laser diode is much smaller than the lateral aperture.Therefore, configuration b) requires a higher precision of the optical alignment to efficientlycouple the optical feedback back into the laser diode cavity and the sensitivity of the setupto mechanical vibrations is higher.

The grating constants dG and the Littrow angles αL for a wavelength of 445 nm arelisted in table 4.2 for different groove densitites of 1800, 2400, and 3600 grooves/mm.Depending on the groove density and the respective beam diameter, the number of illumi-nated grooves varies for configuration a) and b) and is listed in table 4.2 as well. Insertingthese values into equation (4.3) gives the expected spectral bandwidth ∆λ (FWHM) of thegrating feedback for each grating and configuration as listed in the table. The diffractionefficiency DE given by the supplier (Richardson Gratings) at a wavelength of λ = 445 nm[137] is also listed in table 4.2 for each grating and configuration.Only for a grating with 3600 grooves/mm aligned in configuration b), a sufficient spectralresolution of ∆λ‖ = 38 pm in the range of the wavelength acceptance bandwidth of the BBOcrystal can be achieved. Additionally, the diffraction efficiency in this case is approximatelyDE‖ = (15± 3) %, given by the supplier [137]. This amount of optical feedback should besufficient to stabilize the FP laser diode whereas it also assures that as much as 85 % of the

configuration a) configuration b)grooves/mm dG αL N⊥ ∆λ⊥ DE⊥ N‖ ∆λ‖ DE‖

1800 556 nm 23.61◦ 1980 184 pm 45 % 5040 78 pm 72 %2400 417 nm 32.28◦ 2640 138 pm 94 % 6720 58 pm 48 %3600 278 nm 53.23◦ 3960 92 pm 78 % 10080 38 pm 15 %

Table 4.2: Spectral bandwidth ∆λ and diffraction efficiency DE for the three gratingswith different groove densities G with the laser diode polarization E perpendicular to thegrating grooves (∆λ⊥, DE⊥), or parallel (∆λ‖, DE‖). The measurement uncertainty for thediffraction efficiency values is specified by Richardson Gratings to be ±3 % [137].


Figure 4.8: Simulated normalized diffracted intensity D in the first diffraction order for asurface diffraction grating with 3600 grooves/mm in configuration b).

optical output power of the free-running laser diode can be coupled out through the zerothorder reflection - assuming the ideal case of no absorption or stray light losses. In all otherconfigurations, the expected spectral bandwidth of the grating feedback is broader andthe diffraction efficiency in the first diffraction order is much higher, which would result ina broader emission spectrum and significantly less optical output power of the ECDL system.

As expected from these values, the ECDL system with a holographic surface gratingwith 3600 grooves/mm in configuration b) showed the best results with respect to a narrowspectral emission bandwidth and high optical output power. Therefore, only the results forthis ECDL system will be presented in section 4.2.2. For convenience, the simulated normal-ized diffracted intensity D in the first diffraction order for a grating with 3600 grooves/mmand at a wavelength of λ = 445 nm set up in configuration b) is presented in figure 4.8.It is calculated according to equation (4.3) by using the experimental parameters of thesetup from figure 4.6, that are summarized again in table 4.3:

groove distance dG 278 nm

wavelength λ 445 nm

Littrow angle αL 53.23◦

beam radius r 1.4 mm

illuminated grooves N 10080

Table 4.3: Experimental parameters used for the calculation of the grating dispersion(figure 4.8) for a grating with 3600 grooves/mm.


4.2.2 Electro-optical characteristics

In the following, the performance of the ECDL system with a surface grating with3600 grooves/mm in configuration b) (see figure 4.7) is presented. By tilting the sur-face grating around the Pivot point, the Littrow angle and therefore the center wavelengthof the grating feedback is changed so that the ECDL emission wavelength can be tuned overa certain wavelength range. The grating mount has a rotation sensitivity of ∆θ = 0.003◦(10 arc sec). According to equation (4.4) this correspondents to a wavelength tuningsensitivity of ∆λsens ≈ 17 pm.The lower graph in figure 4.9 shows the lasing threshold current of the ECDL systemfor different wavelengths. For each measurement point the grating is tilted by 0.09◦corresponding to wavelength steps of 0.5 nm. The threshold current is determined byslowly reducing the injection current and observing the emission spectrum of the ECDLsystem with a double-echelle monochromator (LTB Lasertechnik Berlin GmbH) with aresolution of approximately 6 pm. For each measurement point, the ECDL alignment isslightly readjusted and the lowest injection current at which stable, single longitudinalmode emission is observed, is taken as the threshold current. The curve clearly followsthe gain curve in figure 4.4, that is again shown in the upper graph of figure 4.9 for bettercomparison. At the maximum of the gain curve at a wavelength of λ = 445 nm, the lowestthreshold current of Ith,ECDL = 115.4 mA is measured. This is due to the fact, that thethreshold carrier density of the free-running laser diode is the lowest at the maximum ofthe gain curve [86, 138]. The dashed red line in figure 4.9 indicates the lasing thresholdfor the free-running laser diode, which is Ith,LD = 164 mA. It can be seen that the ECDLlasing threshold is smaller than the LD threshold for all ECDL wavelengths, which is inagreement with the theory [101] and other experiments [125–127].The highest suppression of the amplified spontaneous emission of the laser diode andtherewith the best ECDL performance can therefore be expected for an ECDL emissionwavelength of λECDL = 445 nm.

Figure 4.9: Upper part: ASE emission spectrum of the laser diode at an injection currentof I = 0.1 A. Lower part: Lasing threshold current versus emission wavelength for theECDL system at a heatsink temperature of 20◦C.


Figure 4.10 shows the optical output power of the ECDL system emitting at 445 nm andof the free-running FP laser diode as a function of the injection current, both at a heatsinktemperature of T = 20◦C. The curve for the ECDL system was only measured up to aninjection current of 600 mA, because of concerns in the beginning of the experiments, thatthe laser diode could be damaged for high levels of feedback at higher injection currents.

FP laser diodeECDL at 𝜆ECDL = 445 nm

Figure 4.10: Optical output power of the ECDL system emitting at 445 nm and the FPlaser diode for a heatsink temperature of 20◦C.

The FP laser diode has a lasing threshold of Ith,LD = 164 mA and a slope efficiency ofSLD = 1.6 W/A. At the maximum studied injection current of 600 mA, the ECDL has anoptical output power of 545 mW and the FP laser diodes output power at this current is690 mW. The ECDL system exhibits a reduced lasing threshold of Ith,ECDL = 130 mA,and a reduced slope efficiency of SECDL = 1.2 W/A.The reduced slope efficiency is also observed in other ECDL systems and can be understoodby reminding the relation between the differential efficiency ηd and the slope efficiencyS given in equation (3.7), which says that S ∝ ηd. From equation (3.6) for the powercharacteristics of a laser diode above threshold the following equation for ηd can be derived(see [86], p.45):

1ηd

= 1ηi

+ αiηi

2ln(

1R1R2

) . (4.5)

If the reflectivity of the front facet R2 is now replaced by an effective reflectivity R2,eff thatis the composite reflectivity of the back facet reflectivity R2 and the reflectivity R3 of theexternal element (in a three mirror cavity model as described for instance in [101]), onecan see that

S ∝ ηd ∝ ln(

1R1R2,eff

). (4.6)

In ECDL systems, R2,eff is typically larger than R2 (R2,eff > R2), and this should also bethe case for the ECDL systems presented in this work. From the proportionality given inequation (4.6) then directly follows that the slope efficiency S is reduced.The deviation in the threshold current to the previous measurement (in figure 4.9) is causedby the high sensitivity to even minor adjustments. Due to thermal effects, the optimumadjustment with respect to a narrow emission bandwidth depends on the operating point


of the injection current and may therefore be different for low and high injection currents.For the power current characteristics, the ECDL system is adjusted for a minimal emissionbandwidth at the maximum injection current of 600 mA. This adjustment then leads to aslightly higher threshold current compared to the previous adjustment for a the minimizedthreshold current.


4.2.3 Spectral emission characteristics

The spectral emission characteristics of the ECDL system are depicted in figure 4.11. Itshows emission spectra of the ECDL system for an ECDL wavelength of λECDL = 445 nmfor different injection currents of I = 0.2 A, 0.4 A, 0.6 A, 0.8 A, 1.0 A, and 1.2 A. Thespectra of figure 4.11.a) are measured with a spectral resolution of 6 pm at 445 nm. Figure4.11.b) shows corresponding emission spectra measured with the OSA with a spectralresolution of 50 pm.The emission has a spectral bandwidth of ∆λ ≤ 15 pm (FWHM) up to an injection currentof 0.6 A, corresponding to an optical output power of PECDL = 545 mW. In this operatingrange, the emssion spectrum is dominated by a single longitudinal mode of the laser diodecavity. However, an increasing number of side modes starts to oscillate around the mainpeak with increasing injection current. These side-modes have a spectral distance to themain peak and to each other, respectively, of ≈ 27 pm and can therefore be identified aslongitudinal modes of the laser diode cavity. The emission spectra for higher injectioncurrents are dominated by more than one longitudinal mode from the laser diode cavity,which is exemplary shown for an injection current of 0.8 A, and 1.0 A.

0.0

0.5

1.0 I = 0.2 A

15 pm

0.0

0.5

1.0

14 pm

I = 0.4 A

0.0

0.5

1.014 pm

I = 0.6 A

0.0

0.5

1.033 pm

I = 0.8 A

Norm

aliz

ed in

tens

ity

0.0

0.5

1.040 pm

I = 1.0 A

444.8 445.20.0

0.5

1.015 pm

I = 1.2 A

Wavelength / nm

-60

-30

0b)

32 dB

24 dB

a)I = 0.2 A

I = 0.4 A

I = 0.6 A-60

-30

0

-60

-30

042 dB

-60

-30

0 I = 0.8 A19 dB

Inte

nsity

/ dB

-60

-30

0 I = 1.0 A 14 dB

435 440 445 450 455-60

-30

0 I = 1.2 A 4 dB

Wavelength / nm

Figure 4.11: Emission spectra of the ECDL system at injection currents of I = 0.2 A,0.4 A, 0.6 A, 0.8 A, 1.0 A, and 1.2 A measured with a) a double-echelle monochromatorwith a spetral resolution of 6 pm and b) an optical spectrum analyzer with a spectral resolu-tion of 50 pm, for 20◦C heatsink temperature.


The spectral bandwidth however does not exceed 40 pm (FWHM), which is almost identicalto the simulated spectral bandwidth of the diffracted intensity from the surface gratingof 38 pm (FWHM, see figure 4.8). At I = 1.2 A, the ECDL bandwidth is smaller thanat I = 1.0 A, which can be explained by the wavelength shift of the gain with respect tothe ECDL wavelength. At I = 1.2 A, the wavelength difference between gain maximumand ECDL wavelength is larger than at 0.8 A or 1.0 A as can be seen in figure 4.11.b).Thus, the laser diode cavity modes around 445 nm experience less gain and the modediscrimination induced by the grating dispersion is therefore strong enough to favor onlyone of the longitudinal laser diode cavity modes. However, the complete emission spectrumat 1.2 A, which can be seen in figure 4.11.b), is dominated by the laser diode cavity modesaround the gain maximum as their suppression is only 4 dB.Figure 4.11.b) shows how the suppression of the amplified spontaneous emission and laserdiode cavity modes depends on the spectral position of the gain maximum with respect tothe ECDL emission wavelength. Here and in the following, this suppression ratio will bedenoted as SR value in decibel.

As shown in figure 4.5, the center wavelength of the free-running laser diode emissionexhibits a shift of 3.9 nm/A. For lower injection currents, the gain takes its maximum atshorter wavelengths than the center wavelength of the feedback at 445 nm. With the shiftof the gain maximum with increasing current, the SR increases and reaches its highestvalue of 42 dB at I = 0.6 A, where the wavelength of the gain maximum coincides withthe ECDL emission wavelength. The onset of the side-modes seen in figure 4.11.a) is dueto the increasing gain of the laser diode cavity modes, that are not sufficiently suppressedby the mode discrimination from the grating feedback for higher injection currents.At the operating point of 0.6 A, corresponding to an ECDL output power of 545 mW,the ECDL system shows the best performance in terms of emission bandwidth reductiontogether with a high SR value. Figure 4.12 shows a spectrum of the ECDL and the laserdiode (LD) at 0.6 A for comparison. An emission bandwidth reduction by a factor of 73

Figure 4.12: Emission spectra of the FP laser diode (grey) and the ECDL system (red) atI = 0.6 A and T = 20◦C.


from ∆λFP = 1.1 nm to ∆λECDL = 15 pm (FWHM) is achieved. Above I = 0.6 A, thegain shifts towards longer wavelengths than the feedback wavelength and the suppressionof the laser diode cavity modes around the gain maximum decreases in comparison to themodes selected by the optical feedback. Therefore, a majority of the ECDL output powercomes from the laser diode cavity modes around the gain maximum and does not lie in thespectral window of the grating feedback anymore.

This is evaluated by linearizing the normalized spectra from figure 4.11.b) and com-paring the integral over the ECDL peak wavelength range from 445.01 nm to 445.29 nm(ECDL peak wavelength is 445.15 nm) with the integral over the whole laser diode emissionfrom 420 nm to 470 nm. This way, the ratio of the optical power PRECDL, that is containedin the ECDL peak, can be calculated and is summarized in table 4.4. The higher the SRvalue, the higher is the ratio PRECDL of the ECDL peak power with a maximum of 99.7 % at0.6 A. This ratio significantly decreases for higher injection currents. At 1.2 A, less than 10% of the total laser diode power is included in the ECDL peak wavelength range. It will beseen in section 5.1.2 that this behavior has a negative effect on the SHG conversion efficieny.

I ∆λ (FHWM) SR PRECDL

0.2 A 15 pm 24 dB 93.4 %0.4 A 14 pm 32 dB 99.0 %0.6 A 14 pm 42 dB 99.7 %0.8 A 33 pm 19 dB 88.9 %1.0 A 40 pm 14 dB 73.3 %1.2 A 15 pm 4 dB 9.9 %

Table 4.4: Summary of the measured spectral parameters of the macroscopic ECDL sys-tem emitting at 445 nm.


4.2.4 Wavelength tuning

By tilting the surface grating and thereby changing the Littrow angle the emission wave-length of the ECDL in Littrow configuration is tuned. The wavelength tuning behaviorof the ECDL system at an injection current of I = 0.6 A can be seen in figure 4.13. Itshows emission spectra of the ECDL system for different Littrow angles corresponding toemission wavelengths of 443 nm, 444 nm, 445 nm, 446 nm, and 447 nm, indicating a coarsetuning range of about 4 nm.While the emission bandwidth of the ECDL peak is well below 20 pm (FWHM) over thewhole tuning range, the suppression of the longitudinal modes of the laser diode cavity(SR) depends on the position of the ECDL wavelength with respect to the gain maximumaround 445 nm.

-60-40-20

0

-60-40-20

0

-60-40-20

0

-60-40-20

0

435 440 445 450 455-60-40-20

0

ECDL = 443 nm 13 dB

ECDL = 444 nm32 dB

I = 0.6 A

ECDL = 445 nm42 dB

Inte

nsity

/ dB

ECDL = 446 nm34 dB

ECDL = 447 nm12 dB

Wavelength / nm

Figure 4.13: Emission spectra of the ECDL system at an injection current of I = 0.6 A fordifferent Littrow angles measured with the optical spectrum analyzer with a resolution of50 pm at 20◦C heatsink temperature.

This again illustrates why the laser diode coupling facet is usually AR-coated in ECDLsystems. Without AR-coating, the oscillating laser diode cavity modes can only be su-pressed efficiently when the feedback is close to the gain maximum, as it is the case atλ = 445 nm for this ECDL system. In general, the tuning range depends on the feedbackstrength and the reflection of the facet through which the optical feedback is coupled backinto the laser diode cavity. An AR-coating would likely enhance the wavelength tuningrange of the ECDL system compared to the ECDL system without AR-coated couplingfacet [127]. Larger feedback also enhances the tuning range, but leads to a lower ECDLoutput power [138].


4.2.5 Spatial emission characteristics

The beam quality of the ECDL system is determined by a caustic measurement accordingto ISO 11146 using the variance and the 90%-knife edge method, respectively. Figure 4.14shows the caustic in the fast (left) and slow axis (right) at an injection current of I = 0.6 A,corresponding to an output power of 545 mW. The variance beam propagation factors areM2

4σ,fast = 4.0 and M24σ,slow = 7.7 in the fast and slow axis, respectively. The 90%-knife

edge beam diameter values are M290%,fast = 2.1 and M2

90%,slow = 4.7 in the fast and slowaxis, respectively.

The values in both directions are slighlty larger than the ones for the free-running laserdiode. Though it must be noted that the laser diode used in the macroscopic ECDL isnot the same that was presented in chapter 3. However, the slightly larger values couldbe attributed to an increase of peripheral intensities around the central beam profile thatmight be caused by random stray light from the surface diffraction grating. As explained insection 3.6, the precise determination of the beam quality for this laser diode is difficult dueto the high amount of peripheral intensities and must therefore interpreted with caution.In conclusion, it is believed that the optical feedback does not have a relevant effect on thebeam quality of the ECDL system.

100 200 300 400 5000.0

0.5

1.0

1.5

2.0

50 100 150 200 250 300 350

M290% = 2.1

M24 = 4.0


Beam

wai

st d

iam

eter

d /

mm

Position z / mm

fast axis I = 0.6 A

M290% = 4.7

M24 = 7.7


slow axis I = 0.6 A

Figure 4.14: Measured caustic in the fast (left) and slow axis (right) of the ECDL systemat an injection current of I = 0.6 A using variance and the 90%-knife edge method.


4.3 Volume-Bragg-Grating stabilized external cavity diodelaser module (µECDL)

The proof-of-concept study presented in section 4.2 has proven the feasibility of a spectralbandwidth reduction of the emission from a high-power GaN based laser diode by usinga surface diffraction grating as wavelength selective optical element in a simple ECDLsystem without an additional AR-coating of the laser diode front facet. It has also beenshown that the spectral bandwidth of the grating dispersion needs to be in the range ofthe desired spectral emission bandwidth of the external cavity system.

In this section, spectral narrowing and wavelength stabilization of the same type of laserdiode is demonstrated in a micro-integrated external cavity diode laser module (µECDL).In this module, the wavelength selective optical feedback is provided by a holographicreflecting volume Bragg grating (VBG). The use of a VBG opens up the possibility of avery compact and robust laser diode module without moveable parts. Reflecting VBGs canhave reflectivities up to 99% in a narrow wavelength range below 100 pm. For GaAs laserdiodes, reflecting VBGs were used with ridge-waveguide diode lasers in a rear externalcavity [139]. Mainly, they are used with reflectivities of 20%-60% for wavelength narrowingand stabilization of broad-area diode lasers and arrays by placing them directly behind thecollimation optics [140, 141], as it will also be presented in this section.First, the working principle of a VBG will be explained. A detailed description of theconcept and the development of the µECDL module follows before the electro-optical,spectral and spatial emission characteristics of the module are analyzed.

4.3 Volume-Bragg-Grating stabilized external cavity diode laser module (µECDL) 57

4.3.1 Working principle of volume Bragg gratings

A holographic VBG is a structure with a uniformly spaced refractive index variation in thevolume of a photosensitive medium. The refractive index modulation is recorded in so-calledphoto-thermo-refractive (PTR) glass by exposure with the interference pattern of UV laserlight, typically with a wavelength between 280 nm and 350 nm, and a consecutive thermaldevelopment at a temperature of around 520◦C. The first recording of such a hologram inPTR glass was realized in the late 80s [142] and resulted in a further development of thetechnology and the production of highly efficient VBGs in the following years [143–145]. Acomprehensive review of the photo-thermo mechanism in PTR glass is given by Lumeauand Zanotto [146]. PTR glass is Na2O-ZnO-Al2O3-SiO2 glass doped with silver (Ag),cerium (Ce), and fluorine (F). It is transparent in a wavelength range of 350 nm - 2500 nmand characterized by very stable thermo-mechanical properties up to a temperature of400◦C. With dn/dT = 5 · 10−8 K−1 the thermal variation of the refractive index is verylow and results in a small thermal shift of the Bragg wavelength of ∆λB/∆T = 7 pm/K[147].

The working prinicple of a VBG is based on the diffraction of light in a Bragg grat-ing structure and was described in detail with a coupled wave theory formulated byKogelnik in 1969 [148]. Figure 4.15 shows a model of a reflecting volume Bragg gratingwith parallel Bragg planes of varying refractive index indicated by dashed lines, that areslanted at an angle φ with respect to the surface of the VBG medium. The grating vector Khas a length of K = 2π/Λ (with Λ being the grating period) and is oriented perpendicularto the parallel Bragg planes. The incident beam Rin hits the grating with the Bragg angleθ to the grating surface normal, experiences diffraction inside the grating and is coupledout via the output beam Sout.

zf

L

K

d

qR

in

Sout

Figure 4.15: Model of a holographic reflecting VBG with thickness d. Rin: incident beam,Sout: output beam, K : grating vector, θ: Bragg angle, φ: slanting angle, Λ: grating period,z: optical axis.

Based on the results obtained with the surface diffraction grating in the macroscopicECDL system, a reflecting Bragg grating with comparable diffraction efficiency and spec-tral selectivity, produced by the company Optigrate Corp., is applied in the µECDL module.


The diffraction efficiency DE for a lossless reflecting volume Bragg grating is

DE =(

1 + 1− ξ2/ν2

sinh2√ν2 − ξ2

)−1

(4.7)

and also includes a description of the angular and wavelength sensitivity of the grating. Incase of an unslanted (φ = 0◦) reflecting VBG and an incident beam perpendicular to thegrating surface (θ = 0◦), the parameters ξ and ν become

ξ = − ϑ · d2(1− λ/(nav · Λ)) , (4.8)

ν = i π · δn · dλ ·√

1− λ/(nav · Λ)(4.9)

withϑ = 2π

Λ− π · λnav · Λ2 . (4.10)

The other parameters in the equations are the grating thickness d, the average refractiveindex of the grating nav, the refractive index modulation δn, and the wavelength λ.

The chosen grating has an aperture of 10 mm x 10 mm and a thickness of 2.7 mm. Itis AR-coated on both sides with R < 0.5% at the resonant wavelength of 445 nm. Asspecified by Optigrate Corp, the VBGs maximum diffraction efficiency is DEmax = 17.9%at its resonant Bragg wavelength λB = 445 nm. By using the parameters summarizedin table 4.5, the diffraction efficiency of the grating as a function of the wavelength iscalculated using equation (4.7) and is depicted in figure 4.16.The grating is unslanted (φ = 0) and has a Bragg angle of θ = 0 for 445 nm. The spectralselectivity of the grating, i.e. the spectral bandwidth of the diffracted light, is specified to be

Parameter Symbol Value

Bragg wavelength λB 445 nmGrating thickness d 2.7 mm

Average refractive index nav 1.4933Grating period Λ 149 nm

Refractive index modulation δn 22 ppm

Simulated diffraction efficiency DEmax 15.7%Simulated spectral selectivity (FWHM) ∆λsim 24 pm

Manufacturer specificationsDiffraction efficiency DEmax 17.9%

Spectral selectivity (FWHM) ∆λ (30 ± 10) pm

Table 4.5: Parameters used to simulate the diffraction efficiency of the reflecting VBG ap-plied in the µECDL module and the obtained values for DEmax at 445 nm and the spectralselectivity ∆λ. Lower part: manufacturer (Optigrate Corp.) specifications for DEmax and∆λ.


∆λ = (30± 10) pm (FWHM). The simulated diffraction efficiency curve shows a maximumDEmax = 15.7% at 445 nm and a spectral bandwidth of ∆λsim = 24 pm (FWHM). Thesimulated spectral bandwidth is well within the uncertainty of the specified spectral band-width and the deviation in the diffraction efficiency can be explained by usual deviationsin the manufacturing process.

The simulated normalized diffraction efficiency as a function of the deviation from theBragg angle is shown in figure 4.17. For a deviation of ±0.48◦ of the incident light beamfrom the Bragg angle, the diffraction efficiency decreases to 50% of its maximum. Thismeans, that the light incident on the VBG needs to be precisely collimated to make use ofthe maximum diffraction efficiency.

Figure 4.16: Simulation of the wavelength dependence of the diffraction efficiency DEaccording to equation (4.7) for a reflecting VBG with the parameters from table 4.5.

Figure 4.17: Simulation of the angle dependence of the diffraction efficiency DE accordingto equation (4.7) for a reflecting VBG with the parameters from table 4.5.


4.3.2 Concept and development of the µECDL module

The concept of the µECDL module is illustrated in figure 4.18. For the µECDL, the laserdiode characterized in section 3.4 is used as gain medium. The protective glass of theTO56 can (W) has a thickness of 250 µm and the laser diode cavity length is 1.2 mm.Based on the lateral and vertical far-field angles of the laser diode emission and opticalsimulations, an aspheric lens (L) with a focal length of f = 2.54 mm and a numeric apertureof NA = 0.66 is used to collimate the laser light, which is then directed onto the VBG. Thelens (R < 0.5%) and the VBG (R < 0.5%) are AR-coated for 445 nm. Figure 4.18 alsoshows the dimensions of the µECDL elements and their distances to each other along theoptical axis z in millimeters.

1.8 2.7

1.6

1.2

W

VBG

L

LD

Linner

Louter

LLD

4.0

z

yx

Figure 4.18: Concept of the µECDL module. Also shown are the distances between thedifferent elements of the module and the three competing resonators of length LLD, Linner,and Louter

.As explained in section 4.1, without AR-coating of the laser diodes front facet, threecompeting resonators can in principle influence the frequency response of the µECDLmodule: the laser diode resonator CLD between the front and end facet, the inner resonatorCinner between the front facet and the VBG, and the outer resonator Couter between theback facet and the VBG. Each resonator has a different optical path length resulting ina different free spectral range (FSR). The optical path length Lopt = ∑

ni · si of eachresonator is the sum of the optical path lengths si · ni of each contributing section, with nibeing the respective refractive index. For the dimensions indicated in figure 4.18, Lopt andthe resulting free spectral range for each resonator are summarized in table 4.6.

Resonator Lopt FSR

CLD 3.7 mm 27 pmCinner 12.8 mm 8 pmCouter 16.5 mm 6 pm

Table 4.6: Optical path length and free spectral range for the three resonators CLD, Cinner,and Couter formed in the µECDL module.


The required precision for the alignment of the optical elements is determined withoptical beam shaping simulations according to ISO 11146 using the program WinABCD,that was developed at the Ferdinand-Braun-Institut by Dr. Bernd Eppich. The simulatedresidual full divergence angle of the beam in the lateral and vertical plane in dependenceof the collimating lens (L) position along the optical axis z is shown in figure 4.19.The lens needs to be aligned at a distance z = (1.575 ± 0.035)mm to the front facet toachieve a residual divergence in both planes that is smaller than the 95%-angular selectivityδθ95% of the VBG, in order to make use of at least 95% of the VBGs maximum diffractionefficiency.

1.50 1.52 1.54 1.56 1.58 1.60 1.62 1.640.0

0.2

0.4

0.6

0.8

1.0

1.2

Full

div.

ang

le /

degr

ee

Lens position z / mm

lateral divergence vertical divergence

VBG- 95%

Figure 4.19: Simulated residual full divergence angle in lateral and vertical direction as afunction of the lens position.

The most critical parameters in the alignment of the µECDL module are the VBG tiltingangles around the x- and y-axis indicated by the coordinate system in figure 4.18. Tiltingof the VBG around the y-axis changes the position of the back-coupled light along thevertical plane of the laser diodes front facet. Tilting around the x-axis influences thecoupling in the lateral plane of the laser diode.

-0.025 0.000 0.0250.0

0.5

1.0

0.1° = 1.7 mradFWHM

95%

vertical axis

Coup

ling

effic

ienc

y

VBG tilt / degree

0.008° = 140 rad

0.002° = 35 rad

-0.2 -0.1 0.0 0.1 0.2

0.028° = 0.5 mrad

lateral axis

VBG tilt / degree

95%

FWHM

Figure 4.20: Simulated coupling efficiency of the back-coupled light in the vertical andlateral axis as a function of the VBG tilt around the x- and y-axis, respectively.


The laser diodes entrance aperture in the simulation is estimated from the measurementsof the far-field angles to be 0.8 µm and 15 µm in the vertical and the lateral direction,respectively. Figure 4.20 shows the simulated coupling efficiency as a function of the VBGtilt in both directions. The smaller vertical aperture leads to a higher sensitivity in thisdirection. For a tilt larger than ±0.001◦ or ±17.5 µrad the coupling efficiency already sinksbelow 95% of its maximum. In the lateral direction, the coupling efficiency is above 95%for tilting angles smaller than ±0.014◦ or ±250 µrad.

Mounting of the µECDL module

A schematic top and side view of the µECDL module is shown in figure 4.21. All elements areassembled on a conduction cooled package (CCP) (5) with a footprint of 25 mm x 25 mm.The CCP is custom-made out of one piece of copper (Cu)1, and electroplated with a250 µm thick gold (Au) layer to protect the surface from oxidation and increase its thermalconductivity. The laser diode in the TO56 can (1) is positioned inside a tube in the CCP,that has a slightly larger diameter than the base plate of the TO56 can. The base plate isclamped onto the CCP with a screw nut with an external thread (4), which is then fixedto the CCP body by UV-curable adhesive. To protect the optical elements the CCP iscovered with a black anodized aluminum plate (6). The beam height of the module ish = 8 mm, so that it is compatible with the fixed beam height of the micro-assembly andlaser diode characterization station that is described in detail in the PhD thesis of MartinMaiwald [149].The lens (2) and the VBG (3) are both aligned during laser operation. For this, theyare hold by a vacuum tweezer, that is mounted on a six-axis positioning stage (H-206,Physik Instrumente (PI) GmbH & Co. KG) with high precision in the translational and

25

mm

top view

side view

25 mm

12

3

8 mm

4

5

6

15

mm

Figure 4.21: Schematic top and side view of the micro-integrated ECDL module: (1) laserdiode in TO56 can, (2) collimating lens, (3) reflecting volume Bragg grating, (4) thread forattachment of the laser diode, (5) conduction cooled package, (6) cover plate.

1 The manufacturing of the CCP was done by the FBH colleagues Thomas Roos, Detlef Grimpe, andBastian Deutscher.


rotational axes. The stage provides a minimal translational step size of 0.1 µm and aminimal rotational step size of 2 µrad, both being sufficient for the required alignmentprecision shown in figure 4.19 and 4.20. For the alignment of the collimating lens (L),the laser diode is operated far below threshold and the amplified spontaneous emission isfocused onto a CCD camera, that is positioned with its chip in the focal plane of a lenswith a focal length of 150 mm. The collimating lens and the lens with 150 mm form atelescope, and the optimal collimation is achieved, when the spot size on the CCD chipbecomes minimal. The lens is then mounted onto the CCP with UV-curable adhesive, thatis treated with UV light for approximately 10 minutes.

The VBG alignment and mounting follows a similar procedure. However, alignmentoptimization is carried out by monitoring the emission spectrum of the module withthe double-echelle monochromator with a resolution of 6 pm at 445 nm. Initial coarsealignment is carried out at a high operating current until an effect of the optical feedbacklike fluctuations in the spectral intensity or even a narrowing of the emission bandwidthis observed in the spectrum. Then, the injection current is decreased and the VBG isre-adjusted. This is done until the smallest injection current is reached, at which an effecton the emission spectrum can be observed.

It must be noted, that minimizing the threshold current may be the optimal alignment forlow injection currents. However, as explained above, the tolerances for the VBG alignmentin this system are extremely small. Therefore, the optimal alignment at high operatingcurrents in terms of a preferably narrow emission bandwidth and a high ASE suppressionmay slightly differ from the optimum at low injection currents. This is due to eventualminimal changes in the position of the optical elements or the laser diode cavity causedby thermal expansion, which can have a direct effect on the coupling efficiency. For thisreason, the injection current is set to the maximum value of 1.2 A and the VBG is againslightly adjusted until the emission spectrum is stable and as narrow as possible. Finally,the VBG is then mounted onto the CCP with UV-curable adhesive, that is treated withUV light for approximately 10 minutes.


4.3.3 Electro-optical characteristics

For all measurements presented below the heatsink temperature is set to T = 20◦C. The op-tical output power versus the injection current for the free-running Fabry-Perot laser diodeand the µECDL module is shown in figure 4.22. The laser diode characterized in chapter 3is used as gain medium. It has a lasing threshold of Ith,LD = 145 mA and a slope efficiencyof SLD = 1.55 W/A. Laser operation for the µECDL starts at Ith,µECDL = 115 mA anda reduced slope efficiency of SµECDL = 1.3 W/A is observed. At the maximum injectioncurrent of I = 1.2 A, the FP laser diode has an output power of PLD = 1.57 W. Due tothe much better spectral characteristics (see section 4.3.4) compared to the macroscopicECDL system, the injection current could be extended until the maximum injection cur-rent of 1.2 A. Here, the µECDL module reaches a maximum optical output power ofPµECDL = 1.43 W. The sawtooth-like power characteristic of the µECDL emission forhigher injection currents can be attributed to mode-hops.

In general, a comparison between the electro-optical parameteres of the macroscopicECDL and the µECDL emission is difficult as different laser diodes are applied. However,the higher relative feedback from the VBG compared to the surface grating should leadto a lower threshold current and lower slope efficiency. Whereas the µECDL thresholdcurrent is indeed lower, the slope efficiency is higher than that for the macroscopic ECDL.This effect seems counterintuitive and could not be conclusively explained in the courseof this work. The adjustment of the optical elements of the µECDL module is carriedout with higher precision than the manual adjustment of the macroscopic ECDL system.Furthermore, the external cavity length of the µECDL module is much shorter than inthe macroscopic ECDL. These aspects probably lead to a higher coupling efficiency of theoptical feedback and to a lower sensitivity to mechanical vibrations for the µECDL module,which consequently results in a better performance of the µECDL module.

Figure 4.22: Optical output power versus injection current for the µECDL module (solidline) and the FP laser diode (dashed line) at a heatsink temperature of T = 20◦C.


4.3.4 Spectral emission characteristics

Without external optical feedback the FP laser diode emits in longitudinal and lateralmulti-mode with a spectral emission width of about 1 nm (FWHM) (see section 3.5 and3.6). Figure 4.23 and figure 4.24 show optical spectra of the laser diode emission and ofthe µECDL emission as a function of the injection current I, respectively. The spectrain figure 4.23 are measured with a spectrometer providing a spectral resolution of 0.6 nm.The measuring interval is ∆I = 20 mA. The spectra in figure 4.24 are measured with thedouble-echelle monochromator providing a spectral resolution of 6 pm at 445 nm, and themeasuring interval is ∆I = 5 mA. Each spectrum is normalized in intensity to 1 and allspectra are presented in a false color contour diagram.

The spectral emission of the free-running laser diode shows a shift of the center wave-length of ∆λC = 3.5 nm/A (see section 3.5). At I = 1.2 A, it has a center wavelength ofλC = 445.3 nm. The spectral emission bandwidth is around 1 nm (FWHM). The spectralemission of the µECDL module is wavelength stabilized for the whole operating range.At P = 0.1 W (I = 0.225 A), the peak wavelength is λ = 445.06 nm and at I = 1.4 W(I = 1.2 A) the peak wavelength is λ = 445.12 nm. With a typical wavelength shift of theBragg wavelength of the VBG of ∆λ/∆T = 7 pm/K [147], the peak wavelength shift wouldcorrespond to an increase of the temperature of the µECDL of about ∆T ≈ 9 K fromP = 0.1 W to P = 1.4 W. The entire µECDL emission stays within a spectral window of250 pm, which is indicated by the white dashed lines in both figures.

0.0 0.2 0.4 0.6 0.8 1.0 1.2435

440

445

450

455

T = 20°C

Wav

elen

gth

/ nm


0 0.5 1

Normalized intensity

250 pm

Figure 4.23: Contour plot of multipleemission spectra of the FP laser diode as afunction of the injection current with mea-surement steps of 20 mA. Each spectrum isindividually normalized in intensity to 1.

Figure 4.24: Contour plot of multipleµECDL emission spectra as a function ofthe injection current with measurementsteps of 5 mA. Each spectrum is individu-ally normalized in intensity to 1.

For a better illustration of the wavelength stabilization, figure 4.25.A shows the full-widthat half maximum emission bandwidth for the free-running LD and the µECDL. The µECDLemission bandwidth is in the range of the spectral selectivity of the VBG of 24 pm (FWHM)up to an output power of about 1 W, corresponding to an injection current of I = 0.9 A.For higher output powers, more longitudinal modes of the laser diode start to oscillate.


0.2 0.4 0.6 0.8 1.0 1.2440

442

444

446

448

450

0.2 0.4 0.6 0.8 1.0 1.20.001

0.01

0.1

1

10A

Peak

wav

elen

gth

peak

/ nm


free-running LD ECDL

B

Band

wid

th

FWHM

/ nm


free-running LD ECDL

Figure 4.25: A: Peak wavelength of the free-running laser diode and of the µECDL emis-sion as a function of the injection current. B: Half-logarithmic plot of the FWHM band-width of the free-running laser diode and the µECDL emission as a function of the injectioncurrent.

However, the FWHM emission bandwidth does not exceed 50 pm up to I = 1.2 A. On aver-age, the emission bandwidth of the free-running laser diode is reduced by almost two ordersof magnitude with the µECDL module. The peak wavelength λpeak of the free-runninglaser diode and the µECDL emission as a function of the injection current are shown infigure 4.25.B. For high operating currents, the emission wavelength of the free-runninglaser diode shifts towards the µECDL emission wavelength (∆λC = 3.5 nm/A).This influences the strength of the ASE and longitudinal laser diode mode suppression(SR) for the µECDL, which becomes noticeable in figure 4.26.A. It shows spectra of thefree-running LD and the µECDL emission at selected injection currents of 0.3 A, 0.6 A,0.9 A, and 1.2 A. The spectra are again measured with the optical spectrum analyzer.

The spectral behavior is comparable to that of the macroscopic ECDL described insection 4.2.3 with the difference that the central emission wavelength λC, i.e. the gainspectrum, of the laser diode used in the µECDL is approximately 2 nm blue-shifted. Ac-cordingly, the gain maximum and the µECDL emission wavelength at 445 nm coincidefor higher injection currents and the suppression of the ASE and the longitudinal laserdiode modes (SR) reaches its maximum with 53 dB at an injection current of I = 0.9 A.Compared to the macroscopic ECDL the SR value is higher over a wider operating range.Above I = 0.9 A, the gain shifts towards longer wavelengths but is still close to the µECDLemission and the SR value only slightly decreases to 48 dB at I = 1.2 A.

Figure 4.26.B shows the spectra of the µECDL emission at the same operating points witha higher resolution of 6 pm. As already mentioned, the spectrum is dominated by onlyone longitudinal FP laser diode mode up to I = 0.9 A, where the emission has a spectralwidth of ∆λFWHM = 22 pm. And a spectral width of only ∆λ95% = 46 pm even contains95% of the intensity. At I = 1.2 A (P = 1.43 W) several other modes occur. However,the emission still has a spectral width of ∆λFWHM = 14 pm and ∆λ95% = 120 pm. Theother modes have a spectral distance of about 27 pm to each other and can therefore be


435 440 445 450 455

-60

-40

-20

0

-60

-40

-20

0

-60

-40

-20

0

435 440 445 450 455

-60

-40

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0

444.5 445.0 445.5

0.0

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0.0

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1.0

0.0

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1.0

444.5 445.0 445.50.0

0.5

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ECDL LD

I = 0.3 A

38 dB

I = 0.6 A

Rela

tive

inte

nsity

/ dB 51 dB

53 dB

I = 0.9 A

I = 1.2 A

Wavelength / nm

48 dB

B

95%

FWHM

I = 0.3 A

11 pm

25 pm

A

I = 0.6 A

Norm

aliz

ed in

tens

ity (a

rb. u

nits

)

FWHM

95%

21 pm

39 pm

I = 0.9 A

FWHM

95%

22 pm

46 pm

120 pm

14 pm

I = 1.2 A

Wavelength / nm

FWHM

95%

Figure 4.26: A: Emission spectra of the free-running LD and the µECDL system at injec-tion currents of I = 0.3 A, 0.6 A, 0.9 A, and 1.2 A measured with a high-dynamic rangeoptical spectrum analyzer with a spectral resolution of 50 pm. B: µECDL emission spectraat I = 0.3 A, 0.6 A, 0.9 A, and 1.2 A measured with a double-echelle monochromator with aspetral resolution of 6 pm.

indentified as longitudinal modes from the FP laser diode.In conclusion, narrowband emission together with a high ASE and longitudinal laser diodemode suppression is achieved for much higher output powers up to 1.4 W with the µECDLmodule. The reasons for this improved behavior were already mentioned, namely the higherdiffraction efficiency of the VBG of 17.9%, the more precise adjustment of the opticalelements, the reduced external cavity length of the µECDL module, and the conincidinggain peak and VBG feedback wavelengths for higher injection currents.


4.3.5 Temporal stability of the µECDL emission

To evaluate the temporal stability of the µECDL emission wavelength and optical outputpower, both are measured over an operation time of one hour with a measurement stepsize of 10 s. The injection current is set constant to I = 575 mA and is controlled using acurrent source with a long-term stability of < 15 µA, as specified by the supplier (Newport,model 560B). Figures 4.27.a) and c) show the peak wavelength and the optical outputpower as a function of the operation time, respectively. Over the whole operation timethe mean peak wavelength of λpeak,mean = 445.10 nm is stable within a spectral windowof 6 pm, corresponding to the spectral resolution of the spectrometer. The mean opticaloutput power is Pmean = 528 mW with a peak-to-peak variation of ±3 mW (< ±1%).Figure 4.27.b) shows an exemplary spectrum after an operating time of top = 30 min. Ithas a spectral width of 14 pm FWHM.

444.5 445.0 445.5

0.0

0.5

1.0

445.08

445.09

445.10

445.11

445.120 10 20 30 40 50 60

b)a)

I = 575 mA

I = 575 mA

Operation time top / min

peak

/ nm

6 pm

0 10 20 30 40 50 60500

510

520

530

540

550

560c)

P opt /

mW

Operation time top / min

14 pm

top

= 30 min

Inte

nsity

Wavelength / nm

Figure 4.27: Temporal stability of the µECDL emission for an injection current of I =575 mA. a) Peak wavelength of the µECDL emission measured over a time period of 1 hour.b) Exemplary emission spectrum at top = 30 min. c) Optical output power of the µECDLover a time period of 1 hour.


4.3.6 Spatial emission characteristics

The beam quality of the µECDL module is determined by a caustic measurement accordingto ISO 11146 using the variance and the 90%-knife edge method, respectively. Figure 4.14shows the caustic in the fast (left) and slow axis (right) at an injection current of I = 0.8 A,corresponding to an output power of 860 mW. The variance beam propagation factors areM2

4σ,fast = 2.7 and M24σ,slow = 7.5 in the fast and slow axis, respectively. The 90%-knife

edge beam diameter values are M290%,fast = 1.1 and M2

90%,slow = 4.5 in the fast and slowaxis, respectively.

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

I = 0.8 A

M24 = 2.7

M290% = 1.1

fast axisBeam

wai

st d

iam

eter

d /

mm

Position z / mm


I = 0.8 A

M290% = 4.5

M24 = 7.5

slow axis


Figure 4.28: Variance and 90%-knife edge caustic measurement of the µECDL emission infast and slow axis for an injection current of I = 0.8 A.

The values in both directions are within the measurement uncertainty in good agreementwith the values for the free-running laser diode. In particular, the 90%-knife edge valuesare precisely the same, indicating that the VBG does not have an effect on the beamquality of the laser diode and therewith also no effect on the subsequent SHG process.


4.4 SummaryTable 4.7 summarizes the electro-optical, spectral and spatial parameters of the free-runninglaser diode (LD), the macroscopic ECDL and the µECDL at the respective maximumstudied injection current.

The macroscopic ECDL only shows narrowband emission < 40 pm (FWHM) together witha high suppression ratio of the ASE and laser diode cavity modes (SR) up to an injectioncurrent of 0.6 A, corresponding to an output power of 0.55 W.The µECDL maintains narrowband emission < 40 pm (FWHM) with an SR ≥ 38 dB forall operating points up to the maximum injection current of 1.2 A, corresponding to anoutput power of 1.43 W. The maximum SR is 53 db at 0.9 A.

The big advantage of the macroscopic ECDL system is its principle ability to tune theemission wavelength. As was shown, the tunability is limited to about 4 nm, because thelongitudinal laser diode cavity modes are not suppressed sufficiently for ECDL wavelengthstoo far away from the laser diode gain maximum.

A poorer beam quality of the macroscopic ECDL in the fast axis is indicated by somewhathigher M2 values. As the laser diodes fast axis is in the plane in which the beam is diffractedby the surface grating, the slightly worse beam quality might originate from aberrationscaused by the diffraction.In the slow axis, the caustic measurements resulted in similar M2 values. Considering all90%-M2 values, which are almost identical (except for the value of the macroscopic ECDLsfast axis), the ECDL setups seem to have no impact on the beam quality. In general, therelatively poor lateral (slow axis) beam quality of the laser diode is a drawback for thesubsequent SHG process as it limits the focusability of the beam.

However, both the macroscopic proof-of-concept ECDL system in Littrow configuration andthe micro-integrated µECDL module have proven to fulfill the phasematching tolerancesfor efficicient frequency conversion in a BBO crystal. In the next chapter, both systemsare applied as pump sources for single-pass SHG down to 222.5 nm.

parameter symbol LD macro ECDL µECDL

maximum injection current Imax 1.2 A 0.6 A 1.2 Amaximum output power P 1.6 W 0.55 W 1.43 W

emission wavelength λ ≈ 445 nm 443-447 nm 445 nmemission bandwidth (FWHM) ∆λ 1...2 nm < 40 pm < 40 pm

coarse tuning range ∆λcoarse - 4 nm -suppression ratio at Imax SR - 42 dB 48 dBbeam quality (fast axis) M2

4σ / M290% 2.8 / 1.1 4.0 / 2.1 2.7 / 1.1

beam quality (slow axis) M24σ / M2

90% 5.9 / 4.5 7.7 / 4.7 7.5 / 4.5

Table 4.7: Summary of the electro-optical, spectral and spatial parameters of the free-running laser diode (LD), the macroscopic ECDL and the µECDL module.

5 Compact deep ultraviolet laser lightsource

This chapter presents the results obtained with single-pass second harmonic generationusing the macroscopic ECDL in Littrow configuation and the micro-integrated µECDLmodule as pump sources.The setup with the macroscopic ECDL system again serves as a proof-of-principle toevaluate the feasibility of the concept and to develop an idea of the critical aspects, thatneed to be considered for efficient frequency conversion. The development of the single-passSHG setup with the macroscopic ECDL as pump source is presented in section 5.1. Thefirst challenge of the experiments arose from the need to develop a setup, with which deepultraviolet output powers even in the lower µW range can be reproducibly detected andmeasured. This aspect is discussed in section 5.1.1. First SHG results and the influenceof different focusing conditions on the conversion efficiency are discussed in section 5.1.2.Here, it is also examined how far the simulated curves for the phase matching tolerancescan be reproduced and the differences between theory and experiment will be discussed.

As the the technical goal of this work is the development of a compact DUV laser lightsource, the more compact µECDL is used as pump source in section 5.2. With a betterunderstanding of the setup, the focusing conditions are now optimized, and the achievedresults are presented in section 5.2.2. It is discussed if and how far the results can becompared with the Boyd-Kleinman analysis for focused Gaussian beams given the fact ofthe elliptical beam profile and the relatively poor beam quality of the laser diode emissionespecially in the lateral axis.

71

72 5 Compact deep ultraviolet laser light source

5.1 Proof-of-concept setup with macroscopic ECDL aspump source

A schematic view of the experimental arrangement is given in figure 5.1. The macroscopicECDL in Littrow configuration emitting at 445 nm described in section 4.2 serves as thepump source for the single-pass frequency doubling setup. The laser diode polarizationis oriented parallel to the grating grooves along the y-axis indicated by the coordinatesystem in figure 5.1. This is also the correct alignment for the type-I phase matchingapplied in this setup where the fundamentals polarization has to be perpendicular to thephase matching plane, which, as indicated in figure 4.2, is the xz-plane in this setup. Thecollimated beam from the ECDL has an elliptic beam shape of 1.1 mm x 2.8 mm in lateral(y-axis) and vertical (x-axis) direction, respectively.

All lenses applied in front of the crystal are AR-coated with R ≈ 10−3 for 450 nm. Lens L1and L2 are two cylindrical lenses with a focal length of f = 22 mm (L1) and f = 76 mm(L2), respectively. They are used as a telescope arrangement to expand the beam in lateraldirection, in the exemplary case shown here to a lateral beam width of 4.6 mm. This isdone for several reasons: First, the relatively high lateral M2 value of M2

90% = 4.7 makesthe lateral expansion necessary to enable a smaller beam waist diameter inside of thecrystal. Here and in the following the 90%-knife edge beam diameter M2

90% values areused, because they seem to be more appropriate in the context of the SHG process (seesection 3.6). Secondly, by changing the magnification factor of the lateral telescope withdifferent lenses L1 and L2, different focusing conditions can be tried out to optimize theSHG conversion efficiency. Moreover, after the lateral beam expansion the beam is focusedinto the BBO crystal by a spherical achromatic lens (L3). The lateral expansion minimizesthe relatively large residual divergence in lateral direction of the ECDLs output beam from0.18◦ to 0.04◦ (simulated with WinABCD). Thereby it is assured that the focal points inlateral and vertical direction after focusing with lens L3 are at the same position along thez-axis inside the BBO crystal.

For lens L3, different focal lengths of 50 mm, 75 mm, 100 mm, and 150 mm are applied togain an idea of the effect of different focusing conditions on the SHG conversion efficiencyand compare the experimentally generated beam waist radii with the expected optimumbeam waist radius and focusing condition predicted by the Boyd-Kleinman analysis.

222.5nm

445 nm

BeamShaping

FocusingLens

CollimationLens

BBO

CaF Prism2

BeamDump

FocusingLens

Photodiode,Spectrometer

ECDL

L1 L2 L3

z

x

y,E445 nm

Figure 5.1: Schematic top view of the single-pass frequency doubling setup with themacroscopic ECDL emitting at 445 nm as pump source.

5.1 Proof-of-concept setup with macroscopic ECDL as pump source 73

The BBO crystal applied here has a length of 7.5 mm and an entrance aperture of4 mm x 4 mm. The front and back facets are AR-coated for the fundamental and thesecond harmonic wavelength, respectively. As specified by the supplier (Cryslaser Inc.), thecrystal was cut at a phase matching angle of θPM = 63.6◦ for a phase matching wavelengthof λ = 449 nm at a temperature T = 25◦C. To assure stable phase matching conditions,the crystal is put into a crystal oven (Eksma Optics), and the temperature is stabilized atT = 50◦C. The oven is mounted on a manual 3-axis alignment stage (Newport Corporation,model: 562F-XYZ + model: SM-25) with a specified sensitivity of 1µm.An additional manual rotation stage (Newport Corporation, model: M-UTR80S) specifiedwith a sensitivity of 4 arc sec or 0.001◦ is used for the alignment of the phase matchingangle. The crystal center is positioned in the beam focus. To achieve phase matching forthe fundamental wavelength of 445 nm at a crystal temperature of 50◦C, the crystal isangle-tuned by 1.4◦ to a phase matching angle of θPM = 65◦.

The radiation behind the crystal is collimated by an uncoated UV fused silica lens witha focal length of f = 75 mm. A 10 mm thick UV fused silica plate is specified by thesupplier (Thorlabs Inc.) to have a transmission of 90.5% at 222 nm. A CaF2 prism with aspecified transmission > 90% at 222 nm (Thorlabs Inc.) is used to spatially separate thefundamental from the generated second harmonic light. The fundamental is blocked by abeam dump and the second harmonic UV beam is focused onto a SiC photodiode by anUV fused silica lens with a focal length of f = 50 mm to determine the generated DUVoutput power. The DUV light generates a photocurrent in the photodiode, that dependson its spectral sensitivity and is measured with a picoammeter (Keithley Instruments).

To measure spectra of the generated DUV light, the SiC photodiode is replaced by aspectrometer (Newport MiniSpec 78355) having a spectral resolution of ≈ 1.3 nm over itsentire wavelength range. Unfortunately, there was no spectrometer with a higher resolutionin the DUV wavelength range available in the laboratory at the time of this work, so thatthe actual emission bandwidth of the generated DUV light could not be resolved.


5.1.1 Detection of deep ultraviolet light

As estimated from the Boyd-Kleinman theory, the expected deep ultraviolet output power,that can be achieved in a single-pass second harmonic generation setup with a BBO crystal,lays between tens and a few hundreds of microwatts. Measuring optical output powers insuch low power regimes is challenging. Additionally, the deep ultraviolet light is invisibleto the naked eye impeding an easy laser beam adjustment to an optical power detector.For the initial coarse alignment of the crystal position and phase matching angle in the fre-quency doubling stage even lower optical output powers in the nW range need to be detected.

For the initial alignment of the phase matching angle of the BBO crystal and for theadjustment of the DUV laser beam, a fluorescence glas filter, that is doped with the highlyfluorescent Rare Earth terbion ion Tb3+ (Lumilass G9, Sumita Optical Glass EuropeGmbH), is used. This material can be excited in a wavelength range from 200 nm to390 nm and has a fluorescent emission in the green spectral range with an intensity peakat λ = 542 nm. As specified by the supplier, the filter has a minimum sensitivity of < 1µW/cm2 for an excitation wavelength around the absorption maximum of about 250 nm.This means that for a spot size of 1 mm x 1 mm an optical power of 10 nW could bedetected. During the experiments it was possible to see green fluorescence on the filterwith the naked eye for an optical output power at 222.5 nm as low as approximately 100 nW.

After a rough adjustment of the phase matching angle the initial DUV light is thenused to adjust the DUV laser beam and direct it onto the photodiode. Fine adjustment ofthe phase matching angle can then be done by monitoring and maximizing the photocurrentthat is generated by the DUV laser light.

Besides the low DUV output powers, that need to be measured, another obstacle to thereproducible and reliable power measurement occured with the experimental arrangement.

Figure 5.2: a) Spectral sensitivity of the used SiC photodiode (sglux SolGel Technologies,model SG01XL) and for comparison of a conventional UV extended Si photodiode (ThorlabsInc., model S130VC). b) Spectral sensitivity of the applied SiC photodiode in logarithmicscale. Data taken from the suppliers calibrations.


Even though the separation of fundamental and second harmonic light by the CaF2 prismworks sufficiently good, there is still random stray light from the fundamental even inthe optical path of the DUV laser beam. This random stray light is measured witha silicon (Si) photodiode positioned in the DUV beam to be in the lower mW range.This is still higher than the expected amount of generated DUV laser light and thereforeimpedes a clear and reliable DUV power measurement with a conventional Si photodetector.

For this reason, a solar-blind photodiode based on silicon carbide (SiC) provided bythe company sglux SolGel Technologies is used for the determination of the generatedDUV output power. The advantage of the SiC photodiode over conventional Si basedphotodiodes for our measurements can be seen in figure 5.2.a). It shows the spectralsensitivity of the used SiC photodiode (sglux, model SG01XL) and for comparison thespectral sensitivity of an exemplary conventional UV extended Si photodiode (ThorlabsInc., model S130VC) between 200 nm and 600 nm. The data for the SiC photodiode istaken from the manufacturer calibration and for the Si photodiode from the website of theprovider [150].

The spectral sensitivity of the Si photodiode is higher over the whole wavelength range.However, the SiC photodiode has its highest sensitivity in the DUV wavelength rangewith a peak sensitivity at 286 nm and has an extremely low sensitivity at the fundamentalwavelength of 445 nm. This can be seen in detail on a logarithmic scale in figure 5.2.b).At the fundamental wavelength λ = 445 nm, the sensitivity is 7.2 · 10−8A/W, compared to6.7 · 10−3A/W at the second harmonic wavelength λ = 222.5 nm, which means a differenceof five orders of magnitude. At 222.5 nm, 100 µW result in a photocurrent of 670 nA.Even an optical power of 100 mW of the fundamental beam would generate a photocurrentof only 7.2 nA, which is about 1% of the photocurrent resulting from 100 µW of DUVlight and could therefore be neglected.

In conclusion, even though the absolute spectral sensitivity of the SiC photodiode islower than that of conventional Si photodiodes, its strong wavelength discrimination or"solar-blindness" enable a reliable DUV power measurement and the fundamental straylight can be neglected.


5.1.2 Investigation of conversion efficiency and phase matchingtolerances

Figure 5.3 shows the optical output power, here named pump power Ppump, versus theinjection current of the ECDL system emitting at 445 nm. The fundamental pump poweris measured behind the mirror, that redirects the output beam of the ECDL. The differenceto the power characteristics curve in figure 4.10, is that the curve shown here is measuredmanually in steps of 20 mA during the SHG experiment. Therefore the values differsomewhat to the earlier power characteristics as the ECDL is adjusted before each DUVpower measurement with the goal of a maximized DUV output power.

At an injection current of I = 800 mA, the ECDL system has an output power of 680 mW.The lasing threshold is IECDL-th = 130 mA and the slope efficiency is measured to beSECDL = 1.0 W/A. Inset A and B in figure 5.3 show optical spectra (see also fig. 4.11) ofthe ECDL emission measured with the optical spectrum analyzer for an injection currentof 0.6 A and 0.8 A, corresponding to an output power of 0.47 W and 0.68 W, respectively.At 0.47 W, the suppression of the ASE and longitudinal laser diode modes (SR) reachesa maximum of 42 dB and decreases from thereon to 19 dB at 0.68 W (see figure 4.11 insection 4.2.3).

Figure 5.3: Pump power Ppump of the macroscopic ECDL system emitting at 445 nm asa function on the injection current. Inset A: ECDL emission spectrum at 0.6 A (0.47 W).Inset B: ECDL emission spectrum at 0.8 A (0.68 W).

The spectrum of the ECDL emission is also observed during the SHG experiment usingthe double-echelle monochromator. For output powers up to 0.4 W corresponding to aninjection current of 0.53 A, the ECDL emission shows a spectral width of ∆λ ≤ 20 pm(FWHM). For higher output powers, the spectral emission of the ECDL exhibits a slightbroadening. Figure 5.4 shows two emission spectra of the ECDL at an output power of a)0.47 W (I = 0.6 A) and b) 0.68 W (I = 0.8 A). The spectral width of the ECDL emissionincreases from 25 pm to 64 pm (FWHM) between the two working points. These spectra


Figure 5.4: Emission spectra of the ECDL system at a) I = 0.6 A and b) I = 0.8 Acorresponding to an output power of I = 0.47 W and I = 0.68 W, respectively at a heatsinktemperature of T = 20◦C.

also represent working points at which the SHG conversion efficiency was maximized byadjusting the ECDL. The comparison between the earlier presented ECDL spectra (seefigure 4.11) leads to the conclusion that a slightly broader pump spectrum with severallongitudinal modes oscillating is favorable regarding a higher conversion efficiency. Thisfinding is also in agreement with observations in other publications, where several longitudi-nal modes within the acceptance bandwidth of the nonlinear crystal caused an enhancementof the frequency doubling efficiency compared to a longitudinal single-mode pump source[151, 152].


Conversion efficiency and focusing conditions

After the lateral beam expansion, the beam diameters are 4.6 mm x 2.8 mm in lateraland vertical direction, respectively. In this setup, the vertical plane of the laser diode isthe phase matching (PM) plane considered by the Boyd-Kleinman analysis to find theoptimum focusing conditions, assuming a non-diffraction limited spherical beam. Thelateral beam expansion effects the conversion efficiency insofar as it leads to a higher powerdensity in the focal region inside the BBO crystal due to the enabled stronger focusingin this direction. The drawback however is a high beam divergence after focusing, whichrestricts the high power density to a relatively small region inside the BBO crystal.

Due to the different beam propagation factors of the ECDL emission in the verticaland lateral plane it is impossible to generate symmetric spherical focusing inside the BBOcrystal, which limits the comparibility of the experimental results with the predictionsfrom the Boyd-Kleinman analysis. The values for the optimum beam waist radius andthe predicted SHG output power obtained from the Boyd-Kleinman analysis can thereforeonly serve as an initial reference point. In general, there is no analytical solution toexactly calculate the optimum focusing conditions and expected SHG output power fora non-diffraction limited and highly elliptical beam as applied in this experiment. Theanalysis of the optimum focusing parameters for this experiment is therefore restricted toa qualitative discussion of the achieved SHG output powers with different beam shapingoptics in comparison to the Boyd-Kleinman theory (see section 2.4).

For a first assessment of the influence of different focusing conditions on the conver-sion efficiency, different focal lenghts for lens L3 (see figure 5.1) of 50 mm, 75 mm, 100 mm,and 150 mm are applied. The resulting beam waist radii are simulated with WinABCDand listed in table 5.1. For the simulation, the beam propagation factors are assumed tobe M2

PM,90% = 2.1 in the PM plane and M2⊥,90% = 4.7 in the plane perpendicular to the

PM plane. These values correspond to the M290% values from the caustic measurements

of the ECDL emission using the 90%-knife-edge beam diameter method (see figure 4.14).The resulting beam waist radii in the PM plane are in the vicinity of the Boyd-Kleinmanpredicted optimum of 15 µm for a BBO crystal with a length of 7.5 mm.

In the perpendicular plane, the resulting diameters are larger. A stronger lateral beamexpansion would result in smaller beam waist radii but also in a larger beam divergence. Asexplained in section 2.4, a trade-off between strong focusing and a maximized interaction

fL3 w0,PM w0,⊥

50 mm 11 µm 15 µm75 mm 16 µm 22 µm

100 mm 21 µm 29 µm150 mm 32 µm 43 µm

Boyd-Kleinman 15 µm 15 µm

Table 5.1: Simulated beam waist radii in the phase matching plane (PM) and perpendicu-lar to the phase matching plane for different focal lengths of lens L3.


Figure 5.5: Generated SHG power at 222.5 nm versus pump power at 445 nm for a focallength of lens L3 of 50 mm, 75 mm, 100 mm, and 150 mm. Dashed curves are the quadraticfits of the data points. The solid black curve shows the expected SHG power for a focusedGaussian beam according to the Boyd-Kleinman analysis.

length has to be found. The lenses applied here can be seen as a first approach to theoptimum focusing condition.

Figure 5.5 shows the second harmonic output power PSHG at 222.5 nm versus the fun-damental pump power Ppump at 445 nm for lens L3 having a focal length fL3 of 50 mm,75 mm, 100 mm, and 150 mm. The experimental data points for each measurement arefitted with the function PSHG = η · P 2

pump according to the quadratic conversion efficiencybehavior without pump depletion [74]. The quadratc fits give values for the normalizedconversion efficiency, that are summarized in table 5.2.

fL3 η

50 mm 2.4 · 10−5 W−1

75 mm 3.4 · 10−5 W−1

100 mm 4.2 · 10−5 W−1

150 mm 4.1 · 10−5 W−1

Boyd-Kleinman 13.8 · 10−5 W−1

Table 5.2: Normalized SHG conversion efficiency η for the different focal lengths of lensL3.

For a pump power of 680 mW, a maximum UV power of 16 µW is generated with lens L3having a focal length of f = 100 mm. The corresponding normalized conversion efficiencyis η100mm = 4.2 · 10−5 W−1. However, a focal length of 150 mm leads to almost the samesecond harmonic output power with a conversion efficiency of η150mm = 4.1 · 10−5 W−1.The theoretically expected conversion coefficient according to the Boyd-Kleinman analysisis ηBK = 13.8 · 10−5W−1 with an expected optimum beam waist radius of w0,BK = 15 µm.


The simulated beam waist radii for fL3 = 100 mm and fL3 = 150 mm in the verticalphase matching plane are w0,PM = 21 µm and w0,PM = 32 µm, respectively. It can beestimated that the optimum beam waist radius in the PM plane for this focusing conditionis somewhere between 21 µm and 32 µm and therefore larger than the theoretical optimumof 15 µm. For fL3 = 50 mm and fL3 = 75 mm, the conversion efficiency decreases as thefocusing in the PM plane is too strong.

The achieved conversion efficiency is a factor of 3.3 smaller than the Boyd-Kleinmanprediction. This is attributed to the fact, that the ECDL beam is highly elliptical afterthe lateral beam expansion (aspect ratio of d0,PM : d0,⊥ = 1.6 : 1) and additionally thebeam is not diffraction limited with different beam propagation factors in both transversedirections. A comparison to the Boyd-Kleinman analysis is therefore somewhat difficultand can only be made in a qualitative manner. In section 5.2, when the µECDL is used aspump source, the optimum focusing conditions for this setup will be discussed in more detail.

In the SHG versus pump power curves in figure 5.5, an increasing deviation of theexperimental data points from the quadratic fit PSHG = η · P 2

pump can be observed for apump power higher than 0.47 W, indicated by the first dashed line (A) in figure 5.5. Thisbehaviour can be explained with the ratio of the ASE and FP mode suppression (SR) ofthe ECDL emission, that decreases significantly between a pump power of 0.47 W (A) and0.68 W as was illustrated in the insets of figure 5.3. As examined in section 4.2.3 the ratioof the optical power PRECDL that is contained in the ECDL peak is therefore reduced from99.7% at a pump power of 0.47 W (0.6 A) to 88.9% at a pump power of 0.68 W (0.8 A).Hence, the actual power within the ECDL peak at the maximum applied current is onlyabout 0.61 W. Using this value with the obtained quadratic fits gives SHG powers thatare close to the experimentally measured DUV output powers at 0.68 W. This indicates,that the reduced SR value is the main reason for the reduced SHG conversion efficiency forhigher pump powers. On the other hand, the slight broadening of the spectral emissionbandwidth of the ECDL for higher pump powers seems to have a comparatively smalleffect on the SHG conversion efficiency.

Figure 5.6: Spectrum of the generated SHG light at λSHG = 222.5 nm and of the residualstray light from the fundamental beam at λpump = 445 nm.


Figure 5.6 shows a spectrum of the generated second harmonic emission at λ = 222.5 nmand the residual stray light from the fundamental at 445 nm at a pump power of 0.4 W,corresponding to a generated DUV power of 7 µW. The intensities at both wavelengths arenormalized to 1. The spectrometer (Newport MiniSpec 78355) has a spectral resolutionof approximately 1.3 nm over the entire wavelength range. Therefore, the actual spectralwidth of the UV emission could not be determined. However, it is known from the analysisof the ECDL emission characteristics, that its spectral bandwidth does not exceed 20 pmfor output powers up to 0.4 W. It is therefore reasonable to assume, that the spectral widthof the generated DUV light is in the range of the spectral width of the ECDL emission ofabout ∆λDUV < 20 pm up to 7 µW and about ∆λDUV < 64 pm up to 16 µW DUV outputpower.

Temperature acceptance bandwidth of the BBO crystal for a focused pumpbeam

For the setup with lens L3 having a focal length of fL3 = 150 mm, the generated SHGintensity as a function of the BBO crystal temperature TBBO is exemplary measured at amoderate pump power of Ppump = 240 mW (I = 380 mA). With the focusing conditonsapplied here, a beam waist in the phase matching plane having a simulated radius of 32 µmis generated. The crystal oven temperature is calibrated using a pre-calibrated PT1000temperature sensor. The generated SHG power is maximized for a crystal temperature ofabout 50◦C. For the measurement, the crystal temperature is then tuned from 30◦C to70◦C in steps of about 1.3 K.

Figure 5.7.a) shows the normalized SHG intensity as a function of the crystal temperatureTBBO. The red dots are the measured data points. As can be seen, the phase matchingtemperature acceptance bandwidth is ∆T = 26 K (FWHM), which is much wider thanthe theoretically expected temperature acceptance bandwidth of ∆T = 4.2 K for a BBO

Figure 5.7: a) Measured normalized SHG intensity at 222.5 nm as a function of the crystaltemperature TBBO for a 7.5 mm long BBO crystal. Experimental data points are repre-sented by the red squares. b) sinc2 fit (solid line) of the experimental data from a) using aneffective crystal length of 1.2 mm.


crystal with a length of 7.5 mm using the known Sellmeier equations for BBO [72] (seefigure 2.7 in section 2.3). This deviation between the experimentally observed and thetheoretically calculated temperature acceptance bandwidth is thought to originate from thespatial walk-off of the second harmonic and the fact that the sinc2 calculations are carriedout in the plane-wave approximation whereas the beam in the experiment is focused. Thisresults in a significantly decreased effective interaction length for focused beams whichleads to wider phase matching temperature, angle, and wavelength acceptance bandwidths.

Boyd and Kleinman have defined an effective aperture length [74] to:

La,eff =√πw0ρ

, (5.1)

where w0 is the beam waist radius and ρ the walk-off angle. For the focusing conditiondiscussed here with w0 = 32 µm and ρ = 4°, this effective aperture length is calculated tobe La,eff = 0.8 mm.The red line figure 5.7.b) shows a sinc2 fit of the data according to equation (2.17) havinga FWHM of ∆T = 26 K with the crystal length set to a value of La,eff = 1.2 mm.

The deviation between the calculation using equation (5.1) and the sinc2 function areattributed to uncertainties in the simulation of the relevant beam waist radius, i.e. therelevant M2 value in the PM plane. For example a beam waist radius of 47 µm (implyingM2

PM = 3) applied in equation 5.1 would lead to an effective aperture length of 1.2 mm.Given the mentioned difficulties with the precise measurement of the M2 values, it seemsjustified to use M2 = 3 and thereby have a good agreement between theory and experiment.This result is also qualitatively in good agreement with the temperature acceptance band-width measured by Kumar et al. of ∆T = 33 K for a beam waist radius of 55 µm inside a10 mm long BBO crystal [153].

Figure 5.8: Phase-matching wavelength acceptance bandwidth (a) and phase matchingangle acceptance bandwidth (b) calculated in the plane-wave approximation for a crystallength of La,eff = 1.2 mm (TBBO = 50◦C).


As a consequence, one must note that the effective interaction length inside the BBOcrystal and thereby the temperature acceptance strongly depends on the chosen focusingconditions. This not only effects the temperature acceptance bandwidth but also thephase matching angle and the wavelength acceptance bandwidth in BBO. Figure 5.7 showsthe calculated phase matching and wavelength acceptance bandwidths in the plane-waveapproximation for a crystal length of 1.2 mm. The phase matching wavelength accep-tance bandwidth in this case would be ∆λ = 250 pm (FWHM), compared to the earliercalculated ∆λ = 42 pm (FWHM). And the FWHM phase matching angle acceptancebandwidth would be ∆θ = 0.08◦ (1.4 mrad), compared to the earlier calculated ∆θ = 0.013◦(0.2 mrad).


Temperature acceptance for a parallel beam in the PM plane

It has become clear on the previous pages that the phase matching temperature acceptancebandwidth of the BBO crystal in the plane-wave approximation only holds true for aparallelized beam in the phase matching plane. This is evaluated using the setup illustratedin figure 5.9.a). The beam from the ECDL system is compressed in the phase matchingplane by a telescope arrangement consisting of two cylindrical lenses L1 and L2 with focallength of fL1 = 73 mm and fL2 = 40 mm, respectively, so that the beam diameter issmaller than the BBO crystals entrance aperture of 4 mm. Another cylindrical lens L3with a focal length of fL3 = 40 mm is used to focus the beam into the crystal in the planeperpendicular to the phase matching plane to achieve a higher power density inside thecrystal.

Figure 5.9.b) shows the normalized SHG intensity as a function of the crystal temperature.The red dots represent the experimental data. The dashed curve represents the calculatedtemperature acceptance bandwidth according to the plane-wave approximation for a BBOcrystal with a length of LBBO = 7.5 mm, having a FHWM of ∆T = 4.2 K. It can indeedbe seen that the experimental data shows good agreement with the theoretically expectedacceptance bandwidth for a parallelized beam. The origin of the higher intensities on thelow temperature tail is not clear yet and still under investigation.

L3

BBO

L1 L2

lateral plane

vertical, PM planeΔT

crystal oven

ECDL

ECDL

photo-diode

a) b)

Figure 5.9: a) Setup for the measurement of the temperature acceptance with a paral-lelized beam in the phase matching plane. b) Resulting phase matching temperature ac-ceptance curve. The dots are the experimental data and the solid curve is the simulatedtemperature acceptance in BBO for a crystal length of LBBO = 7.5 mm.

In conclusion, it can be stated that the stronger the focusing in the phase matchingplane the larger are the phase matching tolerances for the pump wavelength, the crystaltemperature, and the phase matching angle.The increased wavelength phase matching tolerance for a focused beam also implies thatthe decreasing conversion efficiency for output powers above 0.47 W observed in figure 5.5is more likely attributed to the decrease of the SR value than to the slight increase of theemission bandwidth of the ECDL. However, in regard to the targeted applications it is stillfavorable to achieve an emission bandwidth as narrow as possible.

5.2 Micro-integrated ECDL module as pump source 85

5.2 Micro-integrated ECDL module as pump sourceThe single-pass frequency doubling setup with the macroscopic ECDL as pump sourcehas shown, that further optimization of the focusing conditions is necessary to increasethe conversion efficiency of the SHG process. However, these first experiments also givean indication, that the Boyd-Kleinman theory can not be used to precisely calculate theoptimum beam waist diameter inside the BBO crystal and to predict the generated UVpower. This is due to the strong elliptical beam shape of the ECDL emission as well as itsnon-diffraction limited nature in both transverse directions.

5.2.1 Compact deep ultraviolet laser light source

The experimental arrangement for the single-pass frequency doubling with the µECDL aspump source is illustrated in figure 5.10 in a side and top view. Except for the differentpump source and different beam shaping optics, i.e. the lenses have different focal lengths,the setup is the same as the previous one. For simplicity, the detection path with theseparation of fundamental and second harmonic is not included here but is also identicalto the arrangement with the macroscopic ECDL as pump source presented in figure 5.1. Adetailed description of the µECDL and its electro-optical, spectral, and spatial propertieswere presented in section 4.3.

The collimated beam from the µECDL module has an elliptic shape with a beam diameterof dvert = 1.8 mm and dlat = 0.7 mm in the vertical and the lateral plane, respectively.For the critical type-I phase matching applied here, the laser diode polarization has to beperpendicular to the phase matching plane of the BBO crystal. Therefore, the TE-polarized(TE:TM = 100:1) laser diode is positioned with its fast axis (vertical plane) parallel to thephase matching plane, and the vertical plane is denoted phase matching (PM) plane in thefollowing. The pump beam is again expanded by two cylindrical lenses L1 and L2 forminga telescope in the lateral plane, which is perpendicular to the phasemathing plane of theBBO crystal. The beam is then focused into the BBO crystal with a spherical lens (L3).For the longest focal length applied for lens L3 of 150 mm, the optical path length betweenthe laser diode and the output facet of the BBO crystal is approximately Lopt = 30 cm.

LD

L1

VBG

L3

Lopt

BBO

top view(vertical /pm plane)

L1L2

side view(lateral plane)

mECDL

222.5 nm

222.5 nm

445 nm

445 nm

Figure 5.10: Schematic side and top view of the single-pass SHG arrangement with theµECDL module as pump source.


For the experiments with the µECDL, a new crystal oven was manufactured at the FBH1.The BBO crystal is placed into the oven and is this time stabilized to a temperature ofT = 40◦C to assure stable phase matching conditions.

For the convenience of the reader, figure 5.11 again shows the optical pump power of theµECDL emission as a function of the injection current up to I = 1.2 A. The inset infigure 5.11 shows an emission spectrum of the µECDL module at an injection current of1.2 A, corresponding to an output power of 1.4 W with a central emission wavelength of445 nm (measured with an optical spectrum analyzer, Yokogawa AQ6373). The suppressionof the ASE and the longitudinal laser diode modes (SR) is measured to be 48 dB. Thisexemplary emission spectrum illustrates that in contrast to the spectral behavior of themacroscopic ECDL system in Littrow configuration, the SR for the µECDL module staysalmost constant up to the maximum output power of Ppump = 1.4 W (I = 1.2 A), (see alsofigure 4.26.A in section 4.3).

Figure 5.11: Optical pump power of the µECDL module as a function of the injectioncurrent for T = 20◦C heatsink temperature. Inset: Emission spectrum of the µECDLmodule at an injection current of 1.2 A, corresponding to an output power of 1.4 W at theemission wavelength of 445 nm (measured with a high dynamic range optical spectrumanalyzer, Yokogawa AQ6373).

1 The manufacturing of the crystal oven was done by the FBH colleagues Thomas Roos, Detlef Grimpe,and Bastian Deutscher.


5.2.2 Optimization of the focusing conditions

As already mentioned, the asymmetry of the µECDL beam in terms of beam quality andbeam diameter makes it difficult to easily find the optimum focusing condition by simplyrealizing the beam waist radius recommended by the Boyd-Kleinman (BK) analysis using aspherical focusing lens. The optimum beam waist radius in the vertical, i.e. phase matchingplane (PM), may be different to the optimum beam waist radius in the lateral plane inthis experiment. For this reason, three different lateral telescopes are applied to expandthe beam in the lateral plane. As mentioned, the expansion is necessary to reduce theresidual divergence and to enable smaller beam waist radii in the lateral plane inside theBBO crystal. The aim of this approach is to find the best pair of lateral and PM planefocusing conditions and thereby maximize the SHG output power.

Table 5.3 gives an overview of the focal lengths of the cylindrical lenses L1 and L2applied for the three lateral telescopes. Beam expansion (1) leads to a lateral magnificationfactor of Xlat = 4.5, expanding the lateral beam diameter from 0.7 mm to 3.1 mm. Forbeam expansion (2), the magnification factor is Xlat ≈ 10, leading to a lateral beamdiameter of 6.6 mm. And expansion (3) results in Xlat ≈ 13 and dlat = 8.8 mm. Theresulting aspect ratios dPM : dlat are listed in the table as well.

beam expansion fL1 fL2 Xlat dPM dlat dPM : dlat

w/o 1.8 mm 0.7 mm 1 : 0.4(1) 22 mm 98 mm 4.5 1.8 mm 3.1 mm 1 : 1.7(2) 7.55 mm 74 mm 10 1.8 mm 6.6 mm 1 : 3.7(3) 7.55 mm 98 mm 13 1.8 mm 8.8 mm 1 : 4.9

Table 5.3: Overview of the different applied lateral beam expansions. fL1, fL2: focal lengthof lens L1 and L2. Mlat: lateral magnification factor. d0,PM, d0,PM: beam diameter in phasematching and lateral plane. d0,PM : d0,lat: aspect ratio.

After the lateral expansion, the beam is then focused into the BBO crystal by a sphericallens (L3) for which focal lengths of 50 mm, 75 mm, 100 mm, and 150 mm are appliedin each case. For the SHG measurement, the pump power Ppump is increased in steps of100 mW up to the maximum output power of the µECDL of 1.4 W and for each step thethe DUV output power at 222.5 nm is measured with the SiC photodiode.

Figure 5.12 shows the generated SHG power PSHG at 222.5 nm for the three appliedlateral beam expansions and the different focal lengths of lens L3 as a function of the pumppower Ppump at 445 nm. The black solid curve shows the SHG output power predicted bythe Boyd-Kleinman analysis for a beam waist radius of w0 = 15 µm leading to a conversionefficiency of η = 13.8 · 10−5 W−1. The experimental data points for each measurementare again fitted with the function PSHG = η · P 2

pump according to the quadratic conver-sion efficiency behavior without pump depletion [74], and are illustrated by the dashedcurves. The numerical fits give the values for the normalized conversion efficiencies ηsummarized in table 5.4. The highest DUV output of PSHG = 160 µW for a pump powerof Ppump = 1.4 W is achieved with the lateral beam expansion (3) and lens L3 having afocal length of f = 150 mm. The normalized conversion efficiency for this configuration


Figure 5.12: Generated SHG power at 222.5 nm versus the pump power of the fundamen-tal at 445 nm for all applied focusing conditions (TBBO = 40◦C). The dashed lines are thequadratic fits according to PSHG = η · P 2

pump. The black solid curves show the SHG outputpower predicted by the BK analysis.

is η(3),150mm = 7.9 · 10−5 W−1. This is a factor of 1.9 higher than the maximum achievedconversion efficiency with the macroscopic ECDL as pump source (ηmacro = 4.2 ·10−5 W−1)and only a factor of 0.57 below the conversion efficiency predicted by the Boyd-Kleinmananalysis. In contrast to the macroscopic ECDL, the µECDL exhibits no decrease of thesuppression ratio of the ASE and longitudinal laser diode modes for higher pump powers(see figure 5.11) and therefore no deviation between the quadratic fits and the measureddata points at higher pump powers can be observed here.

The beam waist radii in the PM and the lateral plane w0,PM and w0,lat, the Rayleigh lengthin the PM and lateral plane zR,PM and zR,lat, and the consequential focusing parametersin the PM and lateral plane ξPM and ξlat, respectively, are also summarized in table 5.4 onthe following page for all applied configurations. For comparison, the parameters resultingfrom the BK analysis are listed as well.

The geometrical beam parameters are determined by a caustic measurement accord-ing to ISO 11146 at the position of the BBO crystal and by applying the previouslydescribed 90%-knife-edge beam diameter criterion. The Rayleigh lengths inside the BBO


beam expansion fL3 w0,PM w0,lat zR,PM zR,lat ξPM ξlat η / 10−5 W−1

(1) 50 mm 12 µm 25 µm 1.0 mm 1.5 mm 3.8 2.5 7.2(Mlat = 4.5) 75 mm 17 µm 42 µm 2.2 mm 3.4 mm 1.7 1.1 6.4

100 mm 20 µm 55 µm 3.9 mm 6.0 mm 1.0 0.6 5.3150 mm 30 µm 85 µm 8.9 mm 13.5 mm 0.4 0.3 3.8

(2) 50 mm 11 µm 13 µm 1.0 mm 0.3 mm 3.8 12.5 4.4(Mlat = 10) 75 mm 18 µm 19 µm 2.2 mm 0.7 mm 1.7 5.4 5.6

100 mm 23 µm 27 µm 3.9 mm 1.3 mm 1.0 2.9 6.5150 mm 35 µm 42 µm 8.9 mm 2.9 mm 0.4 1.3 7

(3) 50 mm 11 µm 10 µm 1.0 mm 0.2 mm 3.8 18.8 3.4(Mlat = 13) 75 mm 15 µm 13 µm 2.2 mm 0.4 mm 1.7 9.4 4.8

100 mm 21 µm 17 µm 3.9 mm 0.7 mm 1.0 5.4 5.9150 mm 30 µm 29 µm 8.9 mm 1.7 mm 0.4 2.8 7.9

Boyd-Kleinman 15 µm 15 µm 2.7 mm 2.7 mm 1.4 1.4 13.8

Table 5.4: Summary of the focus parameter for the different applied focusing conditions.w0,PM: beam waist radius in the PM plane. w0,lat: beam waist radius in the lateral plane.zR,PM: Rayleigh length in the PM plane. zR,lat: Rayleigh length in the lateral plane.η: SHG conversion efficiency (LBBO = 7.5 mm, TBBO = 40◦C).

crystal are simulated using the parameters from the caustic measurements and assuminga refractive index of BBO at the pump wavelength of 445 nm of nBBO = 1.685. The90%-knife-edge beam propagation factors from the caustic measurements are determinedto be approximately M90%,PM ≈ 1.3 and M90%,lat ≈ 6 in the PM and the lateral plane,respectively. These values are also used for the optical simulations and in general agood agreement between experimentally measured and simulated parameters is observed.Here, it is worth noting that the beam propagation factors in the lateral plane haveincreased compared to the previous µECDL caustic measurements (µECDL at 1.4 W:M90%,lat = 4.5). This is attributed to the lateral telescope in which the small focal length oflens L1 leads to a large beam diameter at lens L2, that probably causes spherical aberrations.

The configuration with the lateral telescope (3) creates nearly the same dimensions inthe beam waist in both planes. For fL3 = 150 mm, that yields the highest conversionefficiency, the beam waist radii are measured to be w0,PM = 30 µm in the PM plane, andw0,lat = 29 µm in the lateral plane. This indicates that compared to the BK analysissomewhat higher radii than w0,BK = 15 µm are beneficial in this experimental arrangement.This is also in accordance to earlier results by Übernickel et al., that made the same ob-servation for non-diffraction limited pump beams for SHG in periodically poled crystals [80].

To further elucidate the optimum focusing conditions, the conversion efficiencies forall applied focusing configurations as the function of the average beam waist radius(w0,PM + w0,lat)/2 are plotted in figure 5.13. The dashed curve represents the predictedconversion efficiency by the BK analysis for a spherical Gaussian pump beam. A cleartendency for higher conversion efficiencies towards beam waist radii in the range between


Figure 5.13: SHG conversion efficiency η as a function of the average beam waist radius0.5 · (w0,PM + w0,lat) for the three different applied lateral beam expansions. For compar-ison, the dashed line represents the simulated Boyd-Kleinman function h for an idealizedGaussian beam.

20 µm and 30 µm can be observed, whereas the efficiency significantly decreases for smallerand larger beam waist radii. This is also in accordance with the results with the macroscopicECDL as pump source. However, the plot as a function of the average beam waist radiusdoes not account for the highly asymmetric nature of the µECDL pump beam and cantherefore only be seen as a rough orientation. Additionally, due to the different lateralbeam expansions, similar beam waist radii generated by different lateral beam expansionconfigurations lead to very different focusing parameters ξ.

ξ is defined as ξ = Lcr/b with the crystal length Lcr and the confocal parameter b = 2zrand is a measure for the focusing strength and therefore also takes the strength of the beamdivergence generated by the focusing into account. b can be seen as the distance over whichthe beams cross sectional area is relatively constant. For a strongly focused beam, b is smalland consequently ξ is large. Accordingly, b is large and ξ is small for a weakly focused beam.

Nonetheless, due to the high lateral beam propagation factor, i.e. the poor lateral beamquality, a high ξ value is necessary to create a sufficiently small lateral beam waist radius.As mentioned earlier, optimum focusing conditions are achieved, if a good trade-off betweena small beam waist radius with high power densities in the crystal center and a sufficientlylong interaction length, i.e. a not to high ξ value or not to high beam divergence, is found.It can be seen that for beam expansion (1) with a moderate lateral magnification factor,the highest conversion efficiency is achieved with lens L3 having a focal length of 50 mm, asin this configuration a beam waist radius in the favorable range between 20 µm and 30 µmis achieved for smaller focal lengths. In the PM plane however, the beam propagationfactor is almost 1, indicating that the BK analysis is more justified in this plane. Thus, thegenerated beam waist of w0 = 12 µm with beam expansion (1) and fL3 = 50 mm seems tobe smaller than the optimum.


Considering all applied focusing conditions but concentrating on the focusing conditionwith the highest conversion effiency (beam expansion (3) and fL3 = 150 mm), a ξ valueslightly larger than the BK prediction but smaller than three (1.4 < ξlat < 3) seems to befavorable in the lateral plane, whereas in the PM plane, a beam waist radius only slightlylarger than the BK prediction of 15 µm but a ξ value smaller than the BK prediction(ξPM < 1.4) seems to be beneficial. This finding is good agreement with previous works thatstudied the optimum focusing for highly elliptical but diffraction limited beams [77–79].It was shown, that weaker focusing in the (PM) plane, reducing the negative walk-offeffect, and strong focusing perpendicular to the PM plane can increase the efficiency inlarge walk-off crystals like BBO. Especially in the paper by Steinbach et al. it was foundthat a ξ value of ξPM = 0.25 in the PM plane and of ξlat = 3.3 perpendicular to the PMplane for the case of strong walk-off (B = 16) resulted in a conversion efficiency evenslightly larger than predicted by the Boyd-Kleinman analysis for focused spherical Gaussianbeams [78]. Taking into account that the beam in the experiment presented in this workis additionally not diffraction limited, this is in suprisingly good agreement with the ξvalues with the highest conversion efficiency derived in this experiment of ξPM = 0.4 andξlat = 2.8 (B = 14.7).

The inherent downside of this setup is that it is difficult to optimize the focusing conditionsin both planes independently of each other. And as there is no straightforward analyticalsolution to this problem, the only feasible approach is to test many different beam geometryconfigurations. Nonetheless, given the fact of the asymmetry of the beam and the relativelypoor beam quality it is believed that the achieved conversion efficiency of η = 7.9·10−5 W−1,which is a factor of 0.57 smaller than the Boyd-Kleinman prediction for a focused Gaussianbeam, is already close to what is achievable with this pump source.

With regard to a preferably compact DUV laser light source however, the configurationwith beam expansion (1) and fL3 = 50 mm results in only a slightly lower maximum opticaloutput power of 142 µW, but a 10 cm shorter optical path length and therefore more

Figure 5.14: a) Emission spectrum of the generated SHG radiation with a DUV powerof 161 µW at 222.5 nm for a pump power at 445 nm of 1.4 W measured with a spectralresolution of ∆λ ≈ 1.3 nm. b) Emission spectrum of the µECDL at a pump power of 1.4 Wmeasured with a spectral resolution of 6 pm.


compact light source compared to the configuration with the highest conversion efficiency.For the sake of completeness, figure 5.14.a) shows a spectrum of the generated DUV lightat λ = 222.5 nm with an output power of 161 µW for a pump power of 1.4 W. It has anemission bandwidth of 1.3 nm again limited by the resolution of the spectrometer (NewportMiniSpec 78355). However, it should be at least in the range of the spectral width of theµECDL emission of ∆λ = 32 pm (FWHM) and ∆λ95% = 152 pm at a pump power of1.4 W shown in figure 5.14.b).

6 Conclusion and OutlookA compact laser light source emitting in the wavelength range between 210 nm and 230 nmis of great interest for numerous applications, especially outside the laboratory environment.In this thesis, a compact laser light source emitting around 222 nm was developed basedon single-pass frequency doubling of a commercially available high-power GaN laser diodeemitting in the blue spectral range. As a pump source for the frequency doubling stage, alaser diode with high optical output power above 1 W with narrowband emission in therange of the acceptance bandwidth of the applied nonlinear BBO crystal was required. SinceGaN based high-power laser diodes exhibit a broad emission spectrum of ∆λ = 1...2 nm,wavelength stabilization and narrowing by the use of an external wavelength selectiveelement was necessary.

First, an external cavity diode laser system with a surface diffraction grating as ex-ternal element was realized as a proof-of-concept study. It was found that feeding asmuch as 15% of the laser diode radiation back into the laser is sufficient for wavelengthstabilization even though the laser diodes facet was not treated with an anti-reflectioncoating. And the suppression of the ASE and longitudinal laser diode modes workedbest when the wavelength of the optical feedback coincided with the maximum of thelaser diodes gain bandwidth. The ECDL setup exhibited a narrow emission bandwidth of∆λ ≤ 20 pm up to an output power of about 470 mW, and ∆λ ≤ 70 pm (FWHM) up toan output power of about 680 mW, at an emission wavelength of 445 nm.

The ECDL system was then used as pump source in a proof-of-concept setup for single-pass second harmonic generation of laser light at 222.5 nm in a BBO crystal. Thereby,narrowband DUV laser light with a continuous wave output power of 16 µW was generatedwith a pump power of 680 mW. The measurement of the phase matching temperatureacceptance bandwidth of BBO revealed, that the phase matching acceptance bandwidthsderived from the plane-wave approximation are not valid, when focused beams are used.Rather is the effective interaction length inside the BBO crystal reduced by the focusing,which leads to a broadening of the temperature, wavelength and angle phase matchingacceptance bandwidths in comparison to the simulations in the plane-wave approximation.

To further reduce the footprint of the complete setup, a micro-integrated ECDL module(µECDL) assembled on a conduction cooled package with a footprint of 25 mm x 25 mmwas developed. Here, a holographic volume Bragg grating served as external wavelengthselective element. Compared to the macroscopic ECDL, the µECDL module showed animproved performance having a narrow emission bandwidth of ∆λ ≤ 50 pm up to anoutput power of 1.4 W at an emission wavelength of 445 nm, and a mode suppression ratioas high as 53 dB over the whole operating range.

With the µECDL module as pump source, the focusing conditions were optimized and itwas found that for the asymmetric and non-diffraction limited µECDL output a larger beam

93

94 6 Conclusion and Outlook

waist radius in the range of 20 µm to 30 µm resulted in the highest conversion efficiencycompared to the Boyd-Kleinman analysis for focused Gaussian beams, that recommendsan optimum beam waist radius inside the BBO crystal of 15 µm.

In conclusion, a novel compact DUV laser light source with a footprint of approximately5 cm x 30 cm delivering a continuous wave output power of PDUV = 160 µW at a wavelengthof λDUV = 222.5 nm was developed. The presented concept enables compact, reliable andportable DUV laser light sources with low power consumption, that are potentially suitablefor new applications outside of laboratory environments.

Outlook

Even though the achieved DUV output power of 160 µW is already sufficient for someapplications, a higher output power is always desirable. The most obvious approach ispower scaling by means of a pump source with higher optical output power. At the timeof this work, the laser diode (OSRAM Opto Semiconductors, model: PLTB 450B) appliedthroughout this work delivered the highest output power (P = 1.6 W) from a commerciallyavailable laser diode. Currently, the highest commercially available output power froma laser diode in the blue spectral range is specified with P = 3.5 W [44, 154] (NichiaCorporation, model: NDB7K75, Osram Opto Semiconductors: model PLPT9 450DE A01).Taking the achieved normalized conversion efficiency of η = 7.9 · 10−5 W−1 as a basis, apump power of 3.5 W in the blue spectral range, could generate a DUV output power ofapproximately PDUV ≈ 968 µW. In the next years, even higher output powers from blueGaN based laser diodes can be expected making the generation of more than 1 mW ofDUV laser light by single-pass frequency doubling in a BBO crystal easily feasible.

Another approach to increase the DUV output power would be the enhancement ofthe effective nonlinear coefficient deff by the use of newly developed crystals. Unfortunately,to the best of the authors knowledge, an alternative nonlinear bulk crystal material, thatexhibits a higher effective nonlinear coefficient than BBO and can be phase matched toSHG wavelengths below 230 nm has not yet been developed. However, some efforts havebeen made to increase the conversion efficiency in BBO itself by implementing waveguidestructures into the crystal [155, 156] or using walk-off compensating arrangements withmultiple successive BBO plates [157, 158]. Hara et al. for example, realized a walk-offcompensating structure consisting of room-temperature bonded BBO plates and achieveda 1.8 times increase of the nonlinear conversion efficiency compared to a bulk BBO crystalwith the same overall length [158]. A further advantage of such a walk-off compensatingstructure is a nearly circular DUV output beam. In contrast, the heavy walk-off in a bulkBBO crystal leads to a strongly elliptical shape of the generated DUV beam, which usuallyrequires additional beam shaping optics in order to generate a more practical circular beamprofile. However, the manufacturing procedures of such structures are very complex andhave not yet reached the maturity for the realization of commercially available structuredcrystals.

Another way to circumvent the disadvantage of the heavy walk-off in BBO, would bethe use of periodically poled nonlinear crystals. Conventional used nonlinear materialslike PPLN and PPLT however show strong absorption in the wavelength range below300 nm and the lowest generated second harmonic wavelength with these crystals is 325 nm

95

generated in PPLT [70]. There is ongoing research towards periodically poled nonlinearcrystals suitable for the DUV wavelength range. Recently, Hirohashi et al. demonstratedthe generation of laser radiation at 266 nm in a periodically poled LaBGeO5 crystal [71].In principle, this material can also be used to generate laser light in the wavelength rangearound 222 nm. However, this has not been achieved up to now.

Although cavity enhanced frequency doubling setups are thought to be too sensitiveand complex for the realization of a compact and robust DUV laser light source, theconcept of a multi-pass SHG in a semi-monolithic concave-plano resonator presented byKlappauf et al. represents an interesting alternative to single-pass frequency doubling [159].Implementing a BBO crystal in such a resonator could lead to a compact and robust DUVlaser light source in the mW range with pump powers far below 1 W.

Besides the power scaling possibilities, there are also some other challenges to solvein the presented setup in order to realize an even more compact and practical DUV laserlight source: First, the footprint of the device can be reduced by using an anamorphicprism pair to shape the pump beam in front of the nonlinear crystal instead of usinga telescope arrangement with a rather long optical path length. Furthermore, it wouldbe desirable to filter the intensity of the fundamental by using a band-pass filter, thatonly transmits the generated DUV light instead of using the prism to spatially separatefundamental and second harmonic beam. This is especially challenging as such filters showrelatively low transmission rates of less than 50% in the DUV, resulting in a high loss ofthe useable DUV output power.

With respect to the GaN based external cavity diode laser as pump source, it wouldbe desirable to implement a laser diode with an AR coated facet into the external cavitysetup. This is believed to result in a further reduced emission bandwidth of the laser diode,an increase in the wavelength tuning range, and a higher spectral stabilitity of the ECDLemission. Consequently, these improvements would also translate into the DUV wavelengthrange. Here, especially a wavelength tuneable DUV laser light source would be of greatinterest for many applications.

A Datasheet information for laser diodePL TB450B (Osram OptoSemiconductors GmbH)

Features

• Typ. emission wavelength 450 nm

• Efficient radiation source for cw and pulsed operation

• TO56 package

• ESD protection diode

• Laser diode isolated against package

Applications

• Projection

• Metrology

• Stage lighting

Electro-optical parameters

Parameter Symbol

Emission wavelength λ 450 nmThreshold current Ith 0.2 AOutput power (I = 1.2 A) Popt 1.6 WOperating current (Popt = 1.6 W) I 1.2 AOperating voltage (Popt = 1.6 W) U 4.8 VBeam divergence (FWHM) θ‖ 7◦

Beam divergence (FWHM) θ⊥ 23◦

Polarization ratio (TE:TM) PR 100 : 1Thermal resistance (junction to case) R‖ 15 K/W

Table A.1: Electro-optical parameters of the laser diode applied in this work for a casetemperature of Tcase = 25◦C. Taken from the datasheet published by OSRAM Opto Semi-conductors GmbH.

97

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https://www.thorlabs.com/thorproduct.cfm?partnumber=S130VC

https://www.thorlabs.com/thorproduct.cfm?partnumber=S130VC



List of Figures1.1 Illustration of the spectral separation of Raman signal and fluorescence

background for DUV excitation using the example of the fingerprint regionof the Raman spectrum of polystyrene. . . . . . . . . . . . . . . . . . . . . . 2

2.1 a) Schematic sketch of a second harmonic generation process inside a non-linear crystal. b) SHG process depicted in an energy level diagram. . . . . . 10

2.2 Dispersion of the ordinary and extra-ordinary refractive index in BBO for acrystal temperature of TBBO = 50◦C according to the Sellmeier equationsfrom [72]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3 Schematic illustration of phase matching via angle tuning for second har-monic generation, top view (adapted from [46], p. 98). . . . . . . . . . . . . 16

2.4 Ordinary refractive index of the fundamental beam and extraordinary refrac-tive index of the second harmonic beam as a function of the angle θ betweenthe optical axis of the crystal and the wave vector k of the extraordinarybeam in BBO (TBBO = 50◦C). . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.5 Schematic illustration of the spatial walk-off occuring with critical phasematching (top view). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.6 Normalized SHG intensity as a function of the pump wavelength in BBO ata crystal temperature of T = 50◦ (LBBO = 7.5 mm). . . . . . . . . . . . . . 18

2.7 Normalized SHG intensity as a function of the crystal temperature in BBOfor a phase matching temperature of T = 50◦ (LBBO = 7.5 mm, λ = 445 nm). 19

2.8 Normalized SHG intensity as a function of the phase matching angle for afixed fundamental wavelength of 445 nm (LBBO = 7.5 mm). . . . . . . . . . 19

2.9 Geometry of a focused beam inside a nonlinear crystal of length Lcr. Thesolid line indicates a weakly focused and the dashed line a strongly focusedbeam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.10 a) Boyd-Kleinman function in dependence of the beam waist radius w0 forSHG in a BBO crystal without walk-off (dashed line) and with walk-off (solidred line) on a logarithmic scale. b) Boyd-Kleinman function in dependence ofthe beam waist radius w0 in case of walk-off on a linear scale. (TBBO = 50◦C,LBBO = 7.5 mm, λpump = 445 nm). . . . . . . . . . . . . . . . . . . . . . . . 21

2.11 Simulated SHG power as function of the pump power according to the Boyd-Kleinman theory for a BBO crystal with TBBO = 50◦C, LBBO = 7.5 mmand λpump = 445 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.1 Photography of the applied laser diode packaged in a TO56 can [43]. . . . . 233.2 Optical microscope images of the applied laser diode with different magnifi-

cation factors (5x (a), 10x (b), 50x (c)). . . . . . . . . . . . . . . . . . . . . 243.3 Schematic illustration of a Fabry-Perot laser diode resonator of length L

with mirror reflectivities R1 and R2 of the rear and front facet, respectively. 25

111

112 List of Figures

3.4 a) Exemplary transverse layer structure of a multiple quantum well separateconfinement heterostructure GaN based laser diode. b) Exemplary transverseenergy band structure for a multiple quantum well separate confinementheterostructure laser diode. Eg is the band gap energy of the quantum wells. 25

3.5 Emission spectrum of the laser diode applied in this work below thresholdat a current of I = 140 mA and at T = 20◦C heatsink temperature. . . . . . 26

3.6 Spectrum of the modal gain and the longitudinal FP modes of a laser diodeat threshold. The modal gain takes its maximum Γgth at λp. . . . . . . . . 27

3.7 Voltage U , optical output power P and electro-optical conversion efficiencyηc as a function of the injection current I for the laser diode applied in thiswork at a heatsink temperature of T = 20◦C. . . . . . . . . . . . . . . . . . 28

3.8 Emission spectrum for an injection current of I = 1.2 A at a heatsinktemperature of T = 20◦C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.9 a) Shift of the central emission wavelength λC versus the heatsink tempera-ture at an injection current of I = 1.2 A. b) Shift of the central emissionwavelength λC versus the injection current at T = 20◦C heatsink temperature. 30

3.10 Geometry of a Gaussian beam propagating in z direction. . . . . . . . . . . 313.11 Intensity profile of the collimated beam from the laser diode applied in this

work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.12 Caustic measurements of the laser diode in fast (left) and slow (right) axis

according to the variance and the 90%-knife-edge diameters at T = 20◦Cheatsink temperature and I = 1.2 A. . . . . . . . . . . . . . . . . . . . . . . 34

4.1 Schematic illustration of an external cavity diode laser as a three-mirrorcavity laser. R1 and R2 are the reflectivities of the back and front facet andR3 is the reflectivity of the external dispersive element. . . . . . . . . . . . . 38

4.2 a) Calculated transmission functions in a three-mirror ECDL: The green lineis the combined transmission function of the the cavities formed betweenmirror R1 and R3 and between R2 and R3. The black line is the transmissionfunction of the laser diode cavity. The red line indicates the dispersion D ofthe surface grating and the blue line the semiconductor gain. b) Spectralresponse of the ECDL system as the product of all dispersive factors (blackline) and grating dispersion D (grey line) for comparison. . . . . . . . . . . 39

4.3 Illustration of an external cavity diode laser in a) Littrow and b) Littman-Metcalf configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.4 ASE emission spectrum of the laser diode at an injection current of I = 0.1 A. 444.5 Emission spectra of the laser diode at an injection current of I = 0.2 A,

0.7 A, and 1.2 A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.6 Schematic view of the ECDL system in Littrow configuration: (1) FP laser

diode, (2) collimating lens, (3) surface grating, (4) mirror. . . . . . . . . . . 454.7 Illustration of two ECDL configurations: a) The laser diode (LD) polarization

is perpendicular to the grating grooves, the grating dispersion occurs in thelateral LD plane. b) The LD polarization is parallel to the grating grooves,the grating dispersion occurs in the vertical LD plane. . . . . . . . . . . . . 46

4.8 Simulated normalized diffracted intensity D in the first diffraction order fora surface diffraction grating with 3600 grooves/mm in configuration b). . . 47

List of Figures 113

4.9 Upper part: ASE emission spectrum of the laser diode at an injectioncurrent of I = 0.1 A. Lower part: Lasing threshold current versus emissionwavelength for the ECDL system at a heatsink temperature of 20◦C. . . . . 48

4.10 Optical output power of the ECDL system emitting at 445 nm and the FPlaser diode for a heatsink temperature of 20◦C. . . . . . . . . . . . . . . . . 49

4.11 Emission spectra of the ECDL system at injection currents of I = 0.2 A,0.4 A, 0.6 A, 0.8 A, 1.0 A, and 1.2 A measured with a) a double-echellemonochromator with a spetral resolution of 6 pm and b) an optical spectrumanalyzer with a spectral resolution of 50 pm, for 20◦C heatsink temperature. 51

4.12 Emission spectra of the FP laser diode (grey) and the ECDL system (red)at I = 0.6 A and T = 20◦C. . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.13 Emission spectra of the ECDL system at an injection current of I = 0.6 Afor different Littrow angles measured with the optical spectrum analyzerwith a resolution of 50 pm at 20◦C heatsink temperature. . . . . . . . . . . 54

4.14 Measured caustic in the fast (left) and slow axis (right) of the ECDL systemat an injection current of I = 0.6 A using variance and the 90%-knife edgemethod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.15 Model of a holographic reflecting VBG with thickness d. Rin: incident beam,Sout: output beam, K : grating vector, θ: Bragg angle, φ: slanting angle, Λ:grating period, z: optical axis. . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4.16 Simulation of the wavelength dependence of the diffraction efficiency DEaccording to equation (4.7) for a reflecting VBG with the parameters fromtable 4.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.17 Simulation of the angle dependence of the diffraction efficiency DE accordingto equation (4.7) for a reflecting VBG with the parameters from table 4.5. . 59

4.18 Concept of the µECDL module. Also shown are the distances betweenthe different elements of the module and the three competing resonators oflength LLD, Linner, and Louter . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.19 Simulated residual full divergence angle in lateral and vertical direction as afunction of the lens position. . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.20 Simulated coupling efficiency of the back-coupled light in the vertical andlateral axis as a function of the VBG tilt around the x- and y-axis, respectively. 61

4.21 Schematic top and side view of the micro-integrated ECDL module: (1)laser diode in TO56 can, (2) collimating lens, (3) reflecting volume Bragggrating, (4) thread for attachment of the laser diode, (5) conduction cooledpackage, (6) cover plate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.22 Optical output power versus injection current for the µECDL module (solidline) and the FP laser diode (dashed line) at a heatsink temperature ofT = 20◦C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.23 Contour plot of multiple emission spectra of the FP laser diode as a functionof the injection current with measurement steps of 20 mA. Each spectrumis individually normalized in intensity to 1. . . . . . . . . . . . . . . . . . . 65

4.24 Contour plot of multiple µECDL emission spectra as a function of the injec-tion current with measurement steps of 5 mA. Each spectrum is individuallynormalized in intensity to 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

114 List of Figures

4.25 A: Peak wavelength of the free-running laser diode and of the µECDLemission as a function of the injection current. B: Half-logarithmic plotof the FWHM bandwidth of the free-running laser diode and the µECDLemission as a function of the injection current. . . . . . . . . . . . . . . . . 66

4.26 A: Emission spectra of the free-running LD and the µECDL system atinjection currents of I = 0.3 A, 0.6 A, 0.9 A, and 1.2 A measured with ahigh-dynamic range optical spectrum analyzer with a spectral resolution of50 pm. B: µECDL emission spectra at I = 0.3 A, 0.6 A, 0.9 A, and 1.2 Ameasured with a double-echelle monochromator with a spetral resolution of6 pm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.27 Temporal stability of the µECDL emission for an injection current of I =575 mA. a) Peak wavelength of the µECDL emission measured over a timeperiod of 1 hour. b) Exemplary emission spectrum at top = 30 min. c)Optical output power of the µECDL over a time period of 1 hour. . . . . . 68

4.28 Variance and 90%-knife edge caustic measurement of the µECDL emissionin fast and slow axis for an injection current of I = 0.8 A. . . . . . . . . . . 69

5.1 Schematic top view of the single-pass frequency doubling setup with themacroscopic ECDL emitting at 445 nm as pump source. . . . . . . . . . . . 72

5.2 a) Spectral sensitivity of the used SiC photodiode (sglux SolGel Technologies,model SG01XL) and for comparison of a conventional UV extended Siphotodiode (Thorlabs Inc., model S130VC). b) Spectral sensitivity of theapplied SiC photodiode in logarithmic scale. Data taken from the supplierscalibrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5.3 Pump power Ppump of the macroscopic ECDL system emitting at 445 nmas a function on the injection current. Inset A: ECDL emission spectrum at0.6 A (0.47 W). Inset B: ECDL emission spectrum at 0.8 A (0.68 W). . . . 76

5.4 Emission spectra of the ECDL system at a) I = 0.6 A and b) I = 0.8 A cor-responding to an output power of I = 0.47 W and I = 0.68 W, respectivelyat a heatsink temperature of T = 20◦C. . . . . . . . . . . . . . . . . . . . . 77

5.5 Generated SHG power at 222.5 nm versus pump power at 445 nm for afocal length of lens L3 of 50 mm, 75 mm, 100 mm, and 150 mm. Dashedcurves are the quadratic fits of the data points. The solid black curve showsthe expected SHG power for a focused Gaussian beam according to theBoyd-Kleinman analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.6 Spectrum of the generated SHG light at λSHG = 222.5 nm and of the residualstray light from the fundamental beam at λpump = 445 nm. . . . . . . . . . 80

5.7 a) Measured normalized SHG intensity at 222.5 nm as a function of thecrystal temperature TBBO for a 7.5 mm long BBO crystal. Experimentaldata points are represented by the red squares. b) sinc2 fit (solid line) ofthe experimental data from a) using an effective crystal length of 1.2 mm. . 81

5.8 Phase-matching wavelength acceptance bandwidth (a) and phase matchingangle acceptance bandwidth (b) calculated in the plane-wave approximationfor a crystal length of La,eff = 1.2 mm (TBBO = 50◦C). . . . . . . . . . . . . 82

List of Figures 115

5.9 a) Setup for the measurement of the temperature acceptance with a par-allelized beam in the phase matching plane. b) Resulting phase matchingtemperature acceptance curve. The dots are the experimental data and thesolid curve is the simulated temperature acceptance in BBO for a crystallength of LBBO = 7.5 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5.10 Schematic side and top view of the single-pass SHG arrangement with theµECDL module as pump source. . . . . . . . . . . . . . . . . . . . . . . . . 85

5.11 Optical pump power of the µECDL module as a function of the injectioncurrent for T = 20◦C heatsink temperature. Inset: Emission spectrum ofthe µECDL module at an injection current of 1.2 A, corresponding to anoutput power of 1.4 W at the emission wavelength of 445 nm (measuredwith a high dynamic range optical spectrum analyzer, Yokogawa AQ6373). 86

5.12 Generated SHG power at 222.5 nm versus the pump power of the funda-mental at 445 nm for all applied focusing conditions (TBBO = 40◦C). Thedashed lines are the quadratic fits according to PSHG = η ·P 2

pump. The blacksolid curves show the SHG output power predicted by the BK analysis. . . 88

5.13 SHG conversion efficiency η as a function of the average beam waist radius0.5 · (w0,PM +w0,lat) for the three different applied lateral beam expansions.For comparison, the dashed line represents the simulated Boyd-Kleinmanfunction h for an idealized Gaussian beam. . . . . . . . . . . . . . . . . . . 90

5.14 a) Emission spectrum of the generated SHG radiation with a DUV power of161 µW at 222.5 nm for a pump power at 445 nm of 1.4 W measured witha spectral resolution of ∆λ ≈ 1.3 nm. b) Emission spectrum of the µECDLat a pump power of 1.4 W measured with a spectral resolution of 6 pm. . . 91

List of Tables1.1 Possible applications for deep ultraviolet laser light, corresponding tech-

niques, wavelength ranges, and required specifications. . . . . . . . . . . . . 31.2 Established DUV laser light sources and their emission wavelength λ, typical

average optical output power Popt, and power consumption Pcon. . . . . . . 31.3 Overview of diode laser based DUV light sources. SHG: second harmonic

generation, SFG: sum frequency generation, TA: tapered amplifier, MOPA:master oscillator power amplifier, SP: single-pass, CE: cavity-enhanced. . . 5

1.4 Targeted specifications for the diode laser based DUV light source. . . . . . 6

2.1 Transmission cut-off wavelength λcut-off, minimum SHG wavelength at whichphase matching with type I SHG can be achieved at room temperature(T ≈ 293 K), and effective nonlinear coefficient deff for selected crystals. . . 12

4.1 ECDL parameters from a selection of publications on low-power GaN basedECDLs in Littrow configuration. PLD: nominal maximum laser diode outputpower, PECDL: maximum ECDL output power, ∆ν: ECDL emission band-width, ∆λtun,contin: mode-hop-free or continuous tuning range, ∆λtun,coarse:manual or coarse tuning range. . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.2 Spectral bandwidth ∆λ and diffraction efficiency DE for the three grat-ings with different groove densities G with the laser diode polarization Eperpendicular to the grating grooves (∆λ⊥, DE⊥), or parallel (∆λ‖, DE‖).The measurement uncertainty for the diffraction efficiency values is specifiedby Richardson Gratings to be ±3 % [137]. . . . . . . . . . . . . . . . . . . . 46

4.3 Experimental parameters used for the calculation of the grating dispersion(figure 4.8) for a grating with 3600 grooves/mm. . . . . . . . . . . . . . . . 47

4.4 Summary of the measured spectral parameters of the macroscopic ECDLsystem emitting at 445 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.5 Parameters used to simulate the diffraction efficiency of the reflecting VBGapplied in the µECDL module and the obtained values for DEmax at 445 nmand the spectral selectivity ∆λ. Lower part: manufacturer (Optigrate Corp.)specifications for DEmax and ∆λ. . . . . . . . . . . . . . . . . . . . . . . . . 58

4.6 Optical path length and free spectral range for the three resonators CLD,Cinner, and Couter formed in the µECDL module. . . . . . . . . . . . . . . . 60

4.7 Summary of the electro-optical, spectral and spatial parameters of thefree-running laser diode (LD), the macroscopic ECDL and the µECDL module. 70

5.1 Simulated beam waist radii in the phase matching plane (PM) and per-pendicular to the phase matching plane for different focal lengths of lensL3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.2 Normalized SHG conversion efficiency η for the different focal lengths oflens L3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

117

118 List of Tables

5.3 Overview of the different applied lateral beam expansions. fL1, fL2: focallength of lens L1 and L2. Mlat: lateral magnification factor. d0,PM, d0,PM:beam diameter in phase matching and lateral plane. d0,PM : d0,lat: aspectratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

5.4 Summary of the focus parameter for the different applied focusing conditions.w0,PM: beam waist radius in the PM plane. w0,lat: beam waist radius inthe lateral plane. zR,PM: Rayleigh length in the PM plane. zR,lat: Rayleighlength in the lateral plane. η: SHG conversion efficiency (LBBO = 7.5 mm,TBBO = 40◦C). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

A.1 Electro-optical parameters of the laser diode applied in this work for acase temperature of Tcase = 25◦C. Taken from the datasheet published byOSRAM Opto Semiconductors GmbH. . . . . . . . . . . . . . . . . . . . . . 97

a deep ultraviolet laser light source by frequency

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