Antireflection coated semiconductor laser amplifier for Bose-Einstein condensationexperimentsSaurabh Pandey, Hector Mas, Giannis Drougakis, Kostas G. Mavrakis, Mikis Mylonakis, Georgios Vasilakis,Vasiliki Bolpasi, and Wolf von Klitzing

Citation: AIP Advances 8, 095020 (2018); doi: 10.1063/1.5047839View online: https://doi.org/10.1063/1.5047839View Table of Contents: http://aip.scitation.org/toc/adv/8/9Published by the American Institute of Physics

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AIP ADVANCES 8, 095020 (2018)

Antireflection coated semiconductor laser amplifierfor Bose-Einstein condensation experiments

Saurabh Pandey,1,2,a,b Hector Mas,1,3,a,c Giannis Drougakis,1,2

Kostas G. Mavrakis,1,2 Mikis Mylonakis,1,2 Georgios Vasilakis,1Vasiliki Bolpasi,1 and Wolf von Klitzing11Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas,Heraklion 70013, Greece2Department of Materials Science and Technology, University of Crete,Heraklion 70013, Greece3Department of Physics, University of Crete, Heraklion 70013, Greece

(Received 10 July 2018; accepted 12 September 2018; published online 20 September 2018)

We present a slave laser highly suitable for the preparation and detection of 87RbBose-Einstein condensates (BEC). A highly anti-reflection coated laser diode servesas an optical amplifier, which requires neither active temperature stabilization nordedicated equipment monitoring the spectral purity of the amplified light. The laserpower can be controlled with a precision of 10 µW in 70 mW with relative fluctua-tions down to 2 × 10−4. Due to its simplicity and reliability, this slave laser will be auseful tool for laboratory, mobile, or even space-based cold-atom experiments. By theway of demonstration this slave laser was used as the sole 780 nm light-source in theproduction of 3 × 104 BECs in a hybrid magnetic/dipole trap. © 2018 Author(s).All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/1.5047839

I. INTRODUCTION

Many ultra-cold atom (Bose-Einstein Condensation) experiments rely on optical methods forboth state preparation (optical cooling) and detection of the atoms. In recent years cold-atom quantumsensors have moved into the ‘real-world’ and even into space.1–3 This imposes much more stringentrequirements on portability, stability, and reliability of the laser systems. 87Rb BEC experiments, forexample, require light for the magneto-optical trap (MOT), optical pumping, and imaging.

Most modern laser systems for ultra-cold atom experiments are based on diode lasers in aso-called master-oscillator-power-amplifier (MOPA) arrangement, with Fabry-Perot laser diodes,4

tapered laser diodes5–7 or fiber lasers.8–10 The power amplifier, often referred to as the slave laser,nearly perfectly copies the coherence properties of its master.11–13

Usually, the output facets of the slave laser diodes exhibit considerable back-reflection, thusforming a Fabry-Perot cavity, thus reducing the lasing threshold of the cavity modes. Consequently,there is a risk of the slave lasing at wavelengths other than the seeded one. In order to preventthis, the cavity has to be carefully tuned into resonance with the seed light, thus necessitating avery tight control on the cavity length and thus its temperature and injection current. Therefore, theoutput power cannot be ramped or modulated except over very narrow ranges, and the output powercan only be stabilized against small external fluctuations.15 The output of the slave laser needs to becontinuously monitored for signs of multi-mode or self lasing, e.g. using an external Fabry-Perot (FP)resonator.

aS. Pandey and H. Mas contributed equally to this work.bEmail: pandey@iesl.forth.grcEmail: hector.mas@iesl.forth.gr

2158-3226/2018/8(9)/095020/7 8, 095020-1 © Author(s) 2018

095020-2 Pandey et al. AIP Advances 8, 095020 (2018)

In this paper, we present a much-simplified laser system for the production of Bose-Einsteincondensates (BECs) of 87Rb atoms in a hybrid magnetic and crossed-beam optical dipole trap. Atthe heart of this laser system lies a slave laser based on a highly anti-reflection (AR) coated laserdiode. The AR-coating frustrates the Fabry-Perot resonator formed by the back and front facets ofthe laser diode. As a consequence, an AR coated diode has a very high self-lasing threshold. Whenseeded, however, it amplifies the incident light for a very large range of parameters and the emittedspectrum contains no other peaks next to the one at the frequency of the seed. The output power ofthe laser can be ramped or modulated widely and is easily stabilized down to the 10−4 level, evenafter optics like fibers or polarization cleaners. The resulting laser system does not require activetemperature stabilization or any dedicated equipment monitoring the spectral quality of the emittedlight.

II. EXPERIMENTAL SETUP

Our laser system is depicted in Fig. 1. The AR-slave laser at the heart of this system is basedon an anti-reflection (AR) coated diode laser (Toptica LD-0790-0120-AR-1) mounted on a copperblock.16 The output beam is collimated with an aspheric lens (Thorlabs C230TM-B, f=4.5 mm), whichis mounted into a copper holder and bonded onto copper pillars using a standard epoxy adhesive.Passive cooling of the laser diode can be enhanced using a Peltier element.17 The AR laser diode isseeded with an injection beam of power of typically 7 mW. We operate the AR-slave at an outputpower of typically 95 mW up to 125 mW, which we couple to the distribution board with 50%coupling efficiency. In order to stabilize the power of the laser beams at the distribution board, wedirect a few percent of the light onto a photodiode and feed the signal back to the current of the diodevia the laser controller (Stanford Research Systems LDC501).

We achieve a power stability of 10−4. Note, that such a system could be used to stabilize the opticalpower at any point in the experiment, or even fed back to other parameters such as the fluorescenceof an atomic cloud.

FIG. 1. The complete 780 nm laser setup. Each box denotes a separate optical breadboard with polarization maintainingsingle mode fibers for the light transport. Starting from the lower left corner, the external cavity diode laser ECDL is stabilizedon a Rubidium spectroscopy based on Zeeman modulation. The next board generates the 6.8 GHz sidebands needed forthe repumping of the atoms in the MOT. In the top left corner is a board with acousto-optic modulators for fast switchingand frequency control. Top right corner is the slave-laser board containing the AR-slave laser. We then guide the light over10 m to the experimental table and its distribution board with one fiber in and 6 fibers out for the 3D magneto-optic trap(MOT) and two fibers for the atom source (a 2D-MOT). On this board we clean the polarization, detect the power of theincoming light and feed the signal back to the current supply of the AR-slave, thus achieving a relative power stabilityof 10−4.

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III. CHARACTERIZATION OF THE AR-DIODE-BASED SLAVE LASER

In order to fully characterize behavior of the slave laser based on AR-diode, we analyzed theoptical spectrum, measured the lasing threshold as a function of temperature and seed power and theoutput power as a function of seed power, injected current, and temperature.

A. Very high lasing threshold

The main effect of the AR coating on the laser diode is to prevent self-lasing up to the maximumallowed injection current. For our laser diode, the lasing threshold is 337 mA at 18◦C and changes by11.8 mA/K. Above 19.1◦C we do not observe lasing for any current below the maximum specifiedvalue (350 mA). Below this threshold the laser diode emits mainly non-coherent light with a broadfeatureless spectrum, a small fraction of which is coupled into the optical fiber. Even just below thelasing threshold at 350 mA at 22◦C only 1 mW of light is coupled into the fiber. Above a criticalinjection current the diode starts to lase on its own and a dominant peak appears at 767.1 nm. Wheninjected with the master laser the free-running peak and the background modes are suppressed andthe spectrum contains only a single peak at the master laser’s wavelength. In the FP-diode based slavelasers the output light can be either an amplified light or originates from self-lasing, depending onthe laser current, temperature and seed power. An external cavity is thus commonly used to monitorthe spectral purity of a FP-diode based laser amplifier (slave laser). The AR coated diode amplifiers,on the other hand, do not lase without injection and thus it suffices to monitor the power in the opticalfiber in order to verify proper functioning of the device.

B. Required seed power

The output power is expected to rise with the seed power up to the maximum optical power thatcan be provided for a given injection current. Fig. 2 shows the output power as a function of seedpower and injection current. We measure the seed power right before the slave laser and the outputpower just after the optical isolator. For a given injection current, the output power shows a rapidincrease first and then saturates as a function of the seed power with the maximum output powerbeing 125 mW.

C. Temperature and injection current

In the complete absence of any reflections, the degree of amplification would be a smooth functionwhich increases with increasing current and decreasing temperature. In the presence of reflections aFabry-Perot cavity forms, which on resonance enhances the amplification and off-resonance decreasesit. For relatively strong feedback (FP-diodes), the amplification looks like a typical cavity spectrumwith regularly spaced strong peaks with little in between.

FIG. 2. Dependence of the emitted power on the seed power and the injection current. Note that the maximum output powerof 125 mW can be reached for a 7 mW seed power only.

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FIG. 3. The output power of the AR-coated slave diode as a function of the diode current for 3 mW of seed power at 20◦C.A modulation in the power is observed, up (black) and down (red) as the current is ramped in two directions. Note that theamplification occurs at any current and power.

In the case of weak feedback (AR-diode) the output power versus the injection current (Fig. 3)has a typical sawtooth like shape. The inset shows two hysteresis loops, note that the step occurs athigher currents for increasing currents (black) and at lower currents for the decreasing case.

Fig. 4 shows the output power of the AR diode as a function of injection current and the tem-perature of the diode mount. Note that, like in Fig. 3, the frequency of the output is identical withthe one of the seed laser. The distance of the steps in current or temperature correspond to one freespectral range of the resonator.

One can clearly see, that the AR-diode can maintain the same longitudinal mode if an increasein temperature is compensated by a decrease in injection current. The inset in Fig. 3 shows aclear hysteresis loop, which finds its origin in the power of light absorbed by the diode itself.The thermal power generated in the diode is the difference between the power of the light emit-ted and the electrical power applied by the injection current, which is approximately constantover the hysteresis loop (a few milliamps). In the lower part of the hysteresis loop the emittedpower is lower, which means more re-absorbed light and thus a higher heat load and thus highertemperature at the active junction. As can be seen in Fig. 4, this in turn shifts the step towardslower currents. The inverse holds for the upper part of the loop. Therefore, scanning the currentupwards will result at steps at higher currents, whereas the downward direction will have the samesteps at lower currents. Note that this hysteresis has a positive effect on the stabilization of theoutput power, since after each jump the diode finds itself far away from the any step in outputpower.

FIG. 4. Plot of the measured output power vs the current and the temperature with an injection beam power of 3 mW. Thedata was generated by choosing a value for the current and then ramping the temperature upwards from 18◦C to 22◦C. Thestep size for the temperature was 0.05◦C and 2 mA for the current.

095020-5 Pandey et al. AIP Advances 8, 095020 (2018)

It is clear from Fig. 3 that stable operation can be achieved for any desired output power atany temperature within its operating range. This makes it possible to stabilize the output powerof the AR-diode laser amplifier at any desired power by a simple feedback to its injection cur-rent. This stands in stark contrast to the FP-diodes, where due to the optical cavity formed by thefacets of the diode, amplification can only be achieved for very narrow ranges of temperatures andcurrents.

D. Power and spectral stability

A particularly interesting aspect of the hysteresis in Fig. 3 is that one can stabilize the outputpower by a feedback to the laser current. One can thus control the power not only at the diode itselfbut also after transmission through an optical fiber or even in response to another parameter such asthe size of a MOT.

At a constant power of 46 mW on the distribution board RMS fluctuations are only ∼ 8 µWor 2 × 10−4. We also investigated ramps on the power set at the current controller. Ramping thepower from 15 mW to 75 mW we measured RMS fluctuations of the power of only 58 µWRMS.For scans below 15 mW, we observed occasionally single jumps in the intensity, which are prob-ably related to the area enclosed by hysteresis loops becoming smaller. In a more stringent test,we applied a sudden jump in the temperature of ∼ 2 K (see right side of Fig. 5b). Despite theseabrupt changes the power set (see right side of Fig. 5a) deviates by less than 0.2% from its setvalue.

Another very important feature of a laser diode is its spectral response to the temperature orcurrent changes. We compare the spectral stability of the AR diode while ramping the current fromzero to its maximum value and we compare it with a standard FP diode (Sharp GH0781JA2C). Usingthe optical grating spectrometer (Ocean Optics HR4000), we integrate the counts in a window of± 0.24 nm around the 87Rb D2 line, i.e. the wavelength of the seeding light. This allows us todistinguish the amplified light from the amplified spontaneous emission (ASE). Fig. 6 shows theintegrated power as a function of the injection current for both the AR diode (red curve) and theFP diode (black curve). Both axes are normalized to their peak value of the respective diodes.The response in power of the AR diode with the injection current corresponds to a horizontal cut inFig. 3. The frequency remains locked to the seed laser frequency at all the times (see an example inthe Fig. 6 inset). This stands in stark contrast to the FP-diode, which amplifies the seed light onlyin narrow ranges of injection current.

FIG. 5. A comparison of the long-term stability of the cooling beam power, temperature of the laser, and the measuredatom number for an ultra-cold thermal cloud. a) deviation of the measure from the set power (46.7 mW) on the MOT dis-tribution board, b) temperature near the AR coated diode, and c) atom number of a at 48 µK atom sample in a magneticquadrupole trap after compression and RF-evaporation. The data left of the dashed line were taken at our standard labora-tory conditions, whereas for the data right of the dashed line we manually introduced temperature fluctuation to the slavelaser.

095020-6 Pandey et al. AIP Advances 8, 095020 (2018)

FIG. 6. Integrated counts vs injection current around the locking frequency for the AR diode (red) and an FP diode (black).The horizontal axis is the percentage of the maximum current of each diode and the vertical axis shows the normalizedintegrated counts recorded by the spectrometer with respect to the peaks’ maxima. In the inset we show the spectrum of theAR diode for a seed power of 5mW and at the maximum operating current (350mA). The maximum operating current for theFP-diode is 167mA.

E. Purity of the optical spectrum

We examine the optical spectrum for any signs of a non-resonant background using an opticalgrating spectrometer (Ocean Optics HR4000). The inset in Fig. 6 shows the spectrum of the outputof the seeded AR-slave laser after a single mode optical fiber. There is strictly no pedestal of ASEvisible with the noise visible being entirely due to detector noise and dark-counts. For the inset inFig. 6 the seed power was 5 mW and the injection current 350 mA, but the spectra do not changesignificantly with current or seeding power.

As a further demonstration of the spectral properties of the of the output light—even under harshthermal conditions—we used the intensity stabilized AR-diode to produce a magneto-optical trapwhilst abruptly changing the temperature of the diode (see Fig. 5b). We did not detect any changein the atom number trapped beyond the standard fluctuations normally present in our apparatuseven under stable environmental conditions. This clearly indicates that the spectral properties of theamplified light are very close to the ones of the seed.

IV. BEC PRODUCTION

In order to demonstrate the versatility and stability of the AR-diode laser amplifier, we usedit as the sole source of 780 nm light for the production of 87Rb BECs in a hybrid magneto-dipoletrap following Ref. 18. The design of the laser system is sketched out in Fig. 1. The light from theAR-diode slave laser is sent via optical fibers to a distribution board from where it is sent to the 2Dand 3D magneto-optical traps (MOT).

The experimental sequence takes ca. 30 s and begins by loading 3 × 108 atoms into a 3D-MOTfrom a 2D-MOT. The atoms are then transferred into a magnetic quadrupole trap and subsequentlycooled by radio-frequency (RF) induced evaporation. We then transfer the atoms into a dipole trap,evaporate further, and finally Bose-Einstein condense the atoms.

With the 780 nm laser system described above using 48 mW of light, we produce 87Rb BECs of3 × 104 atoms in the |1,−1〉 state.

Fig. 5a) and c) show the stability of the system in the presence of variable environmental con-ditions. The atom number fluctuation for a magnetically trapped RF-evaporated thermal sample at48 µK temperature can be seen in Fig. 5c) with fluctuations of about 11%RMS. Fig. 5b shows thetemperature of the AR-coated slave laser enclosure. The left side of Fig. 5 up to the dashed linescontains our standard laboratory conditions with temperature fluctuations of the laser enclosure of∆T ∼ 0.3 KRMS. On the right hand side of Fig. 5 the temperature of the slave laser was activelychanged in steps of up to ∼ 2 K. Nevertheless, the AR slave achieves an excellent stability in thetotal optical power on the distribution board with RMS fluctuations 7 µW and 8 µW on the left andright side of Fig. 5, respectively. The atom number fluctuations are similar in magnitude with ourpreviously reported laser setup14 and are attributable to external factors. Note that the right hand

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part of Fig. 5c shows no additional atom number fluctuations despite artificially large temperaturefluctuations in the AR slave laser.

V. CONCLUSIONS

We presented a very simple and highly stable laser amplifier suitable for the operation of anultra-cold atom and BEC experiment. The AR-coated slave laser does not require active cooling ortemperature stabilization, nor does it – as opposed to traditional laser amplifiers based on FP-diodes– require monitoring of the spectral quality of the amplified light. It achieves an RMS power stabilityof 10−4 over a timescale of days even in the presence of large temperature fluctuations. Using thislaser system, we demonstrated reliable production of 87Rb BECs with more than 3 × 104 atoms. Thesimplicity and reliability of this laser system makes it highly suitable for mobile or even space-basedcold-atom sensors.

ACKNOWLEDGMENTS

This work was supported by the Greek Foundation for Research and Innovation (ELIDEK) in theframework of two projects, Guided Matter-Wave Interferometry under grant agreement number 4823and Coherent Matter-Wave Imaging under grant agreement number 4794 and General Secretariat forResearch and Technology (GSRT).1 S. Kolkowitz, I. Pikovski, N. Langellier, M. D. Lukin, R. L. Walsworth, and J. Ye, Physical Reveiew D 94, 124043 (2016).2 T. Schuldt et al., Experimental Astronomy 39, 167 (2015).3 P. W. Graham, J. M. Hogan, M. A. Kasevich, and S. Rajendran, Physical Reveiew Letters 110, 171102 (2013).4 C. E. Weiman and L. Hollberg, Review of Scientific Instruments 62, 1 (1991).5 V. Bolpasi and W. von Klitzing, Review of Scientific Instruments 81, 113108 (2010).6 H. S. Moon, J. B. Kim, J. D. Park, B. K. Kwon, H. Cho, and H. S. Lee, Applied Optics 35(27), 5402 (1996).7 M. Schmidt, M. Prevedelli, A. Giorgini, G. M. Tino, and A. Peters, Applied Physics B 102(1), 11 (2011).8 F. Lienhart, S. Boussen, O. Carraz, N. Zahzam, Y. Bidel, and A. Bresson, Applied Physics B 89(2), 177 (2007).9 T. Leveque, L. Antoni-Micollier, B. Faure, and J. Berthon, Applied Physics B 116(4), 991004 (2014).

10 S. S. Sane, S. Bennetts, J. E. Debs, C. C. N. Kuhn, G. D. McDonald, P. A. Altin, J. D. Close, and N. P. Robins, OpticsExpress 20(8), 8915 (2012).

11 L. E. Erickson and A. Szabo, Applied Physics Letters 18(10), 433–435 (1971).12 J.-P. Bouyer, C. Breant, and P. Schanne, Frequency-Stabilized Lasers and Their Applications (1993).13 S. E. Park, T. Y. Kwon, and H. S. Lee, IEEE Transactions on Instrumentation and Measurement 52(2), 277 (2003).14 D. Sahagun, V. Bolpasi, and W. von Klitzing, Optics Communications 290, 110 (2013).15 B. Saxberg, B. Plotkin-Swing, and S. Gupta, Review of Scientific Instruments 87, 063109 (2016).16 The laser diode has a quoted maximum output power of 140 mW at an operating temperature of 20◦C. The useful gain

stretches from 770 nm to 800 nm. Companies and part numbers are given for information only. This does not consist of anendorsement of the company or of its products.

17 There is no feedback stabilizing the temperature of the diode. We use the Peltier in our system to operate the diode atdifferent temperature ranges and to increase the temperature fluctuations. Other means of cooling like passive air or watercooling would also be sufficient.

18 Y.-J. Lin, A. R. Perry, R. L. Compton, I. B. Spielman, and J. V. Porto, Physical Review A 79(6), 063631 (2009).