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Master of Science Thesis in Electrical EngineeringDepartment of Electrical Engineering, Linköping University, 2017

Electrical Pulsing of a LaserDiode for Usage inFluorescence Microscopy

Karin Jerner

Master of Science Thesis in Electrical Engineering

Electrical Pulsing of a Laser Diode for Usage in Fluorescence Microscopy

Karin Jerner

LiTH-ISY-EX--17/5023--SE

Supervisor: Andreas ForsbergSyntronic

Examiner: Atila Alvandpourisy, Linköpings universitet

Division of Integrated Circuits and SystemsDepartment of Electrical Engineering

Linköping UniversitySE-581 83 Linköping, Sweden

Copyright © 2017 Karin Jerner

To my parents and boyfriend.

Sammanfattning

En relativt ny applikation för lasern är fluorescensmikroskop. Fluorescensmikro-skopet behöver en lampkälla med hög effekt. Användandet av en laserkälla för-bättrar precisionen hos mikroskopet. En pulsad laserkälla ökar prestandan hosfluorescensmikroskopet och en laser diod kan köras på högre effekt utan att taskada. Denna uppsats undersöker vilka egenskaper laserpulser behöver ha angå-ende pulsbredd, period och spänningsamplitud. Uppsatsen undersöker även hurdessa pulser kan genereras genom användning av elektriska komponenter. Denönskade laserpulsen bör ha en pulsbredd på 100 ps och en pulsperiod på 50 ns.Laserpulsen bör även ha en väldefinierad våglängd, stabil effekt och bör snabbtkunna slås av och på. För att uppnå denna laserpuls, bör insignalen till laserdio-den vara en spänning på 5 V, en ström på 250 mA, en pulsbredd på 100 ps och enpulsperiod på 50 ns. För att genera denna puls borde SRD:n ha låg övergångska-pacitans, låg emballagekapacitans och låg emballageinduktans. MESFET:en öns-kar låg drain ström och borde ha hög transkonduktans och en stor negativ trös-kelspänning.

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Abstract

A relatively new application for the laser is in fluorescence microscopes. The fluo-rescence microscope needs a high power light source input. Using a laser sourceimproves the precision of the microscope. A pulsed laser source enhances theperformance of the fluorescence microscope and a laser diode can be overdrivenwithout being damaged. The thesis investigates which properties of the laserpulses are needed regarding pulse width, pulse period and waveform. The thesisalso investigates which properties are desired for the electrical pulses driving thelaser, and how they can be generated using electrical components. The desiredlaser pulse should have a pulse width of 100 ps and a pulse period of 50 ns. Thelaser pulse should also have a well-defined wavelength, stable output power andit should be able to quickly turn on and off. To achieve this laser pulse, the desiredinput to the laser diode should have an input voltage of 5 V, an input current of250 mA, a pulse width of 100 ps and a pulse period of 50 ns. For generating thispulse the chosen pulse generator, an SRD, should have low junction capacitance,low package capacitance and low package inductance. The chosen amplifier, aMESFET, desires low drain current and should have high transconductance anda large negative threshold voltage.

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Contents

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.4 Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.5 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Part I - Investigation of the Laser Waveform 52.1 Fluorescence Microscopy . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Light Emission . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.2 Required Pulse Waveform . . . . . . . . . . . . . . . . . . . 92.2.3 Laser Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Part II - Investigation of Pulse Generation 173.1 Pulsing Method Choice . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.1 Desired Electrical Pulse . . . . . . . . . . . . . . . . . . . . 183.1.2 Average Power and Peak Power . . . . . . . . . . . . . . . . 18

3.2 Circuit Components . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.2.1 Pulse Generation . . . . . . . . . . . . . . . . . . . . . . . . 193.2.2 Amplification and Narrowing . . . . . . . . . . . . . . . . . 22

3.3 MESFET Amplification and Narrowing of SRD Impulses . . . . . . 243.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Implementation 274.1 LTspice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2 Laser Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2.1 Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2.2 Input Current . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.3 SRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.3.1 Junction Capacitance, Cj0 . . . . . . . . . . . . . . . . . . . 31

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x Contents

4.3.2 Package Capacitance, Cp . . . . . . . . . . . . . . . . . . . . 324.3.3 Package Inductance, Lp . . . . . . . . . . . . . . . . . . . . . 33

4.4 MESFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.4.1 Transconductance . . . . . . . . . . . . . . . . . . . . . . . . 354.4.2 Threshold Voltage . . . . . . . . . . . . . . . . . . . . . . . . 354.4.3 Drain Current . . . . . . . . . . . . . . . . . . . . . . . . . . 36

5 Discussion of Implementation 395.1 Laser Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395.2 SRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.3 MESFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.4.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6 Conclusions 45

A Simulation graphs 49

Bibliography 65

Contents xi

List of figures

Figure 1.1 Overview of the pulsed laser 1Figure 2.1 Overview of a a fluorescence microscope 6Figure 2.2 Singlet and triplet states 8Figure 2.3 Stimulated emission by inserted photon 9Figure 2.4 Overview of a waveform og an ideal pulse 10Figure 2.5 Overview of a waveform of a generated pulse 11Figure 2.6 PN-junction and band gap of laser diode 14Figure 3.1 Overview of the electrical approach 17Figure 3.2 Overview of approach affecting the laser beam 17Figure 3.3 Step recovery diode, impulse output from sinus input 20Figure 3.4 Equivalent circuit of a step recovery diode 21Figure 3.5 Equivalent circuit of a step recovery diode with a diode 21Figure 3.6 Overview of an GaAs nMESFET transistor 22Figure 3.7 The MESFET schematic symbol 23Figure 3.8 Effect of MESFET 23Figure 3.9 The pulse path through an SRD and a MESFET 25Figure 4.1 Simulation circuit of diode 28Figure 4.2 Simulation circuit of SRD 31Figure 4.3 Simulation circuit of MESFET 35

xii Contents

List of tablesTable 4.1 Table over diode output voltage dependence on input voltage 29Table 4.2 Table over diode output voltage dependence on input current 30Table 4.3 Table over SRD output voltage dependence on junction capacitance 31Table 4.4 Table over SRD pulse width dependence on junction capacitance 32Table 4.5 Table over SRD output voltage dependence on package capacitance 32Table 4.6 Table over SRD pulse width dependence on package capacitance 33Table 4.7 Table over SRD output voltage dependence on package inductance 34Table 4.8 Table over SRD pulse width dependence on package inductance 34Table 4.9 Table over MESFET input pulse properties 35Table 4.10 Table over MESFET output voltage dependence on transconductance 36Table 4.11 Table over MESFET output voltage dependence on threshold voltage 36Table 4.12 Table over MESFET output voltage dependence on input current 37Table 4.13 Table over MESFET pulse width dependence on input current 37Table 5.1 Table over preferred properties of inputs and component parameters 42

1Introduction

This thesis focuses on the topic of using a pulsed laser diode as light source of afluorescence microscope. Mainly regarding which pulses are required, and howthey can be generated. Pulsed laser diodes are commonly used in research andclinical diagnosis in the medical area, e.g. in fluorescence microscopy, whichrequire large power laser emissions. The alternative, a constant high power input,can cause damage to the laser and the specimen that is being analyzed. Instead, ifthe laser is only active during a fraction of the time the fluorescence microscopeperformance can be enhanced. This can be done by pulsing the laser with veryshort pulses meanwhile using a slow repetition rate, relative to the pulse width.An overview of the whole system is presented in Fig. 1.1, where the block called’Pulse generation’, and the ’laser beam’ are investigated in this thesis. The outputsignal from the pulse generation block pulses the laser, which in turn produceslight that is sent to the fluorescence microscope. These pulses are not trivial tocreate, but the technique is desired for practical applications. The purpose of thismaster thesis is to investigate which pulses are required and techniques that canbe used to produce these pulses.

Figure 1.1: Overview of a pulsed laser.

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2 1 Introduction

1.1 Background

Ever since the laser was first invented in 1960 the usage has increased more andmore every decade.

At present, lasers are often used in analytical applications, such as for re-search and for clinical diagnosis in the medical area. Applications also exist formaterials science, particle analysis and for quality control in the semiconductorand pharmaceutical industry.

In the medical field, a relatively new application for the laser is in fluorescencemicroscopes. The sample being analyzed in the microscope, absorbs energy caus-ing its atoms to become excited. When the electron drops back to the groundstate it emits a photon and the atom becomes fluorescing. The emitted light is de-tected and a viewable picture is produced, AB [2014]. By using a laser as the lightsource the precision is improved, De and Goswami [2016], which is the reason forusing the laser.

The main reasons for using a pulsed laser source is that the laser diode can beoverdriven without damage and that the fluorescence microscope performanceis enhanced by using a pulsed source. Due to the importance of this, this thesisfocuses on an electrical approach to pulse the laser diode.

Generating subnanosecond pulses is a new field, and generating pulses withlarge amplitude is even more in its starting tracks, Huiskamp et al. [2015], Chen-gou et al. [2015]. For the thesis, the first problem is to investigate what propertiesof the laser pulses are needed regarding pulse width, pulse period and waveform.The second problem is to generate electrical pulses, to create these laser pulses,using electrical components. Because of the complexity of similar problems, andlack of previous reports, this master thesis aims to investigate and resolve thisissue.

1.2 Previous Work

In the article by Aswani et al. [2012], several light sources for the fluorescence mi-croscope are investigated. The comparison is between the long-lived high energyfocused LEDs and the relatively short-lived broad spectrum arc lamps. The arti-cle’s conclusion is that the LEDs are a good investment if the microscope is heav-ily used. The article by De and Goswami [2016] uses a pulsed laser light sourceto enhance the performance of the fluorescence microscope, decreasing photo-damage. Regarding pulse generation the article by Miao and Nguyen [2003], usesa SRD and MESFET to create an impulse with 2.3 V amplitude and a half-powerpulse width of 115 ps.

1.3 Method

This thesis was written at one of Syntronic’s offices. It is based on literature onfluorescence microscopy, pulse generation, amplification and basic electronics.Simulations of a laser diode and chosen circuit components have been carried

1.4 Restrictions 3

out in respect to parameters with impact of the devices. The simulations wasperformed in LTspice using SPICE models and equivalent circuits.

1.4 Restrictions

Circuit implementation is a very broad area with a lot of different aspects toconsider. There are several simulation programs and analysis techniques whichcan be used. Since this master thesis aims to investigate what pulses are neededfor fluorescence microscopes and to find a technique to generate these pulses,the main focus is on finding that information and technique and performing aqualitative simulation. The thesis work and writing was done at Syntronic andthe external supervisors helped with setting the limitations.

The pulse generation will only be implemented using models in this thesisand evaluated using a simulation tool.

1.5 Outline

Following this chapter:

• Chapter 2 presents the investigation of the desired laser waveform, includ-ing discussion.

• Chapter 3 presents the investigation of pulse generation, including discus-sion.

• Chapter 4 describes the implementation that has been made. This part in-cludes the results that have been obtained.

• Chapter 5 discusses the results of this work as well as future improvementand development.

• Chapter 6 contains a conclusion of the thesis in whole.

2Part I - Investigation of the Laser

Waveform

This chapter gives an overview of the fluorescence microscopy and physicalquantities of the laser waveform. The purpose of this part of the thesis isto investigate what properties of the laser pulses is required for the fluores-cence microscopy application.

2.1 Fluorescence Microscopy

As mentioned in the previous chapter, the laser technique is becoming morecommon in the medical area of the fluorescence microscopes, which by be-ing awarded with the Nobel price in Chemistry 2014 proves to be a tech-nique with considerable potential.

The basis of the fluorescence technique is to excite atoms in a specimenand analysing the emitted light. The specimen can, for instance, be a cellmembrane and the technique is often used in medical research. Spring andDavidson [1999] An overview of a fluorescence microscopy is given in Fig.2.1. Fluorophores are added to a specimen binding to certain moleculesof interest, with a corresponding wavelength desired as light input. Thelight goes into the microscope through a filter, which makes sure that onlythe desired frequencies enter the microscope, effectively keeping only thedesired energy levels. The incoming light is reflected down on the specimenthat is being analysed using a beam splitting mirror. Photons from thelight source excites electrons in the specimen which become excited andjump up to a higher energy state. The electrons cool down and jump backto their ground state. While returning to the ground state the atoms emitcharacteristic light, by sending out photons with a characteristic amount of

5

6 2 Part I - Investigation of the Laser Waveform

energy, related to the material in the specimen. The emitted light is emittedspherically and a part of the light travels up through the beam splittingmirror. This mirror is designed in such a way that the wanted frequenciesis not reflected. The light then passes through another filter, making sureno undesired frequencies reach the final step. The light continues up tothe observer or detector creating a picture of the specimen, where it can beanalysed. Spring and Davidson [1999]

Figure 2.1: Overview of a fluorescence microscope.

2.1.1 Light Sources

There are multiple light sources applicable to the fluorescence microscope.For a broad spectrum there are several white light sources available, i.earc lamps. These are high-powered light sources generating intense bandsfor fluorescence excitations covering the entire UV-visible light spectrum.These lamps have a spectral peak where the lamp produces a lot of power.This is also a disadvantage for these lamps due to the fact that they havea non-uniform illuminating across the microscope field of view. Anotherdisadvantage is that the bulb has to be replaced since the lamp intensitydecreases over time.

Another light source usable in the fluorescence microscopy is the light emit-

2.1 Fluorescence Microscopy 7

ting diode, LED. The LED is positioned to take over the market as the lightsource of choice due to several reasons. LEDs are much smaller than thealternatives and can be built into microscope stands. The LEDs generate adiscrete excitation peak at a certain wavelength, making the LEDs limitedat a narrow spectrum of light emission. The LEDs can be switched on andoff in an instant, and have a much longer lifetime than the alternatives. Anumber of LEDs can be used in combination to mimic a white light source,to broaden the LEDs spectrum. Aswani et al. [2012] The fluorescence mi-croscopy lit by a LED uses fluorophores matched for certain wavelengths.Bosse et al. [2015]

Laser diodes combine the benefits of the arc lamps and the LED. They pro-vide intense light, as for arc lamps, but focus the light on a specific point,instead of across the whole field, making the intensity less damaging to aliving cell. The lasers are long-lasting, just as the LEDs, providing lightat specific wavelengths, decreasing the need for filters. Adding these prop-erties together probably makes the laser diodes the best light source forfluorescence microscopes. Lasers can be used in, so called, superresolutionmicroscopes with nanoscale resolution, able to show individual moleculesin unprecedented detail. Although, a disadvantage of the laser is the higherprice. This makes the laser diode still used primarily when higher resolu-tion is needed. Bushwick [2012]

Pulsed Light Source

The traditional light source in a fluorescence microscope is a continuouswave. The continuous wave can lead to excited state absorption causingphoto-damage. Instead of this continuous wave a pulsed light source can beused, achieving significant fluorescence enhancement, due to reduction ofthe photo-damage. There are two main reasons for the pulsed light sourceto be advantageous over the continuous wave. The atoms in the specimenare ensured to go back to ground level before the next light pulse goes into the microscope using a pulse width less than the lifetime of the excited-state. De and Goswami [2016] An ordinary excitation excites an electronfrom the ground state, S0, to a singlet state, e.g. S1, and back to the groundstate again. The excited state molecule may convert into a state where theelectron has changed it spin, called triplet state, T1. The triplet state hasgotten its name due to the fact that it corresponds to three states of equalenergy. Valeur [2013] The excitation and de-exitation of electrons passingthrough a triplet state can be seen in Fig. 2.2. According to De and Goswami[2016], the pulse period shall also be longer than the excite-state tripletlifetime due to the fact that the slowest de-excitation pathway is the tripletstate relaxation.

The fluorescence lifetime is the time for which the electron in the fluo-rophore is excited. In the study, Berezin and Achilefu [2010], and book, byValeur [2013], the fluorescence lifetime goes from around hundred picosec-


Figure 2.2: Singlet and triplet states

onds to a few tens of nanoseconds. According to the study De and Goswami[2016], a shorter pulse width than the fluorescence lifetime is desired, giv-ing that a pulse around 100 ps could be used for most fluorophores. Alsoaccording to De and Goswami [2016], the period of the pulses has to beadapted to the triplet lifetime, giving that a period of around 50 ns shouldalso be appropriate. These requirements are applicable to a wide range ofpossible fluorophores that might be used.

2.2 Laser

Laser is an acronym for: Light Amplification by Stimulated Emission Radia-tion. It was first discovered by Theodore Maiman in 1960, and has sincebecome a crucial device in many applications, both in our every day life,such as in computers, and in highly advanced technical devices used in in-dustries, medical treatment and research. Diels and Arissian [2011]

2.2 Laser 9

2.2.1 Light Emission

The gain medium of the laser consists of atoms well prepared to be excited.The excitation is performed by a pump mechanism. The efficiency is of-ten determined by this mechanism, which is a very important part of thelaser. The excited atoms can emit photons either spontaneously, which isdone in any direction, or stimulated in a predefined direction. The stimu-lated emission, see 2.3, is initiated by the insertion of a few photons whichcontribute to other electrons jumping back to ground level, emitting addi-tional photons of energy Eg , corresponding to the band gap. In an instancethere are double the amount of photons able to stimulate other electronssimilarly. All the photons in stimulated emission follow the first photon’sdirection, creating a sharp and strong light beam, characteristic for lasers.Therefore, the stimulated emission is crucial for the laser to work properly.Diels and Arissian [2011] The time for which this event takes place is calledthe propagation time for the laser, being the time for which the laser turnson. The propagation time creates the lower bound for pulsing of the laser,but is much lower than the pulse around 100 ps, appropriate for the fluo-rophores.

The band gap energy corresponds to a certain wavelength, due to Eg = h · cf ,

where h is Planck’s constant c is the speed of light and f is the wavelengthof the light. A smaller wavelength corresponds to a larger band gap andenergy. Diels and Arissian [2011]

Figure 2.3: Stimulated emission by inserted photon.

2.2.2 Required Pulse Waveform

Laser pulses suited for the fluorescence microscopy technique should befocused, having a well-defined wavelength, to provide light adjusted for a


specific fluorophore. A laser with a specific wavelength removes much ofthe need for filtering, required for wide-spectrum light sources. It also re-duces photobleaching, compared to wide-spectrum light. The laser pulsesshould also be of high intensity, to stimulate as much emission as possiblein the analysed specimen, providing a more intense image of the specimen.The pulse power should be kept stable to provide an even input to the mi-croscope. To make imaging of live cells possible the light source should beable to quickly turn on and off, due to the fact that continous high intensitylight may cause damage, such as phototoxicity and cell death. The light hasto be used efficiently, focusing the laser pulses only on the parts of the spec-imen being analysed, and only during a required time frame. The pulsewidths and repetition rate has to be suited for the fluorophores used for theanalysis. Bushwick [2012], Aswani et al. [2012], Bosse et al. [2015].

An ideal pulse has zero rise and fall time, and the output is constant, as canbe seen in Fig. 2.4. The desired pulse have several similarities to an idealpulse. According to Markettech [2015], aspects such as rise time, fall time,overshoot, undershoot, ringing and reflections can not be neglected for agenerated pulse waveform. Some of these aspects can be seen in Fig. 2.5.

Figure 2.4: Overview of a waveform of an ideal pulse.

To obtain a pulse suited for use in the fluorescence microscopy application,several aspects of the generated pulse has to be regarded. According toMaxim [2000], there are several different distortions caused by the setup ofthe laser that can be solved easily. The impact of the different distortionsare taken under consideration, while regarding the importance to resolvethem.

2.2 Laser 11

Figure 2.5: Overview of a waveform of a generated pulse.

Overshoot

Overshoot of a pulse is when the pulse exceeds the intended amplitude ofthe pulse, e.g. 100 %, as can be seen in Fig. 2.5. Overshoot can be caused byboth the rising edge and a low bias current. If the rising edge of the pulse istoo fast, the rising edge may overshoot the digital one level. This can be re-solved by using a low-pass filter with a frequency cutoff at 75 % of the datarate. This decreases the overshoot by slowing down the rising and fallingedge. If the digital zero level is below the threshold of the laser is may causea delayed rising edge. The delayed edge results in a build-up in potentialcausing the laser to overshoot whilst the threshold is reached. This can beresolved by increasing the bias current above the threshold value. Maxim[2000]

Undershoot

Undershoot of a pulse is when the pulse goes below the intended amplitude,e.g. 100 %, as can be seen in Fig. 2.5. Undershoot can be caused by the risingand/or falling edge not reaching their high or low level within the first halfor the unit interval. This may be resolved by decreasing the damping of theoutput circuit. Maxim [2000]

Ringing

Ringing of a pulse is when the pulse signal oscillates, as can be seen inFig. 2.5. Ringing on rising and/or falling edges may be caused by impedance


discontinuities, large inductances in the circuit or resonance effects of cir-cuit components. For resolving this issue these has to be decreased or re-moved. Maxim [2000]

Reflections

Reflections can appear as overshoot, undershoot, ringing or other distor-tions and are caused by transmission-line impedance discontinuities. For re-solving this issue the discontinuities has to be decreased or removed. Maxim[2000]

Discussion Regarding Distortions

As stated, the desired laser pulse should have a stable output power, fastswitching between on and off, and high intensity for a narrow wavelength-spectrum. These properties has to be kept in order for the fluorescencemicroscopy to work as intended.

Overshooting of the laser pulse creates a higher intensity output for a cer-tain part of the laser pulse. This may result in an output pulse containingtoo much energy. This issue needs to be taken care of so that the laser pulsedoes not damage the specimen being analysed.

Undershooting of the laser pulse creates a lower pulse than desired. Thismay result in that the output pulse does not contain the required amountof energy to excite the specimen in the fluorescence microscope. This issuehas to be taken care of in order for the fluorescence microscope to be ableto create an image of the specimen being analysed.

Ringing can, as for overshoot and undershoot, create differences in poweroutput. Ringing can also cause the output to act like several pulses, if theringing is large. This may cause additional laser pulses to go in to the flu-orescence microscope, causing undesired excitation. Ringing in the formof overshoot and undershoot has the same effect on the fluorescence mi-croscopy analysis as mentioned above, and needs to be reduced accordingly.As for the ringing resulting in several pulses being created, the specimen be-ing analysed in the fluorescence microscope might get excited from severalphotons. This could create additional sources of emission, making the im-age less sharp.

Reflections cause the same problems as the respective distortion it appearsas, and needs to be solved for the same reasons.

An ideal pulse should be the most fit pulse to send in to the fluorescence mi-croscope, but small distortions of the laser pulse should not cause problemsfor the fluorescence microscope. As stated previously, the most commonlight source for the fluorescence microscope has been the arc lamp. Thearc lamp, which decreases in intensity over time and has a continous flow

2.2 Laser 13

of light. The fluorescence microscope is proven to work under these condi-tions, at least with respect to undershoot and ringing. But, this is also thereason why the laser has been proven to be a supreme solution for the flu-orescence microscope, due to the longer endurance of the light source andsharpness of the image. Based on that the lasers are much more expensivethan the alternatives, these problems should indeed be solved so that thefluorescence microscope can work as good as possible, creating really sharpimages of the analysed specimens.

2.2.3 Laser Diode

The laser diode is a semiconductor device consisting of a crystal that hasbeen doped differently on either side, see Fig. 2.6. The PN-junction laser isrelegated to history but serves well to illustrate some of the main principles,according to Sands [2004]. The PN-junction laser is n-doped with donorson one side, which produce an abundance of electrons, and p-doped with ac-ceptors on the other side, with an abundance of holes. This gives the crystaldifferent physical properties on the different sides. The currect flows easilythrough the semiconductor from the p- to the n-side while there is greatresistance if positive voltage is applied to the n-side while negative voltageis applied to the p-side. This is why it is called a ’diode’. In the semiconduc-tor the conduction band, containing the exited electrons, and the valenceband, containing the corresponding holes, are separated by a band gap, be-tween which the electrons can jump during excitation/emission. Diels andArissian [2001]

The voltage drop, V , over the laser diode is, according to Vanzi [2008], ap-proximately equal to the band gap energy, Eg . The relationship betweenthe voltage drop, measured voltage, VE , current, I , and series resistance,RS , can be seen in Eq. (2.1).

VE = V + RS · I (2.1)

When high current density is going through the laser diode the temperaturerises. The temperature is an important factor for the performance of thelaser diode and the laser diode needs to be cooled down or alternativelyoperated under pulsing conditions. According to Sands [2004], pulsed PN-junction laser diodes have, at room temperature, been made operating aswell as constantly turned on diodes.

According to Andrews et al. [2013], the total forward current, IF goingthrough the PN-junction diode can be calculated as in Eq. (2.2). IS is thesaturation current, VF is the forward voltage and, VT is the thermal volt-age. At room temperature the thermal voltage, VT , is approximately 0.026V, as can be seen in Eq. (2.3), where k is Boltzmann’s constant, T is the tem-perature in Kelvin and q is the elementary charge. Together these give therelationship given in Eq. (2.4).


Figure 2.6: PN-junction and band gap of a laser diode.

IF = IS · (eVFVT − 1) (2.2)

VT ≡k · T| q |

=1.380 · 10−23 · 300| 1.602 · 10−19 |

≈ 0.026 (2.3)

IF = IS · (eVF

0.026 − 1) (2.4)

Desired Laser Pulse and Laser Diode

According to Hodgson et al. [2004] the pulsed laser diode receives lowerthreshold current and a slight increase in slope efficiency mainly due tothe decrease in junction temperature. As noted earlier the temperature iscrucial for the laser diode and according to Hodgson et al. [2004], pulsingat a duty cycle of less than one percent, the heating effects are insignificant.The stated pulse width of 100 ps and pulse period of 50 ns has a duty cycleof 0.2 %, implying that the heat is not an issue.

The pulse stated above is applicable for the fluorescence microscope ap-plication based on properties within the laser diode. This gives that therequired pulse is 100 ps wide, with a period of 50 ns.

2.3 Summary 15

In the study, Lopez et al. [2000], the fluorescent dye SYPRO Ruby Pro-tein Gel Stain is used for protein analysis. The SYPRO Ruby Protein GelStain from Sigma, product number S 4942, has optimal excitation at 302and 470 nm. This is a well-suited fluorescent for the analysis made in thisthesis. The laser diode determines the requirements on the pulse shape interms of current and voltage. The fluorescent dye SYPRO Ruby Protein GelStain desires an incoming light source with wavelength 302 or 470 nm. Alaser diode able to excite an 470 nm output is the SMLS14BET from ROHMSemiconductor. The forward voltage of the laser diode is typically 3.2 V witha maximum value of 5 V. The forward current is typically 20 mA with max-imum value of 30 mA. The maximum power dissipation is 117 mW. Thislaser diode is suited for the fluorescence microscope application based onthe application’s requirements on the output of the laser diode.

2.3 Summary

A fluorescence microscope analyses specimens using a high energy lightsource producing light corresponding to an added fluorophore. The per-formance of the fluorescence microscope is enhanced using a pulsed laserdiode as light source. The waveform of the laser pulse should have fast riseand fall times, a stable power output and a narrow wavelength-spectrum.Distortions, such as overshoot, undershoot, ringing and reflection, shouldbe decreased or removed for the fluorescence microscope to work properly.The laser pulse should have a pulse width less than 100 ps and a period ofmore than 50 ns. The laser diode’s wavelength should correspond to a cer-tain fluorophore. The laser diode SMLS14BET is a suitable light source forthe fluorescence microscopy, corresponding to the fluorescent dye SYPRORuby Protein Gel Stain used in protein analysis.

3Part II - Investigation of Pulse

Generation

This chapter gives an overview of the pulsing of lasers. The purpose of thispart of the thesis is to investigate how the desired laser pulse stated in theprevious chapter can be generated.

3.1 Pulsing Method Choice

Pulsing the laser light has two fundamentally different approaches. Thepulsing can either be electrical, turning the laser on and off by targeting thelaser input chain, or by effecting the laser beam itself. The two alternativesare illustrated in Fig. 3.1 and Fig. 3.2, respectively.

Figure 3.1: Overview of the electrical approach.

Figure 3.2: Overview of the approach affecting the laser beam.

For the electrical approach, a circuit that produces pulses is needed. The

17

18 3 Part II - Investigation of Pulse Generation

propagation time of the laser is the time it takes for the laser to turn on. Therise time of the output can not go below the propagation time of the laser,which sets the maximum frequency possible of the pulsing. The ultimatechallenge for this alternative is to reach, but not exceed, the speed of thepropagation time. The propagation time is well below the rise time inves-tigated in this thesis, so this will not be a restriction. Furthermore, higherenergy pulses can be sent through the laser without damaging it due to thefact that the laser is off periodically, reducing the temperature of the laser.Generating the pulse can be done by using a simple pulse generator, whichcan be enough for many applications. An oscillator can, e.g, generate a si-nusoidal wave with 10 V amplitude by itself. The generated pulses mightneed amplification and/or narrowing to reach given requirements, this canbe done by some sort of amplifying circuit. Kilpela [2004], Chengou et al.[2015], Huiskamp et al. [2015] and Zhou et al. [2015].

The second approach is based on adding a device located after the laserbeam, for which the rise time of the outgoing signal is totally dependenton the implementation properties, which can vary between the techniques,and get very short. There are several options available for affecting the laserbeam, such as fiber, mechanical, using mirrors etc.

For the implementations in the second approach the laser is constantlyturned on, which is uneconomical and decreases the lifetime of the laser.The electrical approach can pulse the laser with high energy without dam-aging the laser. Therefore, it should be able to solve the problem by reduc-ing the pulse width, meanwhile being the more energy efficient alternative.Arrigoni et al. [2012], Lin and Lin [2014] and Shaozhen et al. [2012].

3.1.1 Desired Electrical Pulse

The laser pulse stated in the previous part can be achieved by electricallypulsing the laser diode. As stated previously the laser diode is chosen be-cause of its narrow wavelength and stable power output, which is not signif-icantly affected by the input signal. The rise and fall times of the electricalpulses could be considered equal to the laser pulse, due to the laser diode’squick response to the input signal. This also makes the requirement on thepulse width and pulse period equal to the desired laser pulse. As a conse-quence, the desired electrical pulse has a pulse width of 100 ps and a pulseperiod of 50 ns, and the rise and fall times should be short.

3.1.2 Average Power and Peak Power

In order to keep a laser undamaged the average power has to be kept low.The average power of the pulses can be calculated using the period of thepulses, the width of the pulses and the peak power of the pulses. For easiercalculations the pulse is assumed to be an ideal square wave, with straight

3.2 Circuit Components 19

edges. The period of a signal, T , is the duration of time of one cycle in arepeating sequence. This is, for a pulsing signal, the time between e.g. therising edges of two consecutive pulses. The pulse width, Twidth, is the timeduration of the pulse, from rise to fall. The peak power, Ppeak, is the highestpower generated at some point throughout the signal. The peak power ofan ideal pulse signal can be seen as the amount of energy generated duringa single pulse, divided by the pulse width. This is due to the fact that thepower between the pulses is ideally zero. These relationships can be seenin Eq. (3.1).

The average power, Paverage, is the amount of energy generated on a sus-tained basis, over a longer period of time, regardless of pulse duration. Av-erage power is equal to the total energy measured, Etotal, divided by thetotal time in seconds during which the measured amount of energy was ac-cumulated, Ttotal. The average power is, by definition, always equal or lessthan the peak power of a signal. In an ideal case the total energy during aperiod is equal to the energy generated during one pulse, Epulse. The aver-age power from the signal is, approximately, the energy generated duringa single pulse. This is the peak power multiplied with the pulse duration,divided by the period time. These relationships can be seen in Eq. (3.2). Alaser has a limit for the average power going through it, and suffers dam-age above that limit. The peak power needs to be ’high’ so that the definedperformance is maintained. Therefore, the average power can be kept loweither by keeping the pulse width narrow, the period long, or a combinationof the two bringing the average power down to a safe level. Newport [2016]

Epulse = Ppeak · Twidth (3.1)

Paverage =Etotal

Ttotal=Eperiod

T=Epulse

T=Ppeak · Twidth

T(3.2)

3.2 Circuit Components

A circuit that, from a DC voltage input, produces a pulse train can be im-plemented using a few circuit components. From the DC voltage a pulseis generated, amplified to the right level and narrowed to the right widthusing the circuit parts presented in Sec. 3.1. The circuit parts, will be ex-plained in the following sections.

3.2.1 Pulse Generation

Many pulse generators are oscillators, which produce periodical output sig-nals from a DC input. The output signal can be a square wave, sine wave,triangular wave etc. Kazimierczuk [2015]. Although, a very commonly


used device for pulse generation is the step recovery diode, SRD. Miao andNguyen [2003]

Step Recovery Diode

The SRD can be used as an pulse generator by converting a sinusoidal in-put signal into narrow pulses, down to <150 ps. The repetition rate of thepulses is the same as for the sinusoidal signal, due to the fact that the SRDoutputs one pulse per period. An illustration of this is presented in Fig. 3.3.

Figure 3.3: Step recovery diode, impulse output from sinus input.

It is an advantage for the SRD to output one impulse per period of thesinewave, due to the fact that the frequency of the pulses is easily controlled.An SRD is a two-terminal PN-junction with similar DC characteristics as ausual PN-junction diode, but with different dynamic characteristics, usedin switching. Hewlett-Packard [1968]. During positive bias, minority car-riers are inserted on both sides of the PN-junction. The SRD allows theminority carriers to concentrate to a narrow region near the junctions. Theminority carriers have a long lifetime, which means in that they cannot re-combine during positive bias. When positive bias is turned into a negativebias these minority carriers flow in the opposite direction from the junction,which create a strong backward current. When all the minority carriers areextracted the current is quickly reduced to a low level, cutting the diode off,and thereby creating a step voltage. Zhou et al. [2016]

A real SRD experiences impact from package parasitics which may resultin ringing, overshoot and ramping. An equivalent circuit of an SRD canbe seen in Fig. 3.4. The SRD can be seen as a diode, a capacitance and aninductance, seen in Fig. 3.5.

The average value of the output voltage has to be equal to zero during acycle, which restricts the pulse height and the output during cut-off. Theimpulse width, W , is dependent on the SRDs , Cj0 and inductance, Lp, ac-cording to Eq. (3.3), Hewlett-Packard [1968]

W = π√LpCj0 (3.3)


Figure 3.4: Equivalent circuit of a step recovery diode.

Figure 3.5: Equivalent circuit of a step recovery diode as a diode with pack-age parasitics.

To generate a narrow pulse width with high amplitude and low ringing anSRD with small junction capacitance should be used. Zhou et al. [2016]

According to Hewlett-Packard [1968], an SRD can be used to generate animpulse at below 150 ps, which is close to the required pulse stated in theprevious chapter, but not close enough. To further reduce the pulse widthand to increase the amplitude of the impulse generated by the SRD a nar-rowing and amplifying circuit is needed, to simultaneously reach below therequired 100 ps and to keep the voltage relatively high. The fact that theSRD can make an impulse from a sinusoidal signal with the same repeti-tion frequency is a clear advantage for the SRD to be a good candidate as apulse generator, especially due to the fact that a sinusoidal signal is easilygenerated by an oscillator.

The SRD model used for implementation is taken from Hewlett-Packard[1968] and the specific SRD was chosen for its low junction capacitance inaccordance to results in Zhou et al. [2016].


3.2.2 Amplification and Narrowing

The main purpose of an amplifier circuit is to increase the power level ofan incoming signal. Kazimierczuk [2015] Sometimes they can also be usedas narrowing circuit components.

Metal-semiconductor Field Effect Transistor

A metal-semiconductor field effect transistor, MESFET, is a semiconductordevice which can be used to narrow and amplify pulses. The MESFET con-sists of a conducting channel between a source and drain contact region. ASchottky barrier diode is used to isolate the device’s metal gate from thechannel and is used to control the carrier flow in the channel. This is doneby varying the depletion layer width, modulating the thickness of the con-ducting channel. Zeghbroeck [2004] An overview of a Gallium Arsenide,GaAs, nMESFET, n-doped and constructed with a GaAs-semiconductor ma-terial, can be seen in Fig. 3.6, and the schematic symbol can be seen inFig. 3.7, where S represents the source, D the drain and G the gate. Themetal gate-control electrode is connected to the channel, Hudson [2014].This can be compared with the more common MOSFET, abbreviation formetal-oxide semiconductor field effect transistor, which uses an oxide layerto separate the gate and the channel.

Figure 3.6: Overview of an GaAs nMESFET transistor.

Compared to a MOSFET the MESFET has one key advantage, which is itshigher carrier mobility. The higher carrier mobility leads to higher current


Figure 3.7: The MESFET schematic symbol.

and transconductance of the device. The transconductance of the devicecan be calculated according to Eq. (3.4). A disadvantage of the MESFETis that the Schottky metal gate limits the forward bias voltage, so that thethreshold voltage must be lower than the turn-on voltage off the Schottkydiode. Although this is easily tolerated, and the advantages exceed the dis-advantages. For the material used GaAs is advantageous over silicon, dueto its 5 times higher carrier mobility, with a twice as high peak electronvelocity. Zeghbroeck [2004]

gm =dID,sat

dVGS(3.4)

Figure 3.8: Effect of MESFET.

By varying the DC voltage at the gate of the transistor an impulse canbe voltage amplified and compressed, in order to significantly reduce the


broader pulse foot and the strongly nonlinear variation of the transconduc-tance. An overview of the effect of the MESFET is given in Fig. 3.8.

3.3 MESFET Amplification and Narrowing of SRDImpulses

In the article, Lee and Nguyen [2001], an impulse is generated by an SRDand shaped by a MESFET network, including a Schottky diode, a capaci-tor and resistors, at a frequency of 10 MHz. The article claims that thefrequency on such a circuit is limited by the performance of the SRD, be-ing able to manage several hundred MHz. The output pulse from the SRDreaches a width of 300 ps with an amplitude of 2 V, peak-to-peak. The fre-quency limitation by the SRD, which the article suggests, does not seemto place any limitation for our construction, where a frequency of only20 MHz is required.

According to Miao and Nguyen [2003] a circuit containing an SRD and twoGaAs MESFET transistors can create a narrow impulse of 115 ps with anamplitude of 2.3 V. An overview of the pulse path can be seen in Fig. 3.9.The SRD is used as pulse generator generating a 140 ps impulse with anamplitude of 1.1 V. To amplify this pulse a MESFET is used, inverting thesignal as well as amplifying it. The pulse then reaches 2 V at 130 ps, so theMESFET is also narrowing the pulse slightly. The second MESFET is usedprimary for narrowing of the pulse, but is also amplifying the pulse slightly.After this stage the impulse of 115 ps with an amplitude of 2.3 V is reached.

The pulse path going through the SRD and MESFETs can be seen in Fig. 3.9.

The SRD and MESFET devices should be implemented for analysis, to seeif they are fit for providing the desired electrical pulse.

3.4 Summary

The desired pulses can be generated by electrically pulsing the laser usinga circuit implementation. To achieve high energy emission the laser canbe overdriven for a certain time, as long as the average power is low. Theneeded pulses are produced using a pulse generator circuit and an amplifi-cation circuit, which together need to fulfill functionality requirements onthe desired pulse. An SRD could be used to generate a narrow pulse anda MESFET circuit could be used to amplify it. Both the requirements fromthe fluorescence microscopy and the laser diode itself have to be consideredwhen creating a circuit implementation.

3.4 Summary 25

Figure 3.9: The pulse path through an SRD and a MESFET

4Implementation

This chapter includes the implementation of the laser diode and circuit com-ponents chosen to generate the desired pulse from Part I. The implementa-tion in this thesis consisted of several simulations of the chosen componentsregarding input parameters and device properties detected as important inChapter 3, analyzed by simulation over a wide range of values.

Something worth noticing is that the pulse width is the time from 50 %power level of the rising edge to 50 % power level of the following fallingedge. Rise and fall time can have great impact on the pulse width. Theefficiency described in this chapter is the measure of output voltage in rela-tions to the input voltage. The efficiency is given in percent.

Figures of simulations results and graphs can be found in the Appendix.

4.1 LTspice

The simulations are performed in the circuit simulation program LTspice.LTspice is a freeware computer software implementing a SPICE simula-tor. SPICE, Simulation Program with Integrated Circuit Emphasis, is usedbroadly to simulate electrical circuits. Some components have been mod-eled by SPICE models, and some are modeled by an equivalent circuit, asin the case with the SRD. LTspice is not a perfect program and it has itslimitations. Parameters not stated in the models are set to default values byLTspice. Due to the fact that the analyses are only considering the depen-dencies on parameters the models does not have to be exact. Circuit para-sitics are not taken into consideration in simulations, due to the fact thatthe master thesis is only an initial study of the topic. The simulations that

27

28 4 Implementation

are run are transient analysis simulations. These simulations are run over arange of time. The simulated parameters are run over a set of 6-9 differentvalues. These are based on the parameter values of the real devices thatthe simulated models shall correspond to. Values both above and under theactual value is simulated. The values were selected to get a broad spectrumof parameter values. Additional values were simulated, which followed thepresented lines, not needed to be stated in the tables and graphs.

4.2 Laser Diode

The SPICE model of the laser diode is provided from ROHM Semiconductor,given below. These parameter values are used in the laser diode model.

.MODEL SMLS14BET

+ IS=215.94E-21 N=2.9257 RS=13.21 IKF=3.4242E-3 EG=3.5 CJO=41E-12

+ M=.38596 VJ=5.6484 ISR=20.977E-12 NR=10 BV=5 TT=17.4n

The diode implementation was carried out using a circuit schematic shownin Fig. 4.1. The pulse coming in to the laser is assumed to be the desiredpulse stated earlier to have a pulse width of 100ps and a pulse period of50 ns.

Figure 4.1: Simulation circuit of diode.

Properties determining the output of the laser diode is believed to be thecurrent and voltage, and their impact was simulated and analyzed.

To analyze the impact of the input voltage on the chosen diode the currentwas kept constant. To analyze the impact of the input current on the cho-sen diode the voltage was kept constant. This was done by changing thevalue of the connected resistance in series, Rconnected according to Ohm’slaw, which can be seen as R1 in Fig. 4.1. For example, at a voltage of 10 Vand a current of 50 mA, the connected resistance was set to 186.79 Ω, ac-cording to Eq. (4.1) and Eq. (4.2). The efficiency of the laser diode regarding

4.2 Laser Diode 29

Input volt. (V) Output volt. (V) Volt. drop (V) Efficiency [%]1 0.3 0.7 70.03 2.3 0.7 23.35 4.3 0.7 14.07 6.3 0.7 10.010 9.3 0.7 7.015 14.3 0.7 4.630 29.3 0.7 2.3

Table 4.1: Table over diode output voltage dependence on input voltage

voltage was calculated by dividing the voltage drop by the input voltage, tosee how much voltage was used efficiently by the laser diode.

V = R · I = (Rconnected + RS) · I (4.1)

Rconnected =VI− RS =

10 V0.05 A

− 13.21 Ω = 186.79 Ω (4.2)

4.2.1 Input Voltage

During the simulation the current was kept constant at 50 mA by varyingthe connected resistance, to match the voltage. E.g, at the 10 V input theresistance was set to 200 Ω, according to:

R =VI

=10 V

50 mA= 200 Ω (4.3)

Simulation results from a low, 5 V, and a high, 30 V, input voltage can beseen in Fig. A.1 and Fig. A.2.

The interesting result is the voltage drop over the laser diode and the effi-ciency of the laser diode at different voltages, given in percent. This can beseen in Tab. 4.1.

The corresponding graph can be seen in Fig. A.3

4.2.2 Input Current

During the simulation the voltage was kept constant at 10 V by keeping thecircuit’s voltage source constant.

Simulation results from a low, 10 mA, and a high, 0.5 A, input current canbe seen in Fig. A.4 and Fig. A.5.

30 4 Implementation

Input curr. (mA) Output volt. (V) Volt. drop (V) Efficiency [%]10 9.8 0.2 230 9.6 0.4 450 9.3 0.7 7100 8.7 1.3 13150 8.0 2.0 20250 6.7 3.3 33500 3.4 6.6 66

Table 4.2: Table over diode output voltage dependence on input current.

The interesting result is the voltage drop over the laser diode and the effi-ciency of the laser diode at different voltages, given in percent. This can beseen in Tab. 4.2.

The corresponding graph can be seen in Fig. A.6

4.3 SRD

Due to the fact that the SRD has the ability to generate a narrow pulse froma sinusoidal input, the SRD is fed with such a signal. The period of thesignal is 50 ns, according to the desired pulse, due to the fact that the SRDis able to generate one pulse per period of the sinusoidal input signal.

The SRD simulation was carried out using a circuit schematic shown inFig. 4.2. The supporting devices in the schematic are taken from the analysemade in the article by Zhou et al. making the circuit able to analyse in a sim-ilar matter, creating a similar environment for the device. The inductancecreates a low-inductance path to ground for low-frequency signals, keepingthe output node drained. When the SRD fires, a very high-frequency pulseis created. This signal sees the inductance as a high-impedance path, caus-ing a sharp rise in the output node voltage. The output resistance dampenthe circuit and the capacitance is used as a DC blocker.

The diode in the equivalent circuit was modeled by the following SPICEmodel:

.MODEL DMOD D

+ IS=2.3e-15 CJO=0.89e-12 VJ=0.4 FC=0.5

+ BV=42.6 M=0.2 RS=0.8 IBV=10e-6

The impact of the junction capacitance, parasitic package capacitance andparasitic package inductance of the SRD was simulated and analyzed. Forall simulations of the SRD the input voltage sinusoidal wave is much largerthan the output signal. The input signal might look a bit strange because it

4.3 SRD 31

Figure 4.2: Simulation circuit of SRD.

Junction cap. (pF) Output voltage (V) Efficiency [%]0.30 3.40 340.89 4.10 412.00 4.60 463.50 4.85 495.00 5.00 5010.00 5.20 52

Table 4.3: Table over SRD output voltage dependence on junction capaci-tance.

is zoomed in, this is done to be able to see the output better. The efficiencyof the SRD regarding voltage was calculated by dividing the output voltagewith the input voltage.

4.3.1 Junction Capacitance, Cj0

The junction capacitance was simulated to see its dependence on outputvoltage and pulse width.

A low, 0.3 pF, and a high, 5 pF, junction capacitance and their respectiveresults can be seen in Fig. A.7 and Fig. A.8.

Output Voltage

The interesting result is, besides the actual output voltage, the efficiencyat different voltages, given in percent. This can be seen in Tab. 4.3. Thecorresponding graph can be seen in Fig. A.9

32 4 Implementation

Junction cap. (pF) Pulse width (ps)0.30 1500.89 3002.00 5003.50 6005.00 70010.00 1000

Table 4.4: Table over SRD output pulse width dependence on junction ca-pacitance.

Package cap. (pF) Voltage output (V) Efficiency [%]0.01 4.00 40.00.10 4.05 40.50.37 4.11 41.10.50 4.12 41.20.70 4.20 42.01.00 4.25 42.5

Table 4.5: Table over SRD output voltage dependence on package capaci-tance.

Pulse Width

The junction capacitance’s impact on the pulse width can be seen in Tab. 4.4.The corresponding graph can be seen in Fig. A.10

4.3.2 Package Capacitance, Cp

The package capacitance was simulated to see its dependence on outputvoltage and pulse width.

A low, 0.1 pF, and a high, 1 pF, package capacitance and their respectiveresults can be seen in Fig. A.11 and Fig. A.12.

Output Voltage

The interesting result is, besides the actual output voltage, the efficiencyat different voltages, given in percent. This can be seen in Tab. 4.5. Thecorresponding graph can be seen in Fig. A.13

4.3 SRD 33

Package cap. (pF) Pulse width (ps)0.10 1400.37 1500.70 1751.00 1901.50 2052.00 215

Table 4.6: Table over SRD pulse width dependence on package capacitance.

Pulse Width

The impact of the the package capacitance on the pulse width can be seenin Tab. 4.6. The corresponding graph can be seen in Fig. A.14.

4.3.3 Package Inductance, Lp

The package inductance was simulated to see its dependence on output volt-age and pulse width.

A low, 0.5 nH , a middle, 3 nH and a high, 10 nH , package inductance andtheir respective results can be seen in Fig. A.15, Fig. A.16 and Fig. A.17.

To be clearer with the difference of these signals, the input signal has beencut from the simulation figures. The input signals for these cases are similaras in previous SRD simulations.

Output Voltage

The interesting result is, besides the actual output voltage, the efficiencyat different voltages, given in percent. This can be seen in Tab. 4.7. Thecorresponding graph can be seen in Fig. A.18.

Pulse Width

The impact of the package inductance on the pulse width can be seen inTab. 4.8. The corresponding graph can be seen in Fig. A.19.

34 4 Implementation

Package ind. (nH) Voltage output (V) Efficiency [%]0.5 3.72 371.5 4.12 413.0 4.29 435.0 3.97 407.0 3.62 3610.0 3.18 32

Table 4.7: Table over SRD output voltage dependence on package induc-tance.

Package ind. (nH) Pulse width (ps)0.5 2951.5 3003.0 3205.0 3657.0 41010.0 460

Table 4.8: Table over SRD pulse width dependence on package inductance.

4.4 MESFET

The MESFET model used for implementation is taken from the Miao andNguyen [2003]. The supporting devices in the schematic are taken from theanalysis in the article making the circuit able to analyse in a similar matter,by creating a similar environment for the device. The capacitance is usedas a DC blocker, the resistance dampens the circuit and the inductance andcapacitance form the DC bias current.

Simulations of the MESFET was carried out using a circuit schematic shownin Fig. 4.3.

The impact of transconductance, threshold voltage and drain current forthe MESFET was simulated and analyzed. The efficiency of the MESFETregarding voltage was calculated by dividing the output voltage with theinput voltage.

The input to the MESFET was a pulse with properties approximated fromthe SRD output according to Miao and Nguyen [2003]. These propertiescan be seen in Tab. 4.9.

4.4 MESFET 35

Figure 4.3: Simulation circuit of MESFET.

Rise and fall time 10 psPulse duration 200 psInitial voltage 0 VPulse voltage -4 V

Table 4.9: Table of MESFET input pulse properties.

4.4.1 Transconductance

The transconductance was simulated to see its dependence on output volt-age.

A low, 5 mS, and a high, 100 mS, transconductance and their respectiveresults can be seen in Fig. A.21 and Fig. A.22.

The interesting result is, besides the actual output voltage, the efficiency atdifferent voltages, given by percent. This can be seen in Tab. 4.10.

The corresponding graph can be seen in Fig. A.23.

4.4.2 Threshold Voltage

The threshold voltage was simulated to see its dependence on output volt-age.

A low negative, -0.5 V, and a high negative, -10 V, threshold voltage andtheir respective results can be seen in Fig. A.24 and Fig. A.25.


36 4 Implementation

Transcond. (mS) Output volt. (V) Efficiency [%]0.1 0 05.0 0.7 1910.0 1.5 3825.0 3.7 9350.0 7.5 18875.0 11.2 280100.0 15.0 375

Table 4.10: Table over MESFET output voltage dependence on transconduc-tance.

Threshold volt. (V) Output volt. (V) Efficiency [%]0 0 0-0.5 0.15 4-1.2 0.78 20-3.0 3.50 88-4.0 5.40 135-5.0 6.90 173-7.0 8.20 205-10.0 9.00 225

Table 4.11: Table over MESFET output voltage dependence on thresholdvoltage.


4.4.3 Drain Current

A low, 10 mA, and a high, 150 mA, drain current and their respective resultscan be seen in Fig. A.27 and Fig. A.28.

Output Voltage



4.4 MESFET 37

Drain current (mA) Output volt. (V) Efficiency [%]5 37.5 93810 19.8 49530 6.9 17350 4.2 105100 2.1 53150 1.4 35

Table 4.12: Table over MESFET output voltage dependence on drain current.

Drain current (mA) Pulse width (ps)5 2110 4030 11050 180100 219150 219

Table 4.13: Table over MESFET pulse width dependence on drain current.

Pulse Width

The impact of the drain current on the pulse width can be seen in Tab. 4.13.


5Discussion of Implementation

This chapter discusses the results from the previous chapter.

5.1 Laser Diode

According to the simulations a higher input voltage does not increase thevoltage drop over the laser diode. This contributes to the fact that the ef-ficiency decreases significantly with higher voltage applied. This could beexpected due to the fact that the voltage drop is dependent on the band gapof the pn-junction, which can be seen as constant. The input voltage shouldbe kept low due to efficiency reasons, and due to the fact that higher voltageconsumes more power.

The simulations show that a higher current gives a higher efficiency of thelaser diode, creating a higher voltage drop over the laser. The requiredpulse of 100 ps with a period of 50 ns has a duty cycle of 0.2 %, implyingthat the laser diode can be overdriven without damage. The simulationsshow an equal result. For example, while applying a 0.5 A current to thelaser diode model, using a 6.79 Ω resistance in series with the 13.21 Ωlaser diode, the laser is overdriven. This seems to only increase the laserdiode efficiency, which reaches 66 %.

A higher input voltage makes it possible to have a higher input current,according to Ohm’s law, due to the fact that the resistance is limited by theseries resistance of the laser diode.

Combining that a higher voltage result in lower efficiency and high energyconsumption with the fact that a higher voltage leads to possibly higherinput current makes it worth using a bit higher input voltage. Looking at

39

40 5 Discussion of Implementation

the laser diode the maximum forward voltage and current during normaloperation are VF=5 V and IF=30 mA. With a 5 V input voltage the laserdiode can be overdriven with a input current of 250 mA using a 6.79 Ωresistance in series with the laser diode. The input voltage is chosen to be5 V, with an input current of 250 mA. During normal operation the outputpower is 1.25 W, which is much higher than the laser diode’s maximumpower of 117 mW.

At an applied input voltage of 5 V, a pulse width of 100 ps and a pulseperiod of 50 ns the average power is calculated using Eq. (3.2). For ourpulsed laser diode the average power is 2.5 mW, according to Eq. (5.1). Thisis far below the maximum value.

Paverage =5 · 0.25 · 100 ps

50 ns= 2.5 mW (5.1)

5.2 SRD

According to the results the SRD produces a narrow negative pulse fromthe sinewave input.

The simulations shows that a higher junction capacitance gives higher ef-ficiency to the SRD. This is not what the study, Miao and Nguyen [2003],says, where a lower junction capacitance gave a higher amplitude of thegenerated pulse. A reason for this may be that the higher junction capaci-tance makes the SRD able to accumulate more charge. Due to the fact that amore charged capacitance generates a higher amplitude of the output pulse.Also, the SRDs used are not the same and the models are not equal, whichmay impact the results. The simulations also show that a lower junctioncapacitance makes the pulse width shorter, as stated in the study, Miao andNguyen [2003]. The junction capacitance can not get infinitely small, soas stated in the theory the SRD might not get much smaller than the given150 ps. The higher junction capacitance produce more oscillations on theoutput pulse, seen in the simulations. These are highly undesired and inaddition to the shorter pulse widths the choice for the SRD is to have lowjunction capacitance.

The package capacitance is barely giving a dependency on the output volt-age. The simulations show a slightly higher efficiency using a higher capac-itance. This might be due to the fact that the SRD accumulates more chargebefore generating the pulse. Using a lower package capacitance generates apulse with shorter pulse width. A reason for this may be that the dischargecan be made quicker due to less accumulated charge. The capacitance doesnot seem to have any noticable affect on the oscillations. The output voltageis barely affected, and due to the fact that a smaller pulse width is desired,the choice for the SRD fall on a low package capacitance.

5.3 MESFET 41

The package inductance shows a different appearance where a lower effi-ciency is achieved for both high and low inductances. A highest efficiencyis found at around 3 pF. Using a lower package inductance generates a pulsewith shorter pulse width. Higher package inductance generates more oscil-lations to the output pulse. On similar grounds as for the junction capaci-tance, the choice for the SRD falls on a low package inductance.

5.3 MESFET

According to the simulations a higher transconductance gives a higher effi-ciency of the MESFET. The efficiency is approximately linearly dependenton the transconductance, which according to Eq. (3.4) is to be expected. Thechoice for the MESFET falls on having a high transconductance.

A larger negative threshold voltage gives a more effective MESFET. Thechoice for the MESFET falls on having a large negative threshold voltage.

The MESFET device is highly dependent on the drain current added to it,with a lower current making the device more efficient. A lower currentalso makes the pulse width smaller. The drawback is that a lower currentalso creates a negative pulse which has to be filtered out using another cir-cuit component. This could be done using another MESFET which is onlyswitched on during the positive pulse. Due to the fact that the negativepulse can be filtered out using another component the choice for the MES-FET falls on a lower current.

5.4 Summary

As stated in Chapter 2, the requirements from the fluorescence microscopeon the laser diode is that the pulse width is around 100 ps with a pulseperiod of 50 ns. These pulses should increase the performance of the fluo-rescence microscope.

According to the laser diode the input voltage should be around 5 V and theinput current around 250 mA. The average power going through the laserdiode under these conditions is around 2.5 mW. The laser diode should notsuffer any damage from this. In contrary this should increase the lifetimeof the laser and lower the energy used.

The simulated SRD can produce a narrow pulse from a sinusoidal wave. Toproduce a pulse as close to the desired pulse as possible a SRD with lowjunction capacitance, low package capacitance and low package inductanceshould be used. The pulses can get as narrow as 150 ps with an amplitudeof 3.4 V. This is close to the desired pulse, but it needs some additionalamplification and narrowing.

42 5 Discussion of Implementation

Input/Component parameter Preferred propertyDiode input voltage LowDiode input current HighSRD junction capacitance LowSRD package capacitance LowSRD package inductance LowMESFET transconductance HighMESFET negative threshold voltage HighMESFET drain current Low

Table 5.1: Table over preferred properties of inputs and component param-eters.

The simulated MESFET proved to be able to both amplify and narrow thepulse. To produce an high voltage and narrow pulse a MESFET with hightransconductance and a large negative transconductance should be used,fed with low drain current. The simulation results show examples on pulsesas low as 40 ps with an amplitude of almost 20 V, for the MESFET with andrain current of 10 mA. The desired pulse should be able to achieve by mul-tiple variations of transconductance, threshold voltage and input current.The input current has big impact on the MESFET and is easy to change ina circuit, compared to changing parameters of a circuit component. Thisgives great trust in that the desired pulse should be possible to generateusing available MESFETs. The negative pulse generated by the MESFETshould be able to filter out and should not be a problem for the solution.

An additional current source might be needed due to the fact that the MES-FET needs much lower current than the laser diode.

Connecting the SRD and the MESFET should create the desired pulse inthe matter of pulse width, pulse period and voltage. It remains to connectthese, filter out the unwanted pulse from the MESFET and to match thecurrent with the desired value from the laser diode. For the circuit to workas intended, there has to be additional analyses regarding the connectionof components, especially when dealing with these high-frequency pulses.There also has to be considerations taken for the actual circuit, with trans-mission line effect etc. Also, the MESFET has to be analysed specifically totake in to consideration the effect of parasitics, which will probably havegreat effect on these high-frequency pulses.

A summary of the preferred properties can be seen in Tab. 5.1.

5.4 Summary 43

5.4.1 Future Work

To continue the work that has been done in this thesis, the complete circuitcould be simulated and tested. A filter should be added together with acurrent source. The next step would be to produce the circuit for furthertesting, and finally to put the circuit, including the laser diode, as lightsource in a fluorescence microscope.

6Conclusions

According to this study the desired light source should be a laser diode.The waveform provided to the fluorescence microscope should have a wave-length corresponding to the used fluorophore and be fast switching with astable output power. Distortions of the laser pulse should be decreased orremoved in order for the fluorescence microscope to work properly.

To create these laser pulses, the electronic pulse provided to the laser diodeshould be 5 V and 250 mA with a pulse width of 100 ps and a pulse periodof 50 ns. This pulse can be achieved by the chosen laser diode by usingan oscillator together with an SRD as pulse generator and a MESFET foramplifying and narrowing the pulse.

The SRD used should have low junction capacitance, low package capaci-tance and low package inductance. The MESFET used should have hightransconductance, large negative threshold voltage and a low drain current.

The circuit is in need of filtering to remove the unwanted negative pulsegenerated from the MESFET. An additional current source might be usedto raise the current before the pulse enters the laser diode.

45

Appendix

ASimulation graphs

Figure A.1: Diode simulation with input of 5 V.

49

50 A Simulation graphs

Figure A.2: Diode simulation with input of 30 V.

Figure A.3: Graph over diode output voltage dependence on input voltage.

51

Figure A.4: Diode simulation with input of 10 mA.

Figure A.5: Diode simulation with input of 0.5 A.


Figure A.6: Graph over diode output voltage dependence on input current.

Figure A.7: SRD simulation with junction capacitance 0.3 pF.

53

Figure A.8: SRD simulation with junction capacitance 5 pF.

Figure A.9: Graph over SRD output voltage dependence on junction capaci-tance.


Figure A.10: Graph over SRD output pulse width dependence on junctioncapacitance.

Figure A.11: SRD simulation with package capacitance 0.1 pF.

55

Figure A.12: SRD simulation with junction capacitance 1.5 pF.

Figure A.13: Graph over SRD output voltage dependence on package capac-itance.


Figure A.14: Graph over SRD pulse width dependence on package capaci-tance.

Figure A.15: SRD simulation with package inductance 0.5 nH.

57

Figure A.16: SRD simulation with package inductance 3 nH.

Figure A.17: SRD simulation with package inductance 10 nH.


Figure A.18: Graph over SRD output voltage dependence on package induc-tance.

Figure A.19: Graph over SRD pulse width dependence on package induc-tance.

59

Figure A.20: Simulation circuit of MESFET.

Figure A.21: MESFET simulation with transconductance 5 mS.


Figure A.22: MESFET simulation with transconductance 100 mS.

Figure A.23: Graph over MESFET output voltage dependence on transcon-ductance.

61

Figure A.24: MESFET simulation with threshold voltage -0.5 V.

Figure A.25: MESFET simulation with threshold voltage -10 V.


Figure A.26: Graph over MESFET output voltage dependence on thresholdvoltage.

Figure A.27: MESFET simulation with drain current 10 mA.

63

Figure A.28: MESFET simulation with drain current 150 mA.

Figure A.29: Graph over MESFET output voltage dependence on drain cur-rent.


Figure A.30: Graph over MESFET pulse width dependence on drain current.

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