DEVELOPMENT and TESTING of an Er:Yb:GLASS COHERENT LASER RADAR for WIND FIELD MAPPING

DEVELOPMENT AND TESTING OF AN Er:Yb:GLASS COHERENT LASER RADAR FOR WIND FIELD MAPPING

Matthew C. Heintze

Thesis submitted for the degree of Doctor of Philosophy in The University of Adelaide School of Chemistry and Physics February, 2010 For my family Contents

Abstract v

Statement of Originality vii

Acknowledgements ix

List of Symbols xiii

List of Figures xvii

List of Tables xxiii

1 Introduction 1 1.1Coherentlaserradar...... 5 1.1.1 Atmosphericscattering...... 6 1.1.2 Backscatteredpower...... 7 1.2ReviewofcurrentCLR’s...... 8 1.2.1 Eyesafety...... 8 1.2.2 10μmsystems...... 8 1.2.3 1μmsystems...... 12 1.2.4 2μmsystems...... 13 1.2.5 Eye-safewavelengthbandsystems...... 17 1.2.6 Otherwavelengths...... 21 1.2.7 Conclusion...... 21 1.3Projectaim...... 23 1.4Thesisoverview...... 24

2 The slave laser head 27 2.1Introduction...... 27 2.2Phosphateglassgainmediumhost...... 27 2.3Erbiumdopant...... 28 2.3.1 Er:glass...... 29

i ii CONTENTS

2.3.2 Er:Yb:glass...... 30 2.3.3 Dopingoptimisation...... 31 2.4Thermalshockresistance...... 33 2.5KigreEr:Yb:phosphateglass...... 34 2.6Gainmediumpumping...... 35 2.7TheEr:Yb:glassgainmedium...... 37 2.8Gainmediumarchitecture...... 37 2.8.1 CPFSgeometry...... 38 2.8.2 MyCPFSlaser...... 40 2.8.3 Gainmediumpumpdiodes...... 42 2.9Conﬁgurationofthelaserhead...... 44 2.9.1 Laserdiodemounting...... 44 2.9.2 Laser diode collimation ...... 45 2.9.3 Laserslabmounting...... 47 2.9.4 Laserhead...... 48 2.10Conclusion...... 49

3 Laser head characterisation 51 3.1Introduction...... 51 3.2Pumplightabsorption...... 51 3.2.1 Background...... 52 3.2.2 Results...... 53 3.2.3 Chirpingoflaserdiode...... 55 3.3Small-signalgain...... 57 3.3.1 Results...... 58 3.4Upperstatelifetime...... 62 3.5Standing-wavelasertests...... 62 3.6Thermallensing...... 67 3.6.1 Mach-Zehnder interferometer ...... 68 3.6.2 Probebeamdisplacementtechnique...... 74 3.6.3 Summary...... 77 3.7Conclusion...... 77

4 Travelling-wave slave laser 79 4.1Introduction...... 79 4.2Objective...... 80 4.3 Travelling-wave resonator ...... 80 4.3.1 Overallresonatorlayout...... 80 4.3.2 Modelling the travelling-wave resonator ...... 81 4.3.3 Paraxiaresonatordesign...... 83 CONTENTS iii

4.4Gain-switchedoutput...... 86 4.5UnseededQ-switchedoutputresults...... 90 4.6Injection-seeding...... 94 4.6.1 Approach...... 96 4.6.2 Themasterlaser...... 98 4.6.3 Controlsystems...... 101 4.6.4 Injection-seededoutput...... 103 4.7Conclusion...... 107

5 Laser radar measurements 109 5.1Introduction...... 109 5.2 Receiver system ...... 110 5.2.1 Receiver overview ...... 110 5.2.2 Telescope...... 112 5.2.3 Balanced photoreceivers ...... 113 5.2.4 Returnsignalamplification...... 115 5.2.5 Dataacquisition/processing...... 116 5.3Testingofthesystemusingahardtarget...... 120 5.3.1 Stationaryhighlyreflectivehardtarget...... 120 5.3.2 Diffusely-reflectingmovinghardtarget...... 124 5.4AtmospherictestofCLR...... 127 5.4.1 Summary...... 138 5.5Conclusion...... 140

6 Conclusion 141 6.1Summaryofresults...... 141 6.2Futuredirections...... 144

A Publications and presentations 145 A.1Publicationsresultingfromthiswork...... 145 A.2Presentationsresultingfromthiswork...... 145 A.3Presentationsassociatedwiththiswork...... 146

B Laser crystal schematics 147

C Laser diode driver 149

D Laser diode characteristics 151 D.1Laserdiodeperformancespeciﬁcations...... 151 D.2 Conversion of laser diode drive current to incident pump energy . . . 151 D.3Diodedegradation...... 151 iv CONTENTS

E Laser resonator alignment 161 E.1Standing-waveresonatoralignment...... 161 E.1.1HeNealignment...... 161 E.1.2Standing-wavemirrorpositioning...... 163 E.2 Travelling-wave resonator alignment ...... 164

F Circuit diagrams 175 F.1Temperaturecontrolofdiodes...... 175 F.2Pockelscellcontrolelectronics...... 177 F.3Interlock...... 184 F.4Manualseedingelectronics...... 194 Abstract

Doppler or coherent laser radars (CLR’s) can measure range-resolved velocities of distant hard and diffuse targets. Critical applications include wind shear and wake vortex detection, clear air turbulence warning, wind field mapping, and pollution dispersion monitoring. To monitor these at different geographic locations in the atmosphere in real time requires a system with high temporal resolution. A laser transmitter that provides eye-safe, transform-limited energetic pulses with good beam quality and a sensitive transceiver is suitable for such applications. In this thesis I describe the development of an eye-safe coherent laser radar that has a range resolution of 75 m with single-shot velocity resolution of ∼1.5 ms−1. I also present measurements of atmospheric wind speeds using this laser. The laser source is a travelling-wave oscillator that uses a conduction-cooled, Coplanar Pumped Folded Slab (CPFS) with an Er:Yb:phosphate glass gain medium that is side pumped using fast-axis collimated laser diodes. The laser uses polarisation- controlled outcoupling and is injection-seeded to produce eye-safe, transform-limited long duration Q-switched pulses at a frequency close to that of the master laser. This thesis describes the complete characterisation and development of that laser. It also describes the design and development of the monostatic heterodyne receiver used to detect backscattered returns from targets. Measurements validating the performance of the CLR using stationary and moving hard targets are reported. The thesis also presents initial measurements of atmospheric wind speeds using the CLR. Reproducible range-resolved single-pulse measurements to ≥2 km are reported and compared to results from a boundary layer radar.

v vi Abstract Statement of Originality

This work contains no material which has been accepted for the award of any other degree or diploma in any university or other tertiary institution and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the text.

I give consent to this copy of my thesis being made available in the University Library, being made available for loan and photocopying, subject to the provisions of the Copyright Act 1968.

The author acknowledges that copyright of published works contained within this thesis resides with the copyright holder(s) of those works

SIGNED: ......

DATE: ......

Supervisors: Prof. Jesper Munch and A/Prof. Peter J. Veitch

vii viii Statement of Originality Acknowledgements

The PhD journey, in particular writing the thesis at the end, is long and arduous. It has been such a massive part of my life for many years that it’s a surreal feeling to have the task almost over. In any piece of work which takes up a great period of a persons life, there are many people behind them supporting, helping and encouraging them along the way. Here is where I get to say thank you to those people who have gone on this roller coaster ride of ups and downs, and never gave up on me, even though at times it looked like I might never ﬁnish. To begin with, I would like to thank my supervisors, Jesper Munch and Peter Veitch for their support, guidance and knowledge over the years. I am truly grateful for your helpful suggestions, advice and approachability whenever I encountered problems in the lab and your assistance in proof-reading this thesis. I would like to thank Damien Mudge for his helpful comments in designing the laser and teaching me measurement tricks and techniques that saved me many hours by not having to “reinvent the wheel”. I would also like to thank Francois Jeanneret for all his help with the development of the acquisition and post-processing software and David Ottaway who helped me tie up all the loose ends and bring it all together at the end with great advice and assistance in extracting as much information from my results as possible. To the workshop staﬀ: Blair Middlemiss and Trevor Waterhouse. Thank you for using your creativity, enthusiasm and expertise in turning my “handwavey” ideas into reality, and teaching me how to do things like a proper machinist and not like I would if I was “on the farm”. Also I would like to thank the electronics technicians, Neville Wild and Robert Nation, for their help in designing and assembling the various electronics used for my project. To David Hosken, the greatest mate a guy could have. I couldn’t have got through this without all your help. I can not put into words how truly grateful I am for all your support, advice and assistance over the duration of my PhD. I greatly appreciate all those nights you helped me late at night in the lab and for making

ix x Acknowledgements

the mistake of offering (only to be taken up on that offer) to help proof read my thesis. I especially thank you for all those nights you dragged me out to the pub for a beer to watch some sport on the TV and a chat about anything and everything, just to “get away from this place for a bit”. To Aidan Brooks, Alex Hemming and Nikita Simakov. Thank you for answer- ing all manner of stupid questions that I have asked over the years. Your generous disposition made being at uni and the entire PhD experience one which I will be able to look back upon with great memories. To my past and present colleagues in the optics group: Keiron Boyd, Nick Chang, Alex Dinovitser, Miftar Ganija, Ori Henderson-Sapir, Chris Hollitt, Cather- ine Hyndman, Shu-Yen Lee, Sean Manning, Tom Rutten, Ka Wu and the rest of the group. Having such a great environment to work in would not have been possible without all your assistance over the years, both with my project and all the BBQ functions. Thank you also for the many enjoyable discussions in the lunch room where the diversity of topics and opinions always made each one interesting. I also would like to thank Murray Hamilton for his many ideas, some I would never have thought of. To Robert (Bob) Hurn, thank you for your friendship and putting up with all the hours I spent invading your office modifying electronics. I will miss those afternoons where we spent hours talking AFL. I would like to thank Ramona Adorjan for all the computer help over the years. From the most trivial to the sometimes complex and time consuming tasks. To Bronwyn Dolman who went through the write up process at the same time as me and suffered many of the same problem, thank you for the constant source of support and help when I just couldn’t nut out those latex quandaries. I would like to thank Amanda Marshall and Clayton Webb for all that you have done for me over the years. For all the nights you have had me over for dinner and how you have opened up your house to me whenever I have needed a place to stay, I am forever indebted. Friends like you make any ordeal all the easier with your unending support and encouragement. I know you will be right alongside me celebrating when I finally finish. To my mates in the indoor cricket, soccer and basketball teams. Being able to run around and share some laughs and a beer after our games with such a great group of guys (and girls) was thoroughly enjoyable and something I looked forward to every week as a welcome distraction from my PhD studies. In particular I want to thank David and Taryn Mackey for all the times they went to a cricket match with Acknowledgements xi

me, or had me over to watch friday night football, and who never gave up inviting me to various sporting matches, concerts or functions even though I often turned them down when my studies took precedence. To Robert, Amy and the rest of the “Archer crew”, thank you for being such great mates, with whom I have had some great adventures and I hope there are more to come in the future. I am grateful to Stacey Panozzo, Rebecca Allen, Torey Marshall and all my other friends and family who kept encouraging and supporting me that I could ﬁnish even though it was often months or years between catch ups. I would like to express my sincere gratitude to Cassandra Ristic for all her constant love and support to help get me through to the end. You never stopped pushing me to be the best that I could be or let problems sidetrack me from my end goal. Thank you for being so understanding and unresentful of all the late nights I spent at uni, and for knowing when I just needed you to be there. To my siblings Anton, Renata and Kalon and their “better halves” Nat, Matt and Kelly. You have been there whenever I have needed anything. You never lost faith that I could ﬁnish despite the constant ribbing that I enjoyed being a “professional student” too much to do so. Finally I want to thank my Mum and Dad: Dianne and Everard Heintze. This would not have been possible without you being there with all your love, support and encouragement. You have stood by me through all my trials and tribulations, and have always done everything in your power to help me reach any objective that I set myself. All the phone calls to just see how I was going, or the trips to Adelaide to come visit me, where always appreciated. I just hope that one day I can repay you for all that you have done for me. A son couldn’t ask for better parents. The few meager words I have written here will never be enough to express how truly thankful I am to all who have helped me on this journey to grow as both a researcher and a person. xii Acknowledgements List of Symbols

Throughout this thesis, several symbols will be used repeatedly to represent spe- ciﬁc quantities or parameters. The following is a list of these symbols and short descriptions for the readers convenience. This list is not exhaustive but every eﬀort has been made to maintain conformity of symbols used here. Wherever possible standard symbols and notation have been used which appear in most laser texts.

α ... Absorption coeﬃcient

αbl ... Bulk loss coeﬃcient

αext ... Extinction coeﬃcient

αte ... Linear coeﬃcient of thermal expansion β ... Backscatter coeﬃcient δ ... Optical loss due to crystal

δrt ... Round-trip loss η ... Overall system eﬃciency

ηα ... Fraction of incident pump light absorbed

ηqd ... Quantum defect

ηSt ... Storage eﬃciency

θ1 ... TIR angle

θexit ... Beam exit angle out of slab

θn ... Slab nose angle

θsep ... Beam exit separation angle

κtc ... Thermal conductivity λ ... Wavelength

λabs ... Absorption wavelength

λe ... Emission wavelength

μP ... Material Poisson ratio

νd ... Doppler shift

xiii xiv List of Symbols

νLO ... Frequency of the local oscillator radiation

νsig ... Frequency of the signal radiation ρ ... Density

σa ... Absorption cross section

σe ... Stimulated emission cross section

σf ... Surface fracture stress τ ... Output pulse FWHM length

τEr ... Er ﬂuorescence lifetime

υr ... LOS velocity

φLO(t)) ... Phase of the local oscillator radiation

φsig(t)) ... Phase of the signal radiation Δν ... Change in frequency

Δνd ... Doppler shift uncertainty

Δυr ... LOS velocity resolution

Δfp ... Fourier transform spectral width Δ R ... Range resolution ΔT ... Temperature diﬀerence

ΔTPBT ... Pulse build-up time ΔV ... Change in voltage

ar ... Aperture radius

Aλ ... Absorbance

Aeff ... Eﬀective aperture

At ... Telescope collection area c ... Speed of light dn/dt ... Change in refractive index with temperature E ... Young’s modulus

ELO ... Electric ﬁeld of the local oscillator radiation

Esig ... Electric ﬁeld of the signal radiation

fi ... Focal length of object i

fth ... Thermal lens focal length

go ... Small signal gain coeﬃcient G ... Gain factor

Go ... Single pass small signal gain factor h ... Height of gain medium

hp ... Height of the pumped region hν ... Photon energy

hνp ... Pump photon energy List of Symbols xv

Ifl(t) ... Fluorescent emission intensity

Io ... Incident optical intensity

Is ... Saturation intensity

lossbulk ... Bulk scatter loss

lossperTIR ... Loss per TIR bounce

lossTIRscatter ... Surface scatter loss

lg ... Total mode pathlength in the gain region

lp ... Pumped length of gain medium

ls ... Parallel side length of gain medium M 2 ... Beam quality factor 2 Mh ... Beam quality factor (horizontal direction) 2 Mv ... Beam quality factor (vertical direction)

Ms ... Material constant n ... Index of refraction nb ... Number of TIR bounces

No ... Total number of ions

Nj ... Number of ions per unit volume for energy level j P ... Round-trip optical length

Pbs ... Backscattered power

PL ... Slave laser transmitted power

PLO ... Power of the local oscillator radiation

Po ... Incident power

Pp ... Absorbed pump light

Psig ... Power of the signal radiation P (z) ... Power at position z Q ... Heat absorbed per unit volume R ... Range

ROC,i ... Reﬂection percentage of output coupler i

Ropt ... Optimum output coupling reﬂection

ROC,tot ... Total output coupling reﬂection percentage

Rpd ... Responsivity of the photodiodes

Rs ... Thermal shock resistance t ... Time

tpump ... Pump pulse duration

tr ... Cavity round-trip time

Ta ... Average temperature of the pumped region of the slab V ... Volume of the pumped region xvi List of Symbols

Vapplied ... Applied voltage

V1/2 ... Half-wave voltage

wm ... Gaussian beam radius

wp ... Width of the pumped region A/D ... Analog-to-digital AoI ... Angle of incidence AOM ... Acousto-optic modulator BAW ... Brewster-angled wedge BBO ... Beta barium borate CCD ... Charge coupled device CLR ... Coherent laser radar CPFS ... Coplanar-pumped folded slab Er ... Erbium ESA ... Excited state absorption ETE ... Energy transfer eﬃciency FFT ... Fast Fourier transform FSR ... Free spectral range FW ... Forward-wave FWHM ... Full width half maximum HWP ... Half-wave plate LOS ... Line of sight MPE ... Maximum permissible exposure MZ ... Mach-Zehnder PBSC ... Polarising beam splitter cube PBL ... Planetary boundary layer PRF ... Pulse repetition frequency PZT ... Piezoelectric transducer QWP ... Quarter-wave plate RW ... Reverse-wave SNR ... Signal to noise ratio T/R ... Transmit/receive TEC ... Thermoelectric cooler TIR ... Total internal reﬂection Yb ... Ytterbium List of Figures

1.1Rangeconfusionillustration...... 2 1.2FWHMandrangegateillustration...... 3 1.3BlockdiagramlayoutofamonostaticCLR...... 5 1.4 Dependence of maximum single-pulse eye-safe energy on wavelength . 8

2.1Er:Yb:glassenergyleveldiagram...... 29 2.2Er:Yb:phosphateglassabsorptionspectrum...... 35 2 2 2.3 The Yb F7/2 → F5/2 transmission spectrum of Er:Yb:phosphate glass 36 2.4Side-pumpedCPFSschematic...... 38 2.53-Dschematicofthelaserslab...... 38 2.6 Pumping and cooling alternatives looking from the end view of the slab 39 2.7SchematicofCPFSmode...... 41 2.8Endviewschematicoflaserhead...... 44 2.93-Dpumpingsetupoftheslab...... 45 2.10 Image of the collimated pump light incident on the gain medium . . . 46 2.11 Collimating the laser diode ...... 46 2.12Slabmounting...... 47 2.13Topviewofpumpingthegainmedium...... 48 2.14Theslavelaserhead...... 49

3.1 Pump light propagation through the slab ...... 52 3.2Setupusedtotemperaturetunethelaserdiodes...... 53 3.3Typicalpumpabsorptiontuningcurvesforthelaserdiode...... 54 3.4Chirpingoflaserdiode...... 56 3.5Small-signalgainmeasurementsetup...... 59 3.6Single-passampliﬁcationofprobebeamduringpumping...... 59 3.7 Small-signal gain factor as a function of the incident pump energy . . 60 3.8Plotofextrapolatedtuningcurvefora20Ampdiodecurrent.... 61 3.9DeterminingtheErupperstatelifetime...... 63 3.10Schematicofthesetupusedtomeasuretheoutputenergy...... 64 3.11Typicalstanding-waveresonatorconﬁgurationoutput...... 65

xvii xviii LIST OF FIGURES

3.12Standing-waveoutputenergy...... 66 3.13Standing-wave,gain-switchedlaserbuild-uptime’s...... 67 3.14 The Mach-Zehnder interferometer used to measure the thermal lens. . 69 3.15 Typical interferograms for a Mach-Zehnder interferometer ...... 70 3.16 Typical vertical plane wavefront distortion measurement in the pumped slabwhenusingatiltedreferencewavefront...... 71 3.17 A typical parabolic fit to the measured vertical profile data in the pumpedregion...... 72 3.18 Measured vertical thermal lens focal lengths using Mach-Zehnder interferometer...... 72 3.19 A typical parabolic fit to the measured horizontal fringe position in thepumpedregion...... 73 3.20 Measured horizontal thermal lens focal lengths using Mach-Zehnder interferometer...... 73 3.21 Block diagram of beam displacement technique for measuring the thermallens...... 74 3.22Beamdisplacementthermallensmeasurementsetup...... 75 3.23 Measured vertical thermal lens focal lengths using probe beam dis- placementtechnique...... 76 3.24 Measured horizontal thermal lens focal lengths using probe beam dis- placementtechnique...... 77

4.1Theslavelaserconfiguration...... 81 4.2Definitionofpathlengthsfortheresonatordesign...... 83 4.3 Schematic of the slave laser travelling-wave resonator for gain-switched operation...... 87 4.4 A typical gain-switched output pulse for the travelling-wave resonator 87 4.5 Total combined forward and reverse-wave gain-switched energy out- coupledbythePBSCandwedge...... 89 4.6 Total gain-switched output by the PBSC and wedge with the Pockels cellremoved...... 89 4.7UnseededQ-switchedoutputpulse...... 90 4.8 Voltage applied to the Pockels cell by the Q-switch driver for a -6.6 kV setting...... 91 4.9TypicalunseededQ-switchedoutputenergies...... 92 4.10 M 2 ofslavelaserbeam...... 93 4.11Illustrationofaxialmodeselectionwheninjection-seeding...... 95 4.12Schematicoftheinjection-seedinglayout...... 96 4.13Typicalpowerfluctuationsoftheoutputfromthemasterlaser.... 98 LIST OF FIGURES xix

4.14TheFabry-Perot...... 99 4.15Typicalfrequencyvariationofthemasterlaser...... 100 4.16 Schematic of how the power ﬂuctuations of the master laser inﬂuence theFabry-Perotresonance...... 101 4.17 The photodiode voltage shape required for successful seeding of the slavelaser...... 103 4.18ComparisonofunseededandseededQ-switchedoutputpulses....104 4.19 Seeded output pulse dependence on the injected master laser power . 106 4.20 Setup to calibrate photodiode 2 to determine the injection-seeded pulseenergy...... 107

5.1 Schematic of the receiver system...... 111 5.2 Telescope used in the receiver...... 113 5.3 Schematic of a fibre splitter and fibre coupled balanced photoreceiver 113 5.4 Block diagram of the amplifier setup between the balanced photore- ceiverandtheA/Dcard...... 115 5.5 Timing diagram of the “return” arm of the receiver system ...... 116 5.6Flowdiagramofdataprocessing...... 117 5.7ShapeoftheHanningweightingfunction...... 119 5.8Schematicofthesetupforstationarytargetmeasurements...... 121 5.9 Heterodyne beats on the “transmit” and “return” photoreceivers . . . 122 5.10 Fourier spectra when using a high reflectivity stationary hard target . 122 5.11BandwidthmeasurementofFourierspectrum...... 123 5.12 Relationship between injection-seeded pulse build-up time and peak poweronthetransmittedfrequency...... 123 5.13 Schematic of the receiver system to detect Doppler shifts from the beltsander...... 125 5.14Dopplershiftsfromamovinghardtarget...... 126 5.15Schematicofthesetupforatmosphericresults...... 128 5.16 Comparison of the Fourier spectrum of the transmitted pulse and the backscatteredreturnfromtheatmosphere...... 129 5.17 A typical set of spectra at different ranges for a single output pulse . 130 5.18 Atmosphere profile using a VHF boundary layer radar ...... 131 5.19FourierspectraFWHMlinewidthanalysis...... 132 5.20Flowdiagramofprocesstoobtainhistograms...... 133 5.21Histogramanalysisofpeaksinthebackscatterspectra...... 134 5.22Atmosphericwindspeedprofile...... 136 5.23 Reproducibility of spectra ...... 137 5.24Variationinbackscatteredspectrapeakamplitudes...... 139 xx LIST OF FIGURES

B.1CPFSdesignspeciﬁcations...... 148

C.1Diodedriverblockdiagram...... 149

D.1SpeciﬁcationsoftheThomson-CSFlaserdiode...... 152 D.2 Performance of laser diode package, serial # 10854, 1 of 2...... 153 D.3 Performance of laser diode package, serial # 10854, 2 of 2...... 154 D.4 Performance of laser diode package, serial # 10855, 1 of 2...... 155 D.5 Performance of laser diode package, serial # 10855, 2 of 2...... 156 D.6Degradationofthelaserdiode...... 158

E.1 Alignment spike positioning and distances to be measured to calculate beamangles...... 162 E.2HeNeprobebeamalignmenttoCPFS...... 163 E.3 Alignment of the standing-wave mirrors using a HeNe probe beam. . 164 E.4 Travelling-wave resonator alignment 1 of 15...... 167 E.5 Travelling-wave resonator alignment 2 of 15...... 167 E.6 Travelling-wave resonator alignment 3 of 15...... 168 E.7 Travelling-wave resonator alignment 4 of 15...... 168 E.8 Travelling-wave resonator alignment 5 of 15...... 169 E.9 Travelling-wave resonator alignment 6 of 15...... 169 E.10Travelling-waveresonatoralignment7of15...... 170 E.11Travelling-waveresonatoralignment8of15...... 170 E.12Travelling-waveresonatoralignment9of15...... 171 E.13Travelling-waveresonatoralignment10of15...... 171 E.14Travelling-waveresonatoralignment11of15...... 172 E.15Travelling-waveresonatoralignment12of15...... 172 E.16Travelling-waveresonatoralignment13of15...... 173 E.17Travelling-waveresonatoralignment14of15...... 173 E.18Travelling-waveresonatoralignment15of15...... 174

F.1Laserdiodetemperaturecontrolfeedbackservoschematic...... 176 F.2Pockelscelldriverschematic1of5...... 178 F.3Pockelscelldriverschematic2of5...... 179 F.4Pockelscelldriverschematic3of5...... 180 F.5Pockelscelldriverschematic4of5...... 181 F.6Pockelscelldriverschematic5of5...... 182 F.7Pockelscelldrivertimingdiagram...... 183 F.8Interlockschematic1of9...... 185 F.9Interlockschematic2of9...... 186 F.10Interlockschematic3of9...... 187 LIST OF FIGURES xxi

F.11Interlockschematic4of9...... 188 F.12Interlockschematic5of9...... 189 F.13Interlockschematic6of9...... 190 F.14Interlockschematic7of9...... 191 F.15Interlockschematic8of9...... 192 F.16Interlockschematic9of9...... 193 F.17Photodiodeschematic...... 195 F.18DCoﬀsetcircuitusedtoadjustthePZTvoltage...... 196 xxii LIST OF FIGURES List of Tables

1.1 Summary of 10 μmsystems...... 10 1.2 Summary of 1 μmsystems...... 14 1.3 Summary of 2 μmsystems...... 15 1.4 Summary of 1.5 μmﬁbresystems...... 19 1.5Summaryoffree-spaceErsystems...... 22

2.1Gainmediumparameters...... 37 2.2CPFSdimensions...... 41 2.3Laserdiodespeciﬁcations...... 43

3.1ReportedQX/Erabsorptioncoeﬃcients...... 55 3.2Pumpvolumeofslab...... 60

4.1 Component losses for the travelling-wave resonator ...... 82 4.2Resonatormodelparameters...... 85 4.3 Modelled mode size and stability results ...... 86

5.1 Comparison between predicted and measured velocities using a mov- inghardtarget...... 126

D.1 Conversion of the laser diode drive current to the total incident pump energy...... 157

xxiii xxiv LIST OF TABLES Chapter 1

Introduction

The invention of the laser in 1960 by Maiman [1] opened up new methods for atmospheric research. Development of various laser technologies has allowed the laser to become an enabling tool for optical remote sensing of the atmosphere using LIght Detection And Ranging (LIDAR). Generally lidars are used in one of two different detection schemes. A direct (or “incoherent”) detection lidar principally looks at the intensity, time of flight and polarisation of the light backscattered by aerosols and dust. This enables measurements of the atmosphere, including determining the wind speed by tracking the drift of inhomogeneities in aerosol content [2, 3], monitoring the boundary layer structure [4, 5], and detecting changes in the polarisation of the backscattered light from different aerosols [6,7]. “Coherent” lidar (also known as Doppler lidar or Coherent Laser Radar, CLR) detection is capable of measuring the density, time of flight, wind speed and direction of motion of the scatterers. Coherent detection is typically more sensitive than direct detection, but a more complex receiver is necessary. The list of atmospheric phenomena that can, and in most cases have, been investigated by studying the movement of aerosols with CLR’s is extensive. These include wind measurement [8–19], the detection of aircraft wake vortices [20–23] and wind shear [24, 25], and clear air turbulence [26] measurements from airborne and ground based platforms. CLR’s are also used in environmental studies to monitor and predict the dispersion of particles from natural sources such as forest fires [27], dust and volcanic eruptions, and man-made sources such as industrial emissions or chemical/biological toxins. CLR’s have also been used in atmospheric boundary layer research [28,29], and vibrometry [30,31]. Coherent systems directly determine the radial wind speed by measuring the

1 2 CHAPTER 1. INTRODUCTION

change in frequency (Doppler shift) of the light backscattered by the aerosols, relative to that of the transmitted pulse. The frequency change is related to the radial velocity (or the line of sight, LOS, velocity) of the aerosols by:

λν υ = d , (1.1) r 2 where υr is the LOS velocity of the particles, νd is the Doppler shift and λ is the wavelength. The scatterers direction of motion is determined by the sign of the frequency shift. The distance of the aerosols from the CLR is related to the time delay between transmitting a pulse and receiving the backscatter by:

ct R= (1.2) 2 whereRistherange,c is the speed of light and t is the time taken for the light to complete a round-trip.

Scattering volume R1 DR

Solid Range, R angle, 2 A/Rt

Laser beam Telescope collection

area, At

Figure 1.1: Range confusion illustration.

The range resolution, ΔR, speciﬁes the ability of the system to distinguish from where in the atmosphere a return has come. At time, t, after the leading edge 3

of the transmitted pulse is emitted from the CLR system and the return signal is detected, backscatter from the leading edge of the transmitted pulse has travelled a distance R1 (see Figure 1.1). Simultaneously, backscatter from the trailing edge of the transmitted pulse returns from a distance R2. Thus the pulse length is one of the primary factors in determining the range resolution. The return from a signle pulse is a continuous distributed return which can be divided up into range gates. The length of the volume from which backscattered light is received is given by [32, 33]:

cτ ΔR = (1.3) 2 where τ is the full width half maximum (FWHM) of the laser pulse outcoupled from the slave laser or the range gate length of the distributed return as illustrated in Figure 1.2. Therefore, to improve the range resolution, the transmitted pulse length or the range gate length should be shortened.

Transmit Return

Range Gate

FWHM Power of signal Power of signal

Power of signal

TimeTime TimeTime

Figure 1.2: Illustration of the FWHM and the range gate of the transmitted and returned signals.

In the scattering volume in Figure 1.1, turbulence can cause variations in the speed and direction of motion of the aerosols, leading to a varying wind ﬁeld within this volume. This produces a spread in the Doppler shift, Δνd, leading to a radial velocity resolution, Δυr of:

λΔν Δυ = d (1.4) r 2 If the velocity of the aerosols in the scattering volume is uniform, the minimum uncertainty in the Doppler shift is proportional to the inverse of the pulse length:

Δνd∼1/τ [33]. 4 CHAPTER 1. INTRODUCTION

Combining Equation 1.3 and Equation 1.4 gives the range-velocity product [33]:

λc ΔRΔυ = . (1.5) r 4

Thus, the shorter the wavelength, the smaller the range-velocity resolution product. Improving the range resolution, by reducing the pulse duration or the range gate length, would degrade the velocity resolution.

The backscattered signal returning from a distant range will be much weaker than that from a short range. The signal to noise can be increased by averaging many returns, improving the velocity resolution, but the rate at which diﬀerent parts of the sky can be scanned will be decreased.

Atmospheric research could be greatly assisted by a CLR system that can eco- nomically provide accurate, high spatial and temporal resolution wind velocity measurements. Coherent systems for wind measurements such as HF/VHF/microwave radars and CLR’s present advantages over other systems such as radiosondes, as they cause no disturbance of the target, and potentially avoid accessing hazardous or diﬃcult to reach regions.

While VHF radar can be used to sense wind fields to ∼20 km in height, the shorter wavelength CLR’s have several advantages. For the same transmit aperture size, radar pulses will diverge faster (due to diffraction) than CLR pulses, degrading the transverse spatial resolution of the radar relative to the CLR. Additionally, the range-velocity product (Equation 1.5) is larger for radars than a CLR. For a given single-shot velocity resolution, a CLR would use a much shorter duration pulse, giving less range confusion. For radar to obtain a range and velocity resolution comparable to a CLR, it must do more averaging of the return signal. This severely limits the rate at which different parts of the sky can be sampled.

Finally, VHF radar detects returns from “stochastic Bragg scattering” [34] due to ﬂuctuations in refractive index on a scale of half the radar wavelength [35] in clear air, and Rayleigh scattering in precipitation [36]. VHF cannot detect particles of small diameters, making it blind to areas of the atmosphere where there are only small particulates, whereas a CLR can undertake wind measurements even in “clear air”.

This all indicates that a CLR system is a very attractive sensor for atmospheric observations, especially when small velocity and range resolution is required with a fast scan rate. 1.1. COHERENT LASER RADAR 5

1.1 Coherent laser radar

A schematic of a monostatic CLR system is shown in Figure 1.3.

Atmosphere CW master laser

Frequency offset Slave laser

Detector 1

Detector 2

Figure 1.3: Block diagram layout of a monostatic CLR.

The location of the receiver with respect to the transmitter defines whether the system is “monostatic” or “bistatic”. A “monostatic” system uses a single transmitter and receiver, while a “bistatic” system uses a separate transmitter and receiver. These systems are adequate for LOS velocity measurements. However, these systems are unable to reconstruct the 3-dimensional wind field [37]. A “multistatic” system, which uses two or more transmitters and/or receivers with overlapping spatial coverage are capable of creating 3-dimensional maps of the wind field [38]. Heterodyne detection is often used in a CLR system rather than homodyne detection as it allows simpler determination of the direction of motion of the target and a better signal to noise ratio (SNR). Heterodyne detection involves mixing two coherent waveforms with slightly different frequencies to generate a beat frequency, equal to the difference of the two original waveform frequencies. To enable heterodyne detection, light from a low power, CW, master laser is divided into two beams. One beam is frequency offset using an acousto-optic modulator (AOM) to move the heterodyne frequency away from zero, therby allowing the determination of the direction of motion. As shown in Figure 1.3, this beam is used to injection-seed the pulsed slave laser. A small portion of the pulse is picked off before being sent to the atmosphere and is combined with light from the other master laser beam on a photoreciever. The resultant heterodyne beat is digitised and Fourier transformed to yield the frequency of the transmitted pulse relative to the frequency of the master laser reference. Light backscattered from the atmosphere is collected by the telescope 6 CHAPTER 1. INTRODUCTION

and the Fourier spectrum determined. Analysis of the heterodyne beat on the two detectors allows for information about the speed, direction, range and density of the scatterers to be obtained. The diﬀerence between the transmitted and returned frequencies allows the LOS speed and direction to be determined. The range is obtained from the time of ﬂight, and the density from the amplitude of the backscatter.

1.1.1 Atmospheric scattering

Scattering and absorption of light occurs from molecules and aerosols. Molecules are typically smaller than a wavelength, whereas aerosols, consisting of solid or liquid particles such as dust, smoke or ice crystals, are typically a few wavelengths of light in dimension. Once light encounters the molecule or aerosol, it is scattered in all directions, including back along the direction of the incident light. Scattering is described by either Rayleigh or Mie theory. Rayleigh scattering is scattering from molecules and particles that are small compared to the wavelength of the scattered light, and is therefore wavelength dependent. The Rayleigh scattered intensity is proportional to λ−4 [32], and is small at IR wavelengths compared to visible or UV. At IR wavelengths Mie scattering is predominant. Mie scattering describes scattering from particles which are larger, or of a similar order of magnitude to the wavelength of the radiation. Hence Mie scattering is not strongly wavelength dependent [32]. Mie scattering produces a scatter pattern that is more intense in the forward direction. However the distribution of the scattered intensity varies with scatterer size and shape. As the size of the scatterer increases, the intensity of the forward scattered light also increases [39]. If the scatterer is spherical in shape, the polarization state of the linearly polarised laser light is unchanged, whereas de-polarisation occurs for non-spherical scatterers [32]. The atmosphere acts like a distributed target rather than a hard target as aerosols and molecules occur throughout the atmosphere. Therefore, for a transmitted pulse, returns from diﬀerent ranges will occur. CLR returns are dependant on the concentration and size of the diﬀerent scatterers. Returns from the atmosphere in the southern hemisphere are in general weaker than for the northern hemisphere, as the density of scatterers is much lower [40]. This is due to the greater land- mass and population of the northern hemisphere, leading to greater sources of dust, pollution and particulates. 1.1. COHERENT LASER RADAR 7

1.1.2 Backscattered power

For a vertically directed monostatic CLR, the return signals, due to Rayleigh and Mie backscattered radiation incident on the receiver system, are described by the lidar equation [32,41,42]:

R At cτ Pbs(R) = PL O(R)ηβ(R)exp(−2 αext(r)dr) (1.6) R2 2 0 where Pbs(R) is the power received from scattering at a range R. PL is the average transmitted power and τ is the output pulse duration. The cτ/2 term is the length of the sampled volume in the atmosphere from which backscattered light is received at a ﬁxed time, (see Figure 1.1). O(R) describes the overlap between the receiver ﬁeld of view and the outgoing laser beam: O(R) = 1 indicates full overlap.

The overall system efficiency, η, includes the efficiency of all the receiver optics. It takes into account the absorption and scatter of elements such as mirrors, lenses, wave plates and beam splitters, the efficiency of the heterodyne beat (due to degraded interference from mismatches in wavefront curvature and beam sizes), and the efficiency of the detectors.

2 The factor At/R is the acceptance solid angle from which light scattered at range, R, is collected by the lidar. As At is the area of the telescope, there will be a quadratic decrease in the backscattered signal with range [32].

The backscatter coeﬃcient β(R), describes how much of the light scattered by aerosol particles (aer) and molecules (mol), is directed back towards the lidar receiver, where [32]:

β(R) = βaer(R) + βmol(R) (1.7)

R The ﬁnal part of the equation, exp(−2 0 αext(r)dr) indicates the attenuation of the laser pulse as it propagates to the scatterer and back to the lidar receiver.

This term has a value between 0 and 1 [32]. The extinction coeﬃcient αext, describes the loss of energy due to absorption (abs) and scattering (sc) in other directions by aerosols and molecules. It can be described in a similar manner to the backscatter coeﬃcient [32]:

αext(R) = αaer,sc(R) + αaer,abs(R) + αmol,sc(R) + αmol,abs(R) (1.8) 8 CHAPTER 1. INTRODUCTION

1.2 Review of current CLR’s

1.2.1 Eye safety

The maximum permissible exposure (MPE) deﬁnes the amount of energy the eye can be exposed to before damage occurs. Its dependence on wavelength is shown in Figure 1.4.

102 Single-pulse exposure 101

100

10-1

10-2

10-3

-4 Maximum eye-safe energy (J) 10

10-5 Beam diameter: 47 mm Pulse duration: 6 ns 10-6 0.1 1.0 10 Wavelength (m m)

Figure 1.4: Dependence of maximum single-pulse eye-safe energy on wavelength for the conditions listed in the plot. This ﬁgure is reproduced from Spuler et al [43].

Systems operating at 1.4 - 1.8 μm are said to operate in the “eye-safe” band of wavelengths. However the term “eye-safe” is a misnomer. It simply means the eye is able to absorb more laser energy before damage occurs to the cornea or the retina than at other wavelengths. Thus, more energy can be transmitted into the sky safely with systems operating in this wavelength band than systems operating at other wavelengths. This allows a greater range for the CLR.

1.2.2 10μmsystems

Until 1987, all reported CLR systems used CO2 lasers capable of being tuned between 9 and 11 μm. These systems are described below, and their performance characteristics listed in Table 1.1. The ﬁrst pulsed CLR was an airborne system (ADLS), operating at 10.6 μm, which was designed for the measurement of wind ﬁelds in the regions surrounding 1.2. REVIEW OF CURRENT CLR’S 9

severe storms [8]. This system was installed in a NASA Convair 990 to perform airborne demonstrations of the system on terrain-induced wind field variations, and wind field mapping of thunderstorm outflows [44, 45]. In the early 1980’s, NOAA (National Oceanic and Atmospheric Administra- tion) began investigation of a Transverse Excited Atmospheric pressure (TEA) CO2 laser in a hybrid configuration, for use as a pulsed CLR transmitter, with the capability of being tuned to wavelengths between 9.2 to 10.9 μm [9]. This system was successfully employed in field measurements of winds within a long narrow mountain valley [46]. It was upgraded and re-engineered [47] until it was capable of operating at higher and variable pulse repetition frequencies (PRF’s). Two operational setting were available: a low energy, short pulse duration setting, and a higher energy, longer pulse duration setting. It was used for various trials including investigation of smoke from forest fires [27], thunderstorm outflows and boundary collision inter- actions [51], performing evolution and vertical scans of the sea-breeze layer [52,53], and boundary layer and pollutant research [54–57]. From 1992, NOAA, NASA (National Aeronautics and Space Administration) and JPL (Jet Propulsion Laboratory) collaborated to develop a new airborne system called the Multi-center Airborne Coherent Atmospheric Wind Sensor (MACAWS), which employed the TEA CO2 laser used by Post et al [47], now capable of producing outputs up to 1 J. It was installed in a side-looking configuration in NASA’s DC-8 research aircraft and, by using multiple scan planes, mapping of a 3-dimensional volume of atmospheric winds and aerosol backscatter was achieved [14,48].

Research into a pulsed, high repetition rate, monostatic, CO2 CLR that used a master oscillator power ampliﬁer (MOPA) conﬁguration led to the NOAA mini- MOPA system [28]. This system is still taking measurements and is mounted in a shipping container on the back of a semi-trailer to make it portable [49]. The National Center for Atmospheric Research (NCAR) reported on their Airborne Infrared Lidar System (NAILS) [58] which uses an injection-seeded TEA

CO2 laser as its transmitter. However, even though it was designed for airborne deployment, reports indicate it is still being tested from the ground, where it is successfully measuring radial air velocities [50]. The franco-german collaboration, WIND (Wind INfrared Doppler lidar), used an airborne, downward facing, conical scanning CO2 CLR system. It was developed for investigations of mesoscale wind phenomena and as a precursor system for future spaceborne projects [16]. When ﬂown at an altitude of 11 km in a Falcon 20 E5/D- CMET research aircraft, wind proﬁles were successfully obtained over the full 11 km 10 CHAPTER 1. INTRODUCTION 5 8 12 11 10 30 10 2(wet) 10 (dry) Maximum range (km) 10 10 20 1000 Number of pulses averaged 1 0.05 0.6 1.0 0.5 0.4 2.2 > Velocity resolution (m/s) 60 60 60 450 250 300 300 300 300 100 330 > Range resolution (m) 300 20 10 10 110 (Hz) 1000 PRF < 0to30 0.1to20 0.1to20 s) μ 2 2 2 3.0 2.5 2.0 0.4 0.4 3.0 Pulse 0.4 to 1.0 0.5 to 10.0 duration ( m systems. Entries are left blank where no value is quoted in the reference. μ 8 8 10 2.0 1.2 730 300 100 120 1000 (mJ) Energy 40 to 200 [13] [24] [46] [28] [16] [14,48] [50] [9] [47] [8,44,45] et al et al et al et al et al et al et al et al et al Table 1.1: Summary of 10 Author et al (ADLS) (WIND) (NAILS) NOAA [49] (MACAWS) Hall Post Mayor Werner Pearson and Post Arbuckle and Targ Bilbro Rothermel (NASA/FAA MOPA system) (Present NOAA mini-MOPA) (Original NOAA mini-MOPA) (NOAA’s hybrid TEA system) (NOAA’s hybrid TEA system) 1.2. REVIEW OF CURRENT CLR’S 11

distance to the ground and agreed with results taken at the same time with a windproﬁler radar.

In 1992, NASA/FAA (Federal Aviation Agency) used a MOPA CO2 CLR developed by the Lockheed Missiles and Space Company as an airborne windshear threat warning system [24]. This laser, operating at 10.6 μm, could obtain results to a range of 2 km in “wet” weather conditions and up to 10 km in “dry” weather conditions [13]. The decrease in performance in “wet” conditions was attributed to humidity and rain attenuating the 10.6 μmbeam.

There have been a wide range of 10 μm systems reported and the majority of these are able to observe to ranges of ∼10 km, using either a high energy output pulse, averaging the returns of many lower energy pulses, or by a combination of the two. Typically the range resolution is several hundred metres (100 - 400 m), primarily due to the long pulse duration needed to obtain a small velocity resolution. A velocity resolution shorter than that obtained from a single-shot can be obtained by averaging many returns.

It is difficult to compare the quoted velocity and range resolutions of the different systems. This is due to authors often not reporting the velocity resolution, or stating the systems simultaneous range and velocity resolution with the number of averages used. Therefore, the velocity resolutions for given range resolutions are often less than that expected using Equation 1.5. The reports on NOAA’s hybrid configuration TEA CO2 laser system operating in the short pulse duration setting are an example of this. For a pulse with a duration of 0.4 μs, a range resolution of 60 m is expected, (using Equation 1.3) and reported by NOAA. However for this pulse duration, a single-shot velocity resolution of 13 m/s is expected. This is much larger than the reported value of 2.2 m/s, which could only have been achieved by averaging ∼40 return pulses but this is not reported.

CO2 laser systems, whilst able to achieve scatter returns from a range of many kms, do have some disadvantages. They have gas handling requirements and are quite bulky and heavy [29]. The operating wavelength suﬀers from atmospheric water vapour absorption and the detectors are often cryogenic. Also, for a small velocity resolution, the single-shot, range resolution will be quite poor. Finally, as can be seen in Figure 1.4, the maximum allowable, eye-safe, single-pulse energy at 10 μm, is a factor of 100 less than in the 1.4 - 1.8 μm wavelength band. 12 CHAPTER 1. INTRODUCTION

1.2.3 1μmsystems

Solid-state systems, when compared to CO2 systems, are often quite robust and compact. They can have longer operating lifetimes with lower power consumption [59] and the detectors can operate at room temperature. Nd:YAG lasers emitting at 1.06 μm are a very mature technology. The short wavelength, in comparison to CO2 lasers means that, for a given range uncertainty, improved single-shot velocity resolution is obtained. However, the MPE is much lower than in the 1.4 - 1.8 μm band. Thus, Nd:YAG CLR systems were only briefly investigated. The performance characteristics of the systems reviewed, are shown in Table 1.2. The system developed by Kane et al [60] was the first 1.06 μm solid-state CLR system. Its transmitter used a combination of a CW Nd:YAG laser oscillator that was diode pumped, and a laser amplifier that was flashlamp pumped. Though thermal effects in the slab limited the PRF, time delayed heterodyne returns from clouds ∼2.7 km away were observed. No wind velocity measurements were reported. Kavaya et al [61] further developed this system and successfully produced remote wind profile measurements. This system used a single frequency, CW laser, which was frequency shifted and gated before being amplified by a flashlamp pumped, multi-pass, slab amplifier to produce the output pulse characteristics listed in Ta- ble 1.2. A year later Henderson et al [59] reported this same system producing laser output pulses with higher energy and a longer pulse duration, and achieved wind velocity measurements at approximately double the range of that reported by Kavaya et al. In 1993, Hawley et al [11] reported on the CLAWS CLR. This systems hardware is based on the MOPA coherent transceiver developed by Coherent Technolo- gies Inc [61]. The earlier systems output was amplified by passing it through four additional amplifiers, yielding ∼1 J/pulse with an M2<1.5. This system is mounted in a portable trailer and can operate at various output pulse durations, thereby altering the range resolution that can be achieved. When operating with a pulse duration that resulted in a range resolution of 75 m (0.2 μs long pulses), returns from ranges up to 26 km were demonstrated. Chan et al [62] described a Q-switched Doppler system, that used simultaneous direct and heterodyne detection, and could obtain and compare the carrier-to-noise ratio (CNR) of returns from a hard target to a range of 450 m. Though atmospheric measurements were not reported, using heterodyne detection with the quoted 8 ns output pulse length would make any single-shot velocity resolution measurement 1.2. REVIEW OF CURRENT CLR’S 13

very imprecise.

1.2.4 2μmsystems

Solid-state 2μm CLR’s also have advantages over 10 μm systems in terms of weight, size, operating lifetime and detectors. They also have less beam divergence for an equivalent transmitter aperture and better range-resolved, single-shot velocity resolutions. While the range-velocity resolution product is larger at 2 μmthanat 1 μm, the MPE for 2 μmsystemsis∼50x more than for 1 μmsystems,and∼10x that of 10 μm systems (see Figure 1.4). This advantage of higher MPE outweighs all the other advantages of 1 μm systems. These systems are summarised in this section, with the reported characteristics listed in Table 1.3. CTI developed a 2.09 μm flashlamp pumped, Q-switched Cr:Tm:Ho:YAG oscillator to be used in a CLR system [59, 63]. The system obtained wind velocity ranges to 30 km [64, 65], and was also used to detect and measure aircraft wake vortices [21]. CTI also reported on work to produce a diode pumped Tm:YAG Q-switched laser [65]. When incorporated into a CLR system and flown in NASA’s B/737 aircraft, it became the first airborne, diode pumped, 2 μm CLR system to measure wind fields [13]. By the late 1990’s, CTI had developed a range of Tm:YAG CLR’s, each with various output energies, pulse durations and PRF’s. The applications included detecting and measuring wind shear and aircraft wake vortices around airports, clear air turbulence detection, improving the precision of dropping cargo from aircraft and the tracking of aerosols and clouds [66]. Their systems include the Wind Tracer CLR which is a Tm:Lu:YAG, 2.02 μm system that has been successfully tested on ground and aircraft platforms [67]. Hannon [68] used the WindTracer system to collect wind velocity and direction measurements for wind energy applications. The CLR data was compared against anemometer data, giving a correlation of 96 %. The WindTracer system has been used by the German aerospace center (DLR) in meteorological campaigns such a COPS, THORPEX IPY and TPARC [72]. In conjunction with NASA, CTI have also developed the Airborne Coherent Lidar for Advanced In-flight Measurements (ACLAIM). To undertake research from ground, sea or air platforms with more reliable performance and higher spatial, temporal and velocity resolution than their earlier

CO2 systems, NOAA developed the High Resolution Doppler Lidar (HDRL). This 2.022 μm, injection-seeded, laser diode pumped, Tm:Lu:YAG system could detect 14 CHAPTER 1. INTRODUCTION 26 7.5 2.7 3.75 0.45 Maximum range (km) 10 10 100 Number of pulses averaged 1.0 0.42 < Velocity resolution (m/s) 1.0 192 Range 75 to 300 resolution (m) 10 10 10 10 (Hz) PRF s) μ 0.5 1.0 0.008 Pulse m systems. Entries are left blank where no value is quoted in the reference. 0.5 to 7 0.2 to 0.5 μ duration ( 8 12 5.0 100 1000 (mJ) Energy [59] [11] [61] [62] [60] et al et al et al et al et al Table 1.2: Summary of 1 Author (CLAWS) Kane Chan Hawley Kavaya Henderson 1.2. REVIEW OF CURRENT CLR’S 15 7 10 30 11 8.5 11.5 8to10 Maximum range (km) 20 20 20 40 Number of pulses averaged 1.0 0.1 < Velocity 0.1to0.5 resolution (m/s) 75 60 30 96 Range 30 to 75 resolution (m) 5 5 3.2 500 100 200 100 (Hz) PRF 1000 to 100 s) μ 0.5 0.4 0.4 0.2 0.56 0.14 0.22 Pulse m systems. Entries are left blank where no value is quoted in the reference. 0.2 to 0.5 μ duration ( 7 90 22 10 2.4 2.0 0.8 250 (mJ) 1to10 Energy [71] [66] [67] [67] [29] [70] [19] [13] [59,63–65] [18,69] et al Table 1.3: Summary of 2 et al et al et al et al et al et al et al et al et al Author (HDRL) (DAWN) (VALIDAR) Targ Koch Koch Grund and Hannon [68] Ishii Hannon Hannon Huﬀaker (ACLAIM system) (NICT CDL system) (WindTracer system) and Kavaya Henderson 16 CHAPTER 1. INTRODUCTION

to ranges of ∼11 km, achieving 30 m range resolution with a simultaneous velocity precision of ∼10 cm/s [29]. At the National Institute of Information and Communications Technology (NICT) an airborne CLR system was developed. The transmitter was a Q-switched, 2.102 μm Tm:YAG laser. When atmospheric conditions suited, the CLR could obtain wind profiles to altitudes of 8.5 km with a range resolution of 96 m, which were in good agreement with results obtained with VHF radar and radiosondes. This system was then mounted on an airborne platform and flown at a height of 7.2 km to measure wind profiles [69]. The results agreed well with those obtained at the same time with GPS-dropsondes and a windprofiler. The VALIDAR system (named from the concept of a “validation lidar”) is a CLR with a laser transmitter based on diode pumping Ho:Tm:LuLiF. Injection- seeded outputs of 90 mJ with an M2<1.2 were obtained at wavelengths that can be tuned between 2.050 μm and 2.057 μm. Using a 20 pulse average, the highest maximum altitude observed was 11.5 km [19]. The Doppler Aerosol WiNd lidar (DAWN) was created by adding a laser amplifier to the initial VALIDAR system, thereby increasing the output pulse energy. The DAWN system was designed for use on an airborne platform, but has been used in ground based trials to measure vertical and horizontal winds, as well as relative backscatter, with a temporal resolution of 4 s [70,71]. Concept designs for spaceborne CLR systems have also been investigated. The laser transmitters need to be capable of producing joule level output energies to accurately measure the global wind fields [73–75]. As the Koch et al [19] system is scalable, adding additional amplification stages to the output of the pulsed laser could potentially achieve energies approaching the pulse requirements. However a diode pumped Ho:Tm:LuLF Q-switched laser at 2.053 μm, with a side-pumped rod in a master-oscillator-power-amplifier (MOPA) architecture, has produced output pulses with 1.1 J/pulse at a FWHM pulse width of 187 ns [76]. This result makes this laser transmitter a viable option for potential space-based CLR systems. There are a variety of 2 μm CLR systems, all operating with short duration pulses to achieve range resolutions <100m. 2 μm systems are, on average, able to observe to ranges comparable to 10 μm systems, but at much lower energies per pulse. However, as for the 10 μm systems, the lack of information provided in the references makes it hard to compare the velocity resolution and the number of pulse averages used by the various systems. The early Henderson et al system [59,63–65] has by far the best performance in terms of range obtained. Though the outcoupled 1.2. REVIEW OF CURRENT CLR’S 17

energy is high compared to the majority of the other systems, the PRF is low. This is signiﬁcant because although ranges of 30 km can be obtained, 20 pulse averages are required, and the low PRF limits the scan rate of the sky, thereby limiting the temporal resolution that can be achieved. Surprisingly, the VALIDAR system [19], though outcoupling energies 4x higher than the CTI system described by Henderson et al, with an identical number of pulse averages, has a maximum observed range only ∼1/3 that of the CTI system. Though 2 μm systems have shown to be much better than 10 μmand1μm systems, their allowable MPE is 10x less than that allowed with a system emitting in the eye-safe 1.4 - 1.8 μm band. Thus, considerable eﬀort has gone into producing laser systems that emit in that wavelength band as will be discussed in the next section.

1.2.5 Eye-safe wavelength band systems

As discussed earlier in Section 1.2.1, CLR systems operating in the 1.4 - 1.8 μm band are the most eye-safe. As the range drops oﬀ via the inverse square law, a 1.5 μm system with an MPE 10x greater than that of a 2 μm system, would increase the range by a factor of 3 for a single-shot. Further advantages of 1.5 μmsystems is the possibility of using telecommunication components, and the likely decrease in atmospheric absorption of the transmitted light due to the reduced number of absorption lines in the atmosphere at 1.5 μmcomparedto2μm [77]. There are several ways to obtain radiation in the eye-safe band and these are reviewed in the remainder of this section.

1.5 μm erbium-doped ﬁbre laser systems

At the commencement of this project, using conventional single mode erbium fibre lasers [78,79] was a possibility, however they had some disadvantages. The amount of pump light that can be coupled into the fibre is limited, and there are peak power limitations due to the onset of Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS) in the fibre, and fibre facet damage [80–83]. There were also difficulties in obtaining stable, equal-amplitude, pulse trains over extended periods of time at high repetition rates [84–87], due to the susceptibility of fibre lasers to temperature variations and mechanical vibrations. The development of large effective mode area (LMA) fibres provides a partial solution to some of these problems. They can support higher peak powers due to the 18 CHAPTER 1. INTRODUCTION

increase in the limit at which non-linear effects occur [88–90]. Even though higher order transverse modes can propagate in these fibres, fundamental mode operation is still possible using a variety of techniques to introduce a substantial loss for the higher order modes, while not significantly affecting the fundamental mode. These techniques include strongly bending the fibre [80], chirally coupled core fibres [91] and leakage channel fibres [89,92]. Though still able to produce good output beam quality, LMA fibres will eventually run into non-linear loss effects. Additionally like all fibres, LMA fibres have an attenuation of the beam as it travels along the fibre greater than that which would be experienced by an equivalent beam travelling in free space, due to the fibre materials linear absorption loss. However, fibre research is continuing to evolve and the search for fibres with higher power capabilities and better properties continues. Despite its pulse energy limitations, fibre based systems have been produced and are summarised in Table 1.4. Using a polarisation independent fibre optic circulator as the transmit/receive (T/R) switch, allows fibre systems to be simple and compact. The first 1.5 μm all-fibre CLR system was a CW system based around a MOPA configuration [15]. This system was later used by Harris et al [98, 99] for various CLR applications. Pearson et al [81] followed on the work of Karlsson et al [15] and reported a pulsed all-fibre system with an output beam quality of M2 ≈ 1.5, with which returns from aerosols could be measured. Recently Gordienko et al [31] reported a free space Michelson interferometer CLR system, using a 1 W CW fibre laser as the transmitting source. The Mitsubishi Electric Corporation developed an all-fibre 1.5 μm CLR system using single-mode fibre [93], and commercial prototype systems based on a MOPA transmitter operating at 1.54 μm are available. These systems have variable transmitted pulse energies and pulse durations, permitting a variety of range resolutions [94]. ONERA in co-operation with Leosphere developed a pulsed, all-fibre, 1.5 μm CLR. The transmitter is based on the Master Oscillator Power Fibre Amplifier (MOPFA) architecture, and has an M 2 ≈ 1.4 [22]. Leosphere [95] have further developed two CLR’s, the WindCube WLS7 and the longer range WindCube WLS70 for atmospheric wind measurements. The University of Salford reported on their Salford Autonomous Lidar System (SALiS) [96], which measured velocity profiles to a range of ∼1100 m [97].

While fibre systems have been used to measure wind fields, the range is limited 1.2. REVIEW OF CURRENT CLR’S 19 1.1 0.5 2.3 0.4 0.8 2.4 0.06 0.1to4 Maximum 0.04 to 0.2 range (km) 100 CW CW 10 000 20 000 30 000 30 000 20 000 30 000 Number of pulses averaged 0.2 0.2 0.5 3.0 0.1 Velocity resolution (m/s) 25 20 50 150 37.5 Range 30 to 150 Few metres resolution (m) (Hz) 1000 4000 PRF 20 000 50 000 15 000 20 000 s) μ 0.7 0.15 0.15 0.25 Pulse 0.2to1.0 duration ( m fibre systems. Entries are left blank where no value is quoted in the reference. μ 0.01 0.01 0.05 0.06 (mJ) 0.0083 1Watt 1Watt Energy 0.0018 to 0.0046 [22] [93] [31] [15] [81] [96] [97] [94] [95] [95] et al et al et al et al et al et al et al et al Table 1.4: Summary of 1.5 et al et al Author Lolli Lolli Ando Davis (SALiS system) (SALiS system) Bozier Pearson Karlsson (WindCube WLS7) (Mitsubishi system) Gordienko (WindCube WLS70) (Mitsubishi systems) Kameyama Dolfi-Bouteyre 20 CHAPTER 1. INTRODUCTION

in most cases to <1 km. Though the PRF of the fibre systems is kHz, the need to average 10’s of thousands of times in most cases means that the temporal resolution is poor. The system reported by Dolfi-Bouteyre et al [22] is the exception. In that system, the range that can be observed is sacrificed in order to increase the temporal resolution by averaging only 100 times. The reported range resolution agrees with that expected from the pulse duration, however the expected single-shot velocity resolution of 6.1 m/s, is approximately double that reported. The Mitsubishi system of Kameyama et al [93] has ∼2x the range of any of the other reported pulsed systems, however an accurate comparison to systems such as Dolfi-Bouteyre et al [22] is difficult due to the lack of a quoted velocity resolution. As the range resolution of the Mitsubishi system is poor, it can be expected that the velocity resolution will be good. Though it has a 50 kHz PRF, the number of averages used (20 000) results in a poor temporal resolution in comparison to Dolfi-Bouteyre et al [22]. A compromise on the performance characteristic of fibre systems is apparent. Systems can have either good temporal resolution but limited range, or have a range on the order of a couple kilometers, with a modest temporal resolution of the sky.

1.5 μmand1.6μm free-space erbium systems

Free-space, solid-state, erbium (Er) doped lasers are a promising alternative to fibre systems. Though they are more bulky and heavier, they are unaffected by nonlinear effects as in fibres. The first known free-space Er CLR system was reported by McGrath et al [100–102]. This system used a flash-lamp pumped, Er:glass gain material in a standing-wave resonator as its transmitter. It used an FTIR Q-switch to control the outcoupling fraction, and produced injection-seeded pulses at 1.55 μm. How- ever, the lamp pumping resulted in poor efficiency and reliability. The Mitsubishi Electric Corporation developed the first laser diode pumped 1.53 μm CLR [103]. This system used an injection-seeded, Q-switched, Er:Yb:glass solid-state laser as the transmitter. The laser head consisted of two laser rods pumped with four water cooled laser diode arrays. Experimental results showed backscattered returns and wind velocity measurements from aerosols to distances of several km’s [104]. Further development of this system by Mitsubishi [77, 105] led to an upgraded transmitter system, which used a flat/flat standing-wave resonator, 2 m in length (giving a 4 m round-trip). It produced 10.9 mJ pulses from ∼1.27 J pump pulses with a maximum PRF of 15 Hz and M 2 ≈ 1.4 [77]. No range resolution 1.2. REVIEW OF CURRENT CLR’S 21

was reported but it is estimated to be ∼35 m. Measurements of atmospheric wind velocities at similar ranges were subsequently reported [17]. Lockheed Martin Coherent Technologies [68] have developed a Er:YAG 1.62 μm WindTracer system which has undertaken range-resolved velocity measurements. However a detailed analysis of the system could not be performed due to the lack of published results. The 1.53 μm Mitsubishi and 1.62 μm Lockheed Martin Coherent Technologies systems have both reported wind ﬁeld measurements. Both systems report short range resolutions, however the quoted velocity resolutions are achieved through averaging, and are much better than the expected single-shot, range-resolved velocity resolutions using Equation 1.5. Though the 1.6 μm WindTracer system has a much faster PRF than the Mitsubishi system, the Lockheed Martin Coherent Technologies system would need to use signiﬁcantly more averages than the Mitsubishi system to achieve the quoted velocity resolution.

1.2.6 Other wavelengths

Even though most systems operate at either 10 μm, 2 μmor1.5μm, CLR’s using other wavelengths have also been investigated. A frequency doubled Nd:YAG laser (λ = 532 nm), which looked at Rayleigh scattering from air molecules was investigated by Chanin et al [10]. Hanson et al [30] reported on a CLR where they had frequency converted a Nd:YAG NPRO with an OPO to 3.6 μm. While not measuring wind proﬁles, they successfully measured the vibration spectra of trucks idling in a parking lot 15 m from the receiver.

1.2.7 Conclusion

This section reviewed the various reported CLR systems. A broad range of wavelengths have been used, each with their advantages and disadvantages. The 10 μm systems are capable, in general, of ranges >10 km. However, the majority of these systems had a range resolution on the order of 100’s of metres and significant pulse averaging was required to obtain velocity resolutions <1m/s. These systems also suffer from eye-safety concerns when compared to the eye-safe band systems, and the laser transmitters are large and bulky. While 1 μm systems offer the best range-velocity product, and have reported atmospheric returns from a range of 26 km in one case, the very low MPE makes 22 CHAPTER 1. INTRODUCTION 12 > 5 6 8to Maximum range (km) 20 100 Number of pulses averaged 0.72 Velocity 0.1to0.5 resolution (m/s) 30 45 Range resolution (m) 1 40 15 (Hz) PRF 500 to 1000 s) μ 0.3 0.4 0.244 0.228 Pulse duration ( 2.0 10.9 (mJ) Energy 2.0 to 3.0 1.0 to 2.0 [104] [17] [77,105] [103] et al [100–102] et al et al msystem) msystem) msystem) et al et al μ μ μ Author Table 1.5: Summary of free-space Er systems. Entries are left blank where no value is quoted in the reference. Hannon [68] m WindTracer system) Asaka (1.53 (1.55 (1.53 μ and Asaka McGrath Yanagisawa and Yanagisawa (1.62 1.3. PROJECT AIM 23

systems at this wavelength very unattractive. 2 μm systems, like 10 μm systems, have demonstrated ranges of >10 km, but have a better range-velocity product. However, averaging is required to achieve the <1 m/s velocity resolutions. Systems at this wavelength also have a higher MPE than 10 μmand1μmsystems. CLR systems at 1.53 μm include Er-doped fibre lasers and free-space Er:glass and Er:YAG lasers. While fibre systems have weight and volume advantages, the very low pulse energies limits ranges to <2 km, even when tens of thousands of averages are used. The free-space Er:glass CLR reported by Mitsubishi produces 11 mJ pulses, but with an optical efficiency of only ∼1 %. It has demonstrated ∼6 km range after averaging 20 pulses, equivalent to a ∼1 s temporal resolution. The recently reported 1.62 μm Er:YAG system produces a pulse energy of ∼3 mJ and is able to obtain returns from ranges approximately double that of the Mitsubishi system and comparable to systems at 10 μmand2μm. The combination of highest MPE with a small range-velocity product and low diffraction indicates that systems operating in the eye-safe band are the most promising for the development of deployable CLR systems.

1.3 Project aim

Our environmental monitoring collaborators indicate the need for a CLR system that has good single-pulse range and velocity resolutions and a range >1km.This would allow a good temporal resolution to be achieved, which would permit the sky to be scanned quickly. Rapid scanning speeds would allow for the possibility of accurately tracking the passage of pollutants in the sky or alerting pilots to the exact nature of the atmosphere just before they are to fly through it. The aim of this project therefore, is to develop an eye-safe, CLR system to perform Doppler measurements on the atmosphere with a single-shot to map wind fields with a range-resolved velocity resolution of ∼1.5 m/s and a range resolution of ∼75 m. This project continues on from the work of McGrath [100–102] who demonstrated that an Er:glass slave laser can produce outputs suitable for use in a CLR. However a more efficient and robust laser transmitter system was required. There- fore one of the goals of this work is to design a system that is diode pumped rather than flashlamp pumped, reducing the quantity of heat required to be removed from 24 CHAPTER 1. INTRODUCTION

the slave laser and improving the reliability. As portability is desired in any future field-deployable unit a decreased heat load allows the development of a laser head that is convection cooled, creating a compact system. The pulsed laser must be injection-seeded to produce single frequency light at a frequency slightly offset to that of a master laser. A travelling-wave resonator is an ideal arrangement for injection-seeeding as it provides natural isolation from the incoming and outcoupled beams. Therefore in this work a novel travelling-wave resonator capable of producing long duration, Q-switched, linearly polarised outputs with a good beam quality that are single frequency will be investigated. Modelling by Cariou et al [106] of a CLR system operating at 1.55 μmshowed that a pulse energy of ∼1 mJ is required to measure wind speeds at a range of 2 km. This work will therefore investigate and test an appropriately mature solid-state gain material and architecture, that is relatively inexpensive, can be diode pumped and produce outputs of >1 mJ. Though Er:YAG was a possibility, it was not a preferred option, as it was an immature technology at the start of this project. While it had been used to generate CW 1.6 μm radiation [107–109], no Q-switched outputs had been reported and the associated resonant pumping technology was either in its infancy or not available. Further objectives are the design and construction of a receiver capable of performing heterodyne detection, and the development of software capable of acquiring and processing the data. This work will investigate a dual detector arrangement to monitor the transmitted and return signals. Though more complex than a single detector arrangement, it allows greater flexibility in optimising the gain of the detector measuring the backscattered return. The final goal of this work is to perform single-pulse atmospheric measurements from which reproducible pulse to pulse directional and speed information can be retrieved.

1.4 Thesis overview

In this thesis I will describe the development and characterisation of a diode pumped Er:Yb:phosphate glass laser transmitter, and the development of a receiver system which together form a working CLR able to undertake atmospheric measurements. The structure of this thesis is a chronological ordering of the steps involved in the design, development and testing process. In Chapter 2, the choice of gain material, geometry and properties is discussed. The pumping and cooling architecture 1.4. THESIS OVERVIEW 25

of the gain medium and the slave laser head are described. Chapter 3 discusses the optimisation of the pump absorption, and measurement of the single-pass small-signal gain and fluorescence lifetime. Characterisation of the laser head while configured in a standing-wave resonator including measurements of the outcoupled energy and the thermal lens focal lengths, is also described. Chapter 4 discusses the design and performance of the travelling-wave resonator. The layout and control servo used to accomplish successful injection-seeding are described and the injection-seeding performance of the slave laser discussed. Chpater 5 describes the design of the CLR receiver and acquisition/processing software. The results from successfully measuring backscattered returns from hard and atmospheric targets are presented and discussed. Finally in Chapter 6, the performance of the slave laser and how successful the CLR system has been in performing atmospheric measurements is summarised, and potential future refinements that could be incorporated discussed. 26 CHAPTER 1. INTRODUCTION Chapter 2

The slave laser head

2.1 Introduction

In Section 1.2, it was discussed that a solid-state laser operating around 1.5 μm would be the ideal laser transmitter for a CLR. This chapter investigates the design and construction of a laser head that will be part of such a laser system. Two essential parts of any solid-state laser are the gain medium which will allow the radiation of light at the required wavelength, and the pumping mechanism by which atoms are excited into a higher energy level. The chosen gain medium host is discussed in Section 2.2, followed by an investigation in Section 2.3 into the appropriate doping ions for use with our choice of gain medium host to produce 1.5 μm radiation. The gain medium thermal loading limit is investigated in Section 2.4 and an overview of appropriate gain materials available to us from a particular manufacturer is presented in Section 2.5. The choice of the optical pumping device is discussed in Section 2.6 after which in Section 2.7 a summary of the parameters of the gain medium is presented. Section 2.8 describes the gain medium architecture and the pumping geometry. This chapter concludes in Section 2.9 with a description of the arrangement of the gain medium, pumping and cooling elements which form the laser head.

2.2 Phosphate glass gain medium host

Optical properties of the host medium such as homogeneous and inhomogeneous broadening and excited and ground state absorption cross sections are determined by the composition [110]. Fused silica is a very useful host for applications where very long ampliﬁers can be used. It has high strength, low expansion and good

27 28 CHAPTER 2. THE SLAVE LASER HEAD

optical transmission properties in addition to higher chemical stability and better mechanical properties than phosphate glasses [111, 112]. Though small silica laser devices have been shown to work [113], silica based systems suffer from the rare earth concentration being limited by concentration quenching (or “clustering”), low solubility and other undesirable effects [114]. More complex silicate glasses can over- come some of these problems but these types of glasses still have low Er stimulated emission cross sections and low laser efficiencies at 1.54 μm. Phosphate glass is a popular laser oscillator/amplifier material because it has a number of attractive properties compared to fluoride, silicate or other laser glass materials. It has a very high solubility for rare earth ions which allows the use of large concentrations of the active ion with low concentration quenching of the upper state lifetime. Phosphate glasses have a higher phonon energy (up to 1325 cm−1) than silicate glasses (1190 cm−1) [115] which enhances the decay of excited ions to the upper state of the near infra-red lasing transition. They also combine the characteristics of good chemical durability and lower upconversion and excited state absorption (ESA) losses [114]. All this results in a short pump absorption length and compact but efficient laser devices. Since glass is a disordered medium, whenarareearthionisdopedintoaglass host, there are slight differences in the sites of the ions. This broadens the energy level manifolds. Energy levels of ions in phosphate glass are broadened both by homogeneous and inhomogeneous mechanisms [115, 116]; the relative contribution of the two mechanisms is still under investigation. Thus the energy levels are Stark effect broadened, creating broad and sometimes continuous energy bands rather than narrow and separated energy levels found in crystal laser host materials. The broadening in glass can be hundreds of wave numbers (cm−1), ∼1000 times larger than in crystals [116,117]. Unfortunately phosphate glasses have a larger thermal expansion and lower fracture toughness than silicate glasses, and they are therefore more prone to damage [118]. However improvements to the thermal shock resistance (see Section 2.4) and strengthening (see Section 2.5) of this glass have improved this to allow pumping at levels that would previously have led to fracture.

2.3 Erbium dopant

Rare earth ions are an ideal choice as active ions in solid-state laser materials. Various combinations of rare earth ions and host media result in ﬂuorescence in 2.3. ERBIUM DOPANT 29

almost every region of the near infrared electromagnetic spectrum [119]. Erbium is widely used and has a lasing transition at the eye-safe 1.5 μm wavelength region. Here I review the use of erbium doping in glass and discuss its limitations. I then examine the more popular alternative, Er:Yb:glass, and discuss its advantages.

2.3.1 Er:glass

1.5 μm Er:glass lasers were ﬁrst reported in 1965, when Snitzer and Woodcock [120] 4 4 obtained laser emission from the I13/2 - I15/2 transition in silicate glass. Since the advent of using laser diodes as a pump source [121], Er:glass lasers have received renewed interest due to the signiﬁcantly reduced thermal load.

2 H11/ 2 280 ns 4 Multiphonon Excited State S3/ 2 Relaxation Transfer (lifetimes) (Absorption) 60 ns 4 F9/ 2 (1100 nm) 6ns 4 I9/ 2

2 2700 ns 4 F5/ 2 I Energy Transfer 11/ 2 (1130 nm)

8ms 4 Pump I13/ 2 (976 nm) Laser (1535 nm)

2 4 F I15/ 2 7/ 2 Ytterbium Erbium

Figure 2.1: The energy level diagram for Er:Yb:glass [111]. The structure and lifetimes of the Er levels are unaﬀected by the Yb codoping, allowing the Yb portion of the ﬁgure to be ignored when discussing Er:glass.

AsshowninFigure2.1,a1.5μm output is produced by lasing between the 4 4 I13/2 upper level and the I15/2 lower level. Due to the broadening of the energy levels, as discussed in Section 2.2, Er:glass can lase between about 1535 nm and 1552 nm. Since the lower level is the ground state, low Er concentrations are required to minimise reabsorption losses, lower the lasing threshold and to reduce upper- 30 CHAPTER 2. THE SLAVE LASER HEAD

state conversion losses [122]. Unfortunately, Er also has a low absorption cross- section at 976 nm of ∼1x10−21 cm2 [123, 124], making it diﬃcult to absorb the pump energy. To circumvent these problems, Er:phosphate glass is codoped with ytterbium (Yb) [123].

2.3.2 Er:Yb:glass

Er:Yb:glass lasers received a great deal of interest in the early 1990’s [121,125–129], and continue to do so [130–132]. Generally, the gain medium is heavily doped with Yb to produce good pump absorption and lightly doped (about 100x less) with Er. For example, the Kigre QX/Er gain medium used during the course of this work is dopedwith19Wt%Yband0.14Wt%Er. Since the absorption cross section of Yb at 976 nm is ∼1.4 x 10−20 cm2 [133,134] we can neglect the direct absorption of pump radiation by the Er ions. The pump 2 light absorbed by Yb populates the F5/2 level, as shown in Figure 2.1. 4 The stored energy is mostly transferred to the Er I11/2 level because of the 4 2 good spectral overlap of the Er I11/2 level with the Yb F5/2 level. This ensures that the excitation is rapidly and eﬃciently transferred via non radiative energy transfer.

Wu et al [114] reported that for Kigre QX/Er glass doped with 19 Wt % Yb2O3 and 0.22 Wt % Er2O3,theYb→ Er transfer reduces the fluorescence decay time of the Yb from 2 ms to ∼200 μs. This indicates an energy transfer efficiency (ETE) of ∼90 %. With my particular dopings, the Yb lifetime should decrease to ∼560 μs with an energy transfer time of ∼770 μsandsimilarlyhighETE. 4 The I11/2 excitation then decays non-radiatively and quickly (<1 μs [114]) to 4 the I13/2 level due to the high phonon energy of the phosphate glass host. Thus, back transfer from Er to Yb is negligible. Codoping Er:glass with Yb therefore produces good absorption of the pump light without large Er doping, allowing more efficient lasing at 1.54 μm. Unfortunately, loss mechanisms such as excited state absorption (ESA) [83], concentration quenching and cooperative upconversion can occur in Er:Yb:glass, particularly in Q-switched lasers. Concentration quenching is an energy transfer process that appears as a reduction in the upper state lifetime and in the fluorescence intensity. Quenching 4 between active ions in the Er I13/2 level becomes more probable the higher the Er concentration due to the increased possibility that the excited energy will undergo non-radiative relaxation which will ultimately be dissipated [135]. This dissipated energy becomes excess heat and this can also become a problem [83,136]. Concen- 2.3. ERBIUM DOPANT 31

tration quenching of the excited energy is also caused by hydroxyls (OH) and other 4 impurities due to the energy transfer between the Er I13/2 level and the impurities [135,137]. Though there is uncertainty in what is the major loss mechanism in Er:Yb:glass, it is believed that the predominant loss mechanism that limits the lasing performance is due to upconversion [134]. Upconversion loss occurs when an excited Er ion in 4 4 either the I11/2 or I13/2 state absorbs energy from an excited Yb ion. In this process, the energy absorbed by the Yb ion does not lead to an additional Er ion in the upper level of the lasing transition, and an excited Er ion is removed from that state. Upconversion produces green ﬂuorescence when:

2 3+ 4 3+ 2 3+ 2 3+ F5/2(Yb )+ I11/2(Er ) → F7/2(Yb )+ H11/2(Er ) (2.1)

2 4 3+ The H11/2 ion radiatively decays either directly, or via the S3/2(Er ) state, to the ground state, emitting light at 520 nm and 543 nm respectively [126,138,139]. Red ﬂuorescence at 650 nm can also be produced when:

2 3+ 4 3+ 2 3+ 4 3+ F5/2(Yb )+ I13/2(Er ) → F7/2(Yb )+ F9/2(Er ) (2.2)

4 where the F9/2 state radiatively decays to the ground state [126,139]. The green fluorescence is generally stronger when more heavily doped with Er 4 due to additional upconversion of F9/2 (Er) ions, which reduces the red fluorescence and increases the green [140]. 4 Upconversion therefore results in significant population loss from the I13/2 state, resulting in a loss of stored energy. To avoid upconversion losses Er dopings are consequently restricted.

2.3.3 Doping optimisation

Erbium

The Er concentration must be high enough for an eﬃcient ETE from Yb to Er and to store the energy required for output Q-switched pulses. Yet it also must be low enough to give a reasonable lasing threshold and to limit the possibility of upconversion and concentration quenching. Wu et al [141] reported that Er doping concentrations in the Kigre Er:Yb:glass 32 CHAPTER 2. THE SLAVE LASER HEAD

couldbemadeashighas7Wt%(∼6.4 x 1020 ions/cm3)1 and still have the ﬂuo- rescence lifetime of erbium remain at 8 ms. Taccheo et al [142] reported that negligible upconversion is obtained for Er concentrations lower than 3 to 10 x 1019 ions/cm3 (∼0.3to1.0Wt%)inaCW laser system. Setzler et al [83] reported that the doping concentration of Er should generally be <1x1020 ions/cm3, however it is unclear if this is for a pulsed or CW laser system. This is important as Q-switched lasers require lower doping levels of Er than CW lasers due to the much larger upper-state population.

Ytterbium

Optimising the Yb concentration entails maximising the Yb→Er transfer rate, yet maintaining a balance between efficient pump light absorption and uniform gain medium pumping. Increasing the Yb concentration results in an increased absorption of pump energy and non-radiative energy transfer from Yb to Er, thus improving the Er laser efficiency. Too high a Yb concentration will lead to a nonuniform pump profile distribution in the gain volume, and will also cause an increase in the back transfer rate from Er to Yb. Gapontsev et al [123] found that the concentration of Yb ions for Er:Yb:glass could exceed 1.5 x 1021 cm−3 (∼16.9 Wt %)2.Jianget al [143] established that for QX/Er, the Yb concentration can be >3.0 x 1021 ions/cm−3 for diode pumped lasers and that the Yb concentration in most phosphate glasses is limited due to devitrification (i.e. it becomes more prone to crystallisation). However the maximum Yb doping concentration before devitrification occurs was not specified. Taccheo et al [142] suggested a Yb upper limit concentration of <5x1021 ions/cm3.Thisnot only limited the onset of Yb - Yb energy migration followed by some quenching phenomena, it also avoided too strong a pump absorption in the gain medium region that pump light initially encounters which would then prevent the latter regions reaching inversion.

1Doping percentages of ions are expressed in either Wt % or the number of ions per cubic centimeter. For Kigre QX/Er glass, to convert from Wt % Er to Er ions/cm3 [111]: 3+ Wt% 2.9 g/cm3 2Er 6.02 x 1023 ions/mole = ions/cm3 100 382.52 g/mole

2For Kigre QX/Er glass, to convert from Wt % Yb to Yb ions/cm3 [111]: 3+ Wt% 2.9 g/cm3 2Yb 6.02 x 1023 ions/mole = ions/cm3 100 394.08 g/mole 2.4. THERMAL SHOCK RESISTANCE 33

2.4 Thermal shock resistance

The thermal loading capability of the gain medium depends on the thermal shock resistance and the percentage of the pump energy dissipated as heat in the medium. The thermal shock resistance, also known as the rupture strength or thermal loading limit, Rs is given by [119,144–146]:

σf κtc(1 − μP ) Rs = , (2.3) αteE where σf is the surface fracture stress (sometimes also denoted σmax), κtc is the thermal conductivity, μP is the materials Poisson ratio, αte is the linear coeﬃcient of thermal expansion and E is Young’s modulus. Rs is the surface fracture stress (commonly called the tensile yield stress) at which the material will fail. However, calculating Rs is not simple as σf is dependent on the surface ﬁnish. Krupke et al [147] stated that for phosphate glass σf ∼87 MPa. Using the values for the gain medium summarised in Table 2.1, gives Rs = 110 W/m.

The fracture strength of a glass gain medium is dependent on the polish quality of the surface, as surface damage limits the resulting fracture strength of the material. It is widely recognised that ﬂaws generated during the grinding and polishing steps of fabrication are primarily responsible for surface and subsurface damage in glass [145, 146]. Because of their relatively poor mechanical properties, phosphate glasses are particularly vulnerable to a phenomenon known as slow crack growth (also known as stress corrosion cracking or sub critical crack growth). This is where a crack can propagate from an existing ﬂaw at stresses less than that at which critical failure occurs [118].

The material constant Ms given by [144]:

(1 − μP )κtc Ms = (2.4) αteE is therefore used as a more common material parameter instead of Rs.Here,Pois- son’s ratio, thermal conductivity, and Young’s modulus do not change significantly with glass composition [133]. Thus, Ms is heavily dependent upon the thermal expansion coefficient. Materials with large Ms can operate well above threshold, (which is important for high efficiency), without catastrophic failure [148]. 34 CHAPTER 2. THE SLAVE LASER HEAD

2.5 Kigre Er:Yb:phosphate glass

A variety of Er:Yb:phosphate glass gain media are referred to in the literature. QE-7 is four times more efficient in lasing performance than erbium doped silicate glass [149]. Subsequently QE-7S, which is also doped with Cr3+,wasdevelopedto improve the thermal loading limit for lamp pump systems. Kigre QX/Er glass is also doped with chromium but has improved thermal conductivity and a lower thermal expansion coefficient than QE-7 and QE-7S, giving a higher thermal shock resistance [143]. To improve further the thermal loading limit, QX/Er glass is strengthened after polishing, as will be discussed later. In addition to its superior thermo-mechanical properties, QX/Er has improved athermal properties [115]. In athermal materials, the optical path length change with temperature is zero, due to the counterbalance between the refractive index dependence on temperature and the thermal expansion coefficient [133]:

αte(n − 1) = −dn/dt (2.5)

where αte is the thermal expansion coeﬃcient, n is the index of refraction at the lasing wavelength and dn/dt is the change in refractive index with temperature. For this type of material, the mode size in the gain medium is less dependent on the pump power.

Strengthening

Laser glass fails due to tensional stresses and the fractures generally originate at the surface of the glass [150]. A strengthening treatment introduces a compressive surface residual stress, increasing the amount of thermal stress required to fracture the glass. Increasing the thermal loading capability of laser glass can be achieved using methods such as tempering, ion-exchange strengthening or surface coatings. The best known method is tempering, in which the glass is cooled at a rapid rate causing the surface layer to solidify before the interior of the glass. A compressive stress is created as the interior of the glass cools [150]. However, for optical quality glass this is not an attractive method. QX based glasses are capable of being chemically strengthened via an ion- exchange process. They are comprised of a large mole percentage of Li2O. In one- for-one ion exchange when QX/Er glass is placed in a KNO3 and NaNO3 molten salt bath, the larger Na+ and K+ ions in the salt replace the smaller Li+ ions in the glass surface, and the Li+ ions migrate from the glass surface into the salt 2.6. GAIN MEDIUM PUMPING 35

bath [146, 150]. The concentration distribution of the larger ions diffused into the glass is determined by Fick’s second law [151] which predicts how the concentration of ions changes with time. The salt bath temperature has to be high enough to result in an efficient ion-exchange process while remaining low enough to prevent stress relaxation due to glass structural rearrangement at the ion-exchange surface, and corrosion of the glass surface [150]. A large Na+/K+ ratio is required since Na+ is the predominant exchanging ion. However, Na+ is quite corrosive, and in applications where the preservation of surface quality is important, a tradeoff has to be made [146]. Ion-exchange strengthening can be performed without loss of optical surface quality. A 24-hour treatment produces an ion-exchange layer on the glass surface that is approximately 10 to 20 μm thick. This layer should maintain its integrity if an air cooling system is used for the laser and the thermal stress distribution in the interior is approximately uniform [150]. Such a layer has increased the thermal load capacity of QX/Er glass to values of Rs ≈ 40 000 psi, which is a factor of 4 increase over the unstrengthened QX material [133].

2.6 Gain medium pumping

The gain medium can be optically pumped by using either flash-lamps or laser diodes, however lamp pumping has serious drawbacks. The absorption spectrum of Er:Yb:phosphate glass is shown in Figure 2.2. The broad emission wavelengths of noble gas flashlamp discharges [152] do not match the spectrum very well making flashlamp pumping inefficient and requiring high pumping levels. The low thermal conductivity and poor thermal shock resistance of the glass would thus prevent laser operation at high repetition rates [143]. a.u.

Wavelength (nm)

Figure 2.2: Er:Yb:phosphate glass absorption spectrum. This curve is a reproduction from the paper by Song et al [139]. 36 CHAPTER 2. THE SLAVE LASER HEAD

% Transmission

Wavelength (nm)

2 2 Figure 2.3: The Yb F7/2 → F5/2 transmission spectrum of Er:Yb:phosphate glass. This curve is a reproduction from the paper by Wu et al [114].

The transmission spectrum of Yb, shown in Figure 2.3, demonstrates the optimum pump wavelengths for Er:Yb:phosphate glass. Flashlamps [125, 153] have generally been replaced with InGaAs laser diodes pumping the 960 nm - 980 nm band. The more efficient conversion of pump power into excited atoms when using diode pumping significantly reduces thermal loading of the gain medium, resulting in smaller thermal wavefront distortion in the gain medium, improved beam quality [119] and enabling higher repetition rates. Additionally, flashlamps are typically liquid cooled, and laser diodes can be water or conduction cooled. Using conduction cooling rather than water cooling to remove the heat allows for a simplified laser design. Due to the broad absorption band of Yb, laser diodes of various wavelengths have been used to pump Er:Yb:glass: 935 nm [154], 954 nm [155], 959 nm [156] and 964 nm [157]. The more popular region for pumping is in the 970 nm - 980 nm region [77, 121, 126, 158–161]. ESA is negligible in the 940 nm - 980 nm pumping 4 region because these wavelengths do not match any transition from the I13/2 Er upper laser level. Pumping at 1.48 μm also avoids ESA, but 980 nm diode lasers have advantages over 1.48 μm diode lasers as they have lower noise figure, reduced temperature 3 sensitivity and higher laser efficiency [121]. However the quantum defect , ηqd,is

3The quantum defect is the ratio of the pump photon energy (pump wavelength) to the laser photon energy (laser wavelength) [162]. This gives the conversion eﬃciency from pump energy to lasing energy. 2.7. THE ER:YB:GLASS GAIN MEDIUM 37

Parameter Value Er concentration 1.28 x 1019 ions/cm3 Yb concentration 1.69 x 1021 ions/cm3 Refractive index 1.5331

Absorption wavelength, λabs 976 nm −20 2 Absorption cross section, σa 1.7 x 10 cm −20 2 Stimulated emission cross section, σe 0.8 x 10 cm Fluorescence lifetime, τEr ∼7.9 ms Emission wavelength, λe 1535 nm Quantum defect, ηqd 0.64 Thermal conductivity coeﬃcient, κtc 0.85 W/m K Poisson Ratio, μp 0.24 −6 −1 Thermal expansion coeﬃcient, αte 7.6 x 10 K Young’s modulus, E 67 GPa Density, ρ [143] 2.90 g/cm3 Surface fracture stress, σf [147] 87 MPa Table 2.1: The parameters for the QX/Er Er:Yb:glass used in this work. All values are from Kigre [133] unless otherwise referenced. larger leading to more heat being deposited in the gain medium.

2.7 The Er:Yb:glass gain medium

The QX/Er glass used in this research is QX/Er alpha 17, a Cr:Er:Yb:phosphate glass. The parameters of this material are listed in Table 2.1.

2.8 Gain medium architecture

There are a variety of gain medium architectures used in solid-state laser systems. Most can be described as either rod, slab or disc geometry, however not all of these are suitable for our particular application. Er lasers have low gain, due to the low doping required to avoid up-conversion and the small σe, thus a long path length inside the gain medium is needed. Therefore, the disc geometry, which is typically only a few hundred microns in length, is not suitable. Side-pumped rod architectures are often used for Er lasers at 1.5 μm, as they provide a longer path length and the diameter of the rod is similar to the pump absorption length. However the overlap of the mode and gain region is not optimised, and they can suﬀer from stress induced biaxial focusing and birefringence [119]. Typically these also require extra optical elements in the laser resonator to obtain 38 CHAPTER 2. THE SLAVE LASER HEAD

polarised outputs. The zigzag slab gain architecture provides longer path lengths and better overlap with the pumped gain [163]. Additionally, in the plane of the zigzag path the thermal eﬀects are averaged out, reducing the thermal focusing in that plane and the birefringence induced by thermal stress [144,164].

2.8.1 CPFS geometry

We chose to use the side-pumped coplanar pumped folded slab (CPFS) geometry [163, 165], shown in Figures 2.4 and 2.5. CPFS combines the scalability and simplicity of side pumping with the good overlap between the pumped volume and the lasing mode [165]. It has succesfully been used to produce pulsed and CW Nd:YAG lasers with diﬀraction limited outputs [163,166,167].

Laser diode Laser diode

CPFS

Fast-axis collimating lenses

Figure 2.4: Schematic of the side-pumped CPFS slab. The whole length of the slab needs to be pumped due to the three level nature of Er:Yb:glass. The fast-axis collimating lenses collimate the vertical (fast-axis) pump radiation from the laser diodes to a height, hp, in the pumped volume.

w p

h y z

x ls

Figure 2.5: 3-D schematic of the laser slab. 2.8. GAIN MEDIUM ARCHITECTURE 39

For a rectangular slab uniformly pumped and cooled through the same surface, as shown in Figure 2.6(a), the maximum thermal power per unit length that can be absorbed by a slab at the stress fracture limit is given by [119]: w P /l =12R p , (2.6) h p s h where h is the height of the slab and wp the width of the pumped region. The maximum pump power allowed before fracture, can be increased by decreasing h and increasing wp. However, then the eﬃciency of the pump absorption would decrease and the zigzag of the mode would need to be in the horizontal direction. Thus gain near the top and bottom surfaces would not be extracted.

w w p p

Pumped region Pumped region h of slab of slab hp h

(a) (b)

Figure 2.6: Pumping and cooling alternatives looking from the end view of the slab. (a) Pumping and cooling through the same top and bottom interfaces, (b) Pumping from the side and cooling through the top and bottom interfaces.

Thesolutionistouseasmallh and large wp and side pump using a collimated beam as shown in Figure 2.6(b). This also reduces the central temperature of the slab and ensures good overlap between the pumped region and the mode. However, to avoid clipping the pump beam and to minimise losses due to thermal birefringence, the height h of the slab should be greater than the height of the pump beam, hp, and large enough so that the top and bottom slab surfaces are spatially separated from the pumped region [165]. The laser mode is incident at Brewsters angle on the entrance and exit faces to provide sufficient polarisation discrimination so as to produce a linearly polarised output. The mode traverses the slab in a zigzag optical path, and is confined by total internal reflection (TIR). 40 CHAPTER 2. THE SLAVE LASER HEAD

The total pathlength in the gain region, lg,isgivenby:

2lp lg ≈ (2.7) sin(θ1) where lp is the slab length and θ1 the TIR angle. Thus the path length is double that of a standard zigzag slab [119], and nearly four times that of a straight-through slab or rod. This maximises the gain-length product, golg, improving the ease of operation of the laser. golg is the exponential gain per round-trip which needs to be large for a ring resonator with unavoidable losses. Additionally the planar nature of the zigzag path is well matched to the gain region that results from side-pumping the gain medium. Top-bottom cooling is used for the CPFS architecture. Unfortunately, the cooling through the top and bottom faces forms a thermal gradient in the vertical plane, which leads to a distributed cylindrical lens (see Section 3.6). However, since the polarisation of the mode is perpendicular to the thermal gradient, the birefringence and hence the depolarisation loss is minimised [163].

2.8.2 My CPFS laser

THE CPFS dimensions used are a replication of a DSTO design [168] and the resonator discussed in Chapter 4 was optimised around this slab. Since Er:Yb:glass is a 3-level gain medium, the lower lasing transition level is the ground state, and light emitted from the lasing transition is also reabsorbed by unpumped regions of the gain medium. To prevent such reabsorption, all parts of the slab through which the laser mode propagates must be pumped. Therefore as the slab is side-pumped its width can not be too large, but it must be made large enough to provide suﬃcient pump absorption. The pump absorptionlengthcalculatedusingσa and the Yb concentration in Table 2.1 is 0.3 mm. In practice, using the measured absorption coeﬃcient reported in Section 3.2.2 gives a pump absorption length of ∼1.3 mm. The discrepancy in absorption length is attributed to the broad linewidth of the laser diode and frequency chirp during pumping. The frequency chirp is discussed in greater detail in Section 3.2.3. The length of the slab matches that of the diode and thus it is 1 cm. The slab has a height of 2 mm. As discussed previously in Section 2.8.1, the slab height is chosen to minimise the central slab temperature but to be larger than the collimated pump light diameter (discussed in Section 2.9.2) to avoid clipping. The gain medium was initially polished by a local polisher, then returned to 2.8. GAIN MEDIUM ARCHITECTURE 41

Parameter Value

Width, wp 3.62 mm ± 0.01 mm Length, lp 10.10 mm ± 0.01 mm Parallel side length, ls 7.06 mm ± 0.01 mm Height, h 2.03 mm ± 0.01 mm Pump height, hp 1040 μm ± 50 μm (see Section 2.9.2) ◦ ◦ Nose angle, θn 11.85 ± 0.02 Total number of bounces, nb 5

Table 2.2: CPFS dimensions.

Kigre Inc for chemical strengthening. It was chemically treated to a depth of ∼20 μm and then returned to the local polisher to remove ∼5 μm from the pump and TIR surfaces. This removed surface discoloration and improved the surface ﬁnish. A two-TIR per side mode path is used for the CPFS. This required a TIR angle ◦ ◦ (θ1)of44.2 , ∼3.5 larger than the critical angle. The dimensions of the slab required to obtain the desired bounce solution are shown in Appendix B. For this design no optical coatings are required. The dimensions of the ﬁnished slab, measured using a travelling microscope and an auto-collimator, are listed in Table 2.2. This results in a total pathlength inside the gain medium of lg = 24.1 mm.

Figure 2.7: Schematic of CPFS mode. The blue region shows the maximum width of the mode.

The bounce solution inside the slab is shown in Figure 2.7. It exits the slab at ◦ ◦ an angle (θexit)of21.25 and has a separation angle (θsep)of42.5 . Transverse mode discrimination in the horizontal plane is accomplished by clip- 42 CHAPTER 2. THE SLAVE LASER HEAD

ping higher order modes on the hard apertures formed by the edges of the Brewster angled windows [163], defined by edges A1 and A1*inFigure2.7,aswellastheaper- ture created by the blunt end of the slab. This creates an effective aperture (Aeff ) for the beam outside the slab. Using the dimensions in Table 2.2, Aeff = 1.63 mm. Mode discrimination in the vertical plane is provided by the self-aperturing effects arising from the unpumped regions of the gain medium and therefore the mode is limited to the height of the pumped volume.

Gain medium loss

There are two main mechanisms that cause loss in zigzag gain media [169]. The ﬁrst is due to absorption by impurities and scatter from crystal imperfections [170].

This bulk loss, lossbulk is given by:

−α l lossbulk =1− e bl g (2.8) where αbl is the bulk loss coefficient, which for Kigre laser glass materials has a typical value of ∼0.001 cm−1 [111]. The other loss mechanism is scatter due to roughness of the TIR surfaces. Significant scatter loss caused by surface roughness can seriously degrade the performance of a glass gain medium in a zigzag slab geometry, as a small loss per TIR can quickly lead to a sizeable overall loss. Typically losses are <0.2 % per surface reflection for a good quality polished glass surface finish [111,171]. The loss due to surface scatter is [169]:

nb lossTIRscatter =1− [1 − lossperTIR] (2.9)

where nb is the number of TIR bounces and lossperTIR is the loss per TIR. The overall optical loss in the laser crystal, δ, due to absorption, scattering and reﬂection is thus expected to be:

δ = lossbulk + lossTIRscatter (2.10)

For the slab used in this work, and using the parameters listed above, δ ≤1.24 ± 0.49 %.

2.8.3 Gain medium pump diodes

Since the width of the slab, wp > the absorption length and we want a uniform population inversion, we need to pump the CPFS from both sides. We chose to use 2.8. GAIN MEDIUM ARCHITECTURE 43

Parameter Value Pulse width 5ms Pulse repetition rate 20 Hz Optical eﬃciency 45 % Spectral width ≤ 9nm Slope eﬃciency 0.8 W/A Δλ/ΔT ∼0.3 nm/K

Table 2.3: Laser diode specifications. two Thomson-CSF (now known as Thales Laser Diodes) laser diodes (Device type TH-Q5401-A1/064) with a central emitting wavelength of ∼976 nm. These diodes have a 1 cm wide single strip of emitters, and can emit a peak power of 100 W from ∼130 Amps and 2 Volt supply. The radiation emitted from these laser diode arrays has an asymmetric shape, with a FWHM beam divergence of 10◦ in the plane of the emitting junction and 34◦ perpendicular to that plane. Additional characteristics of the laser diodes are listed in Table 2.3. At a 5 ms pulse width, the maximum power is reduced to ∼80 Watts (for which the diodes require ∼100 Amps) and produces a maximum of 400 mJ per pulse, per diode. Operating the laser array at higher power increases the efficiency but decreases the lifetime [172], whereas operating at a lower output power, referred to as “derating”, helps to increase the lifetime. The overall efficiency of the pumping is affected by the frequency chirp of the pump light during the pump pulse, the duration of the pump pulse compared to the fluorescence lifetime of the upper state, and bleaching of the ground state. Measurements of the chirp will be reported in Section 3.2.3.

The storage eﬃciency, ηSt, of the pumping for a pump pulse of duration tpump is given by [119]:

[1 − exp(−tpump/τEr)] ηSt = (2.11) tpump/τEr where τEr is the fluorescence lifetime and is 7.9 ms for our material. Thus for a 5 ms pulse, the storage efficiency is 0.74. Reducing the duration of the pump pulse further would increase the efficiency, but reduce the stored energy. The low density of Er ions used in the gain medium to reduce upconversion losses limits the number of Er ions available to absorb the pump energy. This can result in all the ions being in the excited state while pumping is still occurring. This effect is referred to as bleaching. Measurement of the effect of pump bleaching will 44 CHAPTER 2. THE SLAVE LASER HEAD

be discussed in Section 3.3.1.

2.9 Conﬁguration of the laser head

The side-pumped, top/bottom cooled laser head geometry seperates the pumped and cooled surfaces. This simpliﬁes heat removal from the gain medium, as using the same surface for pumping and cooling places considerable engineering constraints on the laser head design. A schematic of the laser head is shown in Figure 2.8. The diode array and Doric lens collimating elements are in close proximity to the sides of the gain medium.

Figure 2.8: End view schematic of the side pumped, top/bottom cooled laser gain medium.

2.9.1 Laser diode mounting

Laser diode cooling and temperature stabilisation is accomplished by mounting each diode package on a copper heat sink using thin indium foil (125 μm) to improve the thermal conductivity of the interface between the laser diode and the copper block. Copper was used as the mounting block to remove heat from the diode package as it has a much better thermal conductivity (∼ 385 W/m.K) than for materials such as aluminium (∼ 205 W/m.K), brass (∼ 110 W/m.K), or stainless steel (∼45 W/m.K). The copper heat sink was then mounted using thermal grease, which aids in trans- ferring heat between the surfaces, on a Melcor thermoelectric cooler (TEC), model CP1.4-127-06L. The TEC acts as a heat pump to move the heat away from the copper block, depositing it in the aluminium heat sink. The aluminium heat sink is 2.9. CONFIGURATION OF THE LASER HEAD 45

cooled by natural convection to minimise undesireable vibration sources that could be coupled into the resonator when water cooling. A thermistor is positioned inside each copper block, as close as possible to the bottom of the diode package to monitor its temperature. This permits the use of a proportional, integration, diﬀerentiation (PID) feedback circuit to stabilise the temperature of the copper black that the laser diode is mounted on. This enables the laser diode to be temperature tuned, thereby adjusting the pump wavelength, to optimise the absorbtion of the pump light in the gain medium. This is discussed further in Section 3.2.

2.9.2 Laser diode collimation

The pump beam is fast-axis collimated to optimise the overlap of the pumped region of the gain medium with the zigzag mode, thereby minimising the threshold for the onset of lasing and increasing the laser eﬃciency [173–175]. This is achieved using Doric Gradient-index Cylindrical Lens’s (DGCL), as shown in Figure 2.9. These lenses are speciﬁcally manufactured to collimate diode lasers. The refractive index of a gradient index (or graded index) lens material decreases continuously from a higher refractive index value at the centre to a lower refractive index toward the outer edge of the lens.

Laser Diode

Doric Lens Laser Slab Pumped Region Doric Lens Laser Diode

Figure 2.9: A 3-D view of the pumping setup of the slab.

The DGCL used to collimate the laser diode packages is 1.5 mm in diameter, 25 mm in length and AR coated for 980 nm. They are manufactured with a ﬂat chamfer on the top and bottom to a height of 1.2 mm. It has a working distance (distance from diode package bar) of 0.28 mm and an expected collimated output beam height of 1.03 mm.

The height of the collimated pump beam, hp, at the slab was measured by imaging the pump light onto a CCD. A typical result is shown in Figure 2.10. 46 CHAPTER 2. THE SLAVE LASER HEAD

Figure 2.10: Image of the collimated pump light incident on the gain medium. The gap is due to imaging a piece of wire of known thickness to deﬁne the location of the conjugate plane and to obtain the magniﬁcation factor.

Figure 2.11: Collimating the laser diode using a DGCL. The DGCL is positioned in front of the laser diode using a holder placed on the same copper block as the laser diode. 2.9. CONFIGURATION OF THE LASER HEAD 47

The 1/e2 collimated pump beam heights of the two diodes are 1040 ± 40 μmand 1070 ± 40 μm, which is in good agreement with the DGCL speciﬁcations. The laser diode packages have no obvious place to aﬃx the DGCL. A holder was made and placed on the copper heatsink on which the diodes are mounted, as shown in Figure 2.11. This holder has a slight (3◦) chamfer on its end to allow its end to be aligned to the DGCL before bonding. The holder and Doric lens are then gluedinplace.

2.9.3 Laser slab mounting

Natural convection cooled Al heat sinks on the top and bottom faces of the CPFS were used to conduction cool the slab, as shown in Figure 2.12. To ensure a good

Macor ceramic Laser spacers slab

Al heatsink

(a) (b)

Figure 2.12: Mounting of the slab using Macor spacers. (a) shows the 3 ceramic spacers placed to form a plane defined by 3 points, (b) shows the ceramic spacers setting the spacing between the top and bottom heat sink, so that the crush of the indium onto the slab is reproducible. thermal contact between the heat sink and the slab, a layer of indium foil was cut to the same dimensions as the laser slab and placed on the areas of the slab that come in contact with the heat sink. Uniform crush of the indium is required to prevent the creation of horizontal or vertical thermo-refractive wedges inside the gain medium, which could refract the mode out of the gain region and lead to a decrease in efficiency. To aid in defining the amount of crush of the indium so that it was reproducible, and to prevent overtightening of the screws holding the top and bottom sections of the laser slab holder together, Macor ceramic spacers were placed between the heat 48 CHAPTER 2. THE SLAVE LASER HEAD

sinks as shown in Figure 2.12(b). Using three Macor spacers provides a three point mounting system, deﬁning a plane that can be replicated each time the slab was remounted.

(a) (b)

Figure 2.13: Top view of pumping the gain medium. (a) schematic showing the slab position in front of the diodes to enable the whole length to be pumped, (b) picture of the actual setup.

The gain medium is positioned between the two diodes so that the entire length can be pumped, as shown in Figure 2.13. The positioning of the slab was achieved by positioning it on alignment indicators pre-marked on the bottom heat sink, and looking for leakage of pump light at either end of the gain medium at low pump power.

2.9.4 Laser head

A picture of the laser head is shown in Figure 2.14. Later in this thesis, results will be shown where the laser head is used in both a standing-wave resonator (Chapter 3) and travelling-wave resonator conﬁguration (Chapter 4). The head proved to work well at low repetition rates (≤2Hz)forlong periods, but high pulse repetition rates could only be maintained for short periods. The cause of this was discovered to be that natural convection could not remove suﬃcient heat from the aluminium heat sinks attatched to the TECs. After a period of time, thermal control of the laser diodes was lost as the control servo would no longer function as designed and heat would begin to build-up in the copper blocks that the laser diodes were mounted on. For this thesis, the slave laser was operated at a 2 Hz repetition rate. 2.10. CONCLUSION 49

Figure 2.14: The slave laser head.

2.10 Conclusion

This chapter has described the design and construction of the laser head used in the laser transmitter. The gain medium is a Kigre QX/Er phosphate glass, codoped with Er and Yb, which lases at 1.535 μm. It uses a zigzag CPFS geometry, cooled via the top and bottom surfaces and optically pumped from both sides in the plane of the zigzag mode using laser diodes emitting at 976 nm. The characterisation of the laser head, using a standing-wave resonator, will be discussed in Chapter 3. 50 CHAPTER 2. THE SLAVE LASER HEAD Chapter 3

Laser head characterisation

3.1 Introduction

The development of an eﬃcient and reliable laser using the laser head described in Chapter 2 relies on adequate absorption of the pump light and reliable cooling of the slab without producing strong thermal lensing. In this chapter, I describe the temperature tuning of the diodes and measurement of the small-signal gain, in Sections 3.2 and 3.3 as a means of investigating the population inversion. In Section 3.4 I also report measurement of the lifetime of the upper excited state from which it can be determined if any detrimental quenching is occurring. The characterisation of the laser head in a standing wave conﬁguration is presented in Section 3.5, with this chapter concluding with a description of the pump induced thermal lens measurements in the slab, in Section 3.6.

3.2 Pump light absorption

The emission wavelength of the pump diodes depends on the drive current and the temperature of the laser diode package due to the ohmic heating of the junction. Thus, optimum pump absorption occurs at a diﬀerent laser diode package temperature for diﬀerent drive currents [176]. In this section I describe measurements of pump absorption as a function of diode package temperature and current, which allows the diode operating parameters for the standing-wave and travelling-wave lasers to be determined.

51 52 CHAPTER 3. LASER HEAD CHARACTERISATION

3.2.1 Background

The power of the pump light after propagating a distance z through the gain medium is given by [177,178]:

−αz P (z)=Poe (3.1) where α is the absorption coeﬃcient for the material, P (z)isthepoweratdepthz and Po is the incident power. This absorption coeﬃcient is a measure of the decrease in intensity of the incident energy as it passes through the gain medium1.

Gain medium

Po P1 P2 PA

Fresnel Fresnel reflection reflection loss loss w p

Figure 3.1: Propagation of pump light through the slab and the loss experienced by the pump light due to Fresnel reﬂections.

To obtain an accurate value for the absorption coeﬃcient, we must know the pump power just after it has entered the slab and just before it exits the slab. As illustrated in Figure 3.1, this is due to the 4.4 % loss of power caused by the Fresnel reﬂection of the pump light at the air/gain medium interface, and then again at the gain medium/air interface. The power of the pump light that has entered the gain medium P1, and the power of the pump light about to exit the gain medium P2 are thus given by:

P1 =0.956Po (3.2)

−αwp P2 =1.046PA = P1e (3.3)

The pump power after the slab, PA, is measured using the setup in Figure 3.2. An aperture was positioned so that only the uniform depth/width portion of the

1 In Equation 3.1 z = wp once the pump light has traversed fully through the gain medium. 3.2. PUMP LIGHT ABSORPTION 53

Power meter

Aperture Laser slab

Laser diode

Figure 3.2: Diagram of the setup used to determine pump absorption and to temperature tune the laser diodes. slab was analysed and to prevent refraction of pump light onto the power meter by the Brewster-angled faces. The incident power on the slab, Po, was measured by removing the slab.

The absorption eﬃciency, ηα, which gives the fraction of pump light absorbed by the gain medium, is calculated by:

P2 ηα =1− (3.4) P1

3.2.2 Results

Typical results in which the temperature of the laser diode package was tuned to achieve maximum absorption, are shown in Figure 3.3. For diode currents of ∼100 Amps and ∼110 Amps, there is no obvious sign of rollover as the laser diode temperature is decreased. This indicates that peak absorption may not have been reached, if the shape for the supply current of ∼70 Amps is representative. However, to conﬁrm this, lower operating temperatures would be necessary. Due to the danger of atmospheric water condensing on the semiconductor laser facets this was not investigated. This is of particular concern if the set point temperature is lower than the dewpoint and the temperature controller is operating without the diode lasing (and thus generating heat). When the temperature of the emitters is optimised to obtain maximum ab- 54 CHAPTER 3. LASER HEAD CHARACTERISATION

100 %

a 90 h

Absorption efficiency, ~70 Amps drive current ~100 Amps drive current ~110 Amps drive current 70 15 20 25 30 35 Laser diode heatsink temperature (oC)

Figure 3.3: Typical pump absorption curves showing the percentage of pump light absorbed by the laser crystal from a 5 ms long pump pulse as a function of laser diode heat sink temperature for diﬀerent diode currents. 3.2. PUMP LIGHT ABSORPTION 55

Author (all are et al) Wu [141] Yanagisawa [105] Hansson [161] Cai [179] Geometry Rod Square rod Microchip Microsphere Yb doping (ions/cm3) 15 x 1020 18 x 1020 Pump wavelength (nm) 975 975 975 976 Absorption % 92 >70 80 Material length (mm) 1.7 1.7 1 57 x 10−3 α (cm−1) 14.8 7.1 16.0 4-5

Table 3.1: Reported QX/Er absorption coeﬃcients. Spaces are left blank where no value was reported.

sorption for diode currents of 70 Amps and 100 Amps, ηα = 0.934 ± 0.004 and the absorption coeﬃcient, α =7.5± 0.2 cm−1. The absorption coeﬃcients for Kigre QX/Er glass reported by other authors are listed in Table 3.1. The measured value is in good agreement with those of Yanagisawa et al [105] and Cai et al [179] who use similar Yb doping concentrations.

The optical density or absorbance, Aλ, of an optical medium can also be de- ﬁned, where for a rectangular slab

P (z) Aλ = −ln = αwp (3.5) Po

Levoshkin et al [180] found that the maximum storage efficiency for a rectangular Er:Yb:glass slab, transversely pumped by collimated beams from two opposite faces, occurs for an optical density of 2.5 - 3.0, and gives a storage efficiency of ∼85 %. Using the measured value for the absorption coefficient and pumped slab width in Equation 3.5, an optical density value of ∼2.7 is obtained. This indicates that the slab should provide an optimum storage efficiency.

3.2.3 Chirping of laser diode

Pump light absorption will be aﬀected by chirping of the pump light frequency as the temperature of the juncture changes and thus the wavelength changes during the pump pulse. This was investigated by replacing the power meter shown in Figure 3.2 by a photodiode. The results plotted in Figure 3.4(a) show more rapid chirping for higher pump currents. For currents greater than ∼100 Amps, the chirp is so rapid that while the diode may emit more energy, less of it will be absorbed. Thus, the pump pulse width could be reduced with little change in the laser pulse energy. 56 CHAPTER 3. LASER HEAD CHARACTERISATION

(a)

(b)

Figure 3.4: Transmission of the laser diode pump light through the CPFS showing the frequency chirp of the laser diode during the pump pulse. (a) Using a constant diode set point temperature of 15.5◦C and varying the current to the diode. (b) Using a constant diode drive current of 100 Amps and varying the diode set point temperature. 3.3. SMALL-SIGNAL GAIN 57

Figure 3.4(b) shows that for a given current, the optimum heatsink temperature will provide the best overlap of the emission and absorption bands. For the remainder of this thesis, the maximum drive current to the laser diodes will be 83.5 Amps to increase the diode lifetime. The laser diode set point temperature is ﬁxed at ∼16◦C for all drive currents used and therefore a pulse duration of 5msisnotexcessive.

3.3 Small-signal gain

The gain factor of an amplifier, G, is the magnitude which the input intensity is amplified after a pass through the gain material. The intensity dependence of the gain factor is given by [170]: g l G =exp o g (3.6) (1 + I/Is) where go is the small-signal gain coefficient, lg is the geometric pathlength in the gain region, I is the intensity and Is is the saturation intensity. The small-signal gain coefficient is defined by [178,181]: g2 go = σe N2 − N1 (3.7) g1 where g1 and g2 are the degeneracy of levels 1 and 2 respectively, N1 is the population of the lower laser level and N2 is the population of the upper-state level, in which the total number of ions No = N1 + N2. The saturation intensity for a three level laser system is given by [170]:

hν Is = (3.8) 2σeτ where hν is the photon energy, σe the stimulated emission cross section and τ is the 2 decay time of the upper laser level. For our Er:Yb:glass material Is ≈ 100 W/cm .

For intensities Is, Equation 3.6 can be simpliﬁed to [170,181]:

golg Go =e (3.9)

where Go is the small-signal gain factor. Since the gain factor can be deﬁned in terms of the input intensity, Io, and the output intensity after passing through the 58 CHAPTER 3. LASER HEAD CHARACTERISATION

gain medium, Iout, such that [181,182]

I G ≡ out , (3.10) Io therefore Equation 3.9 can be rewritten as [181,182]:

golg Iout = Ioe (3.11)

The incident and output powers of the probe beam are thus related by:

golg Pout = Poe (3.12)

The gain was measured using the setup shown in Figure 3.5. A low power

(Is) collimated CW master laser beam, π-polarised using the PBSC, was used to determine the gain. The power in the beam was monitored using photodiode #1 to ensure that it remained constant during the measurement. Photodiode #2 was then used to measure the power incident on it before, and after passing through, the gain medium, thereby removing uncertainties associated with the calibration of the photodiode. In each instance a lens was used to focus the beam to be smaller than the photodiode size and the position of the photodiode was adjusted to intercept the whole beam and thus obtain the largest possible voltage. It is assumed that no birefringence effects in the gain medium occur. A typical voltage on photodiode #2 for a 5 ms pump pulse is shown in Fig- ure 3.6. Before the pump pulse there is no population inversion and therefore the probe beam is attenuated. The attenuation decreases as energy is pumped into the gain medium and, for a sufficiently energetic pump pulse, the probe beam is eventually amplified above the incident power. The gain provided by the gain medium is calculated by dividing the probe beam peak photodiode voltage after the slab by that before the slab.

3.3.1 Results

The gain is plotted as a function of incident pump energy in Figure 3.7. The conversion of diode current to incident pump energy is shown in Table D.1 of Appendix D. As Er:Yb:glass is a three-level system, at least half the available Er ions must be excited for lasing to occur [103, 138, 142]. Figure 3.7 shows that the incident pump energy required to excite 50 % of the ground state population, as required to 3.3. SMALL-SIGNAL GAIN 59

Figure 3.5: Schematic of the layout to measure the small-signal gain. The insert picture shows the setup to measure the power before the laser slab (Insert Position A) and after the laser slab (Insert Position B) using the same photodiode.

480 Pump pulse Probe beam power before slab 440 Probe beam power after slab

400

360

Amplitude (mV) 320

280

0 102030405060 Time (ms)

Figure 3.6: Single-pass ampliﬁcation of the probe beam during pumping with a diode current of ∼50 Amps (approximately 315 mJ total incident pump energy). Note that with the probe beam the photodiode outputs 0 V for no incident power. 60 CHAPTER 3. LASER HEAD CHARACTERISATION

1.3 o

1.2

1.1

1.0

0.9 Single-pass small-signal gain factor G

0.8 0 100 200 300 400 500 600 Incident pump energy (mJ)

Figure 3.7: Plot showing the small-signal gain factor changing as the incident pump energy is increased.

Parallel side section Nose section

Width pump region, wp 3.6 mm Width pump region, wp 3.6 mm Parallel side pump length, ls 7.0 mm Pump length nose, l-ls 3.1 mm Pump height region, hp 1.04 mm Pump height region, hp 1.04 mm ◦ Nose angle, θn 11.85 Pumped volume 0.026 cm3 Pumped volume 0.01 cm3 Total pumped volume of gain material, V 0.036 cm3

Table 3.2: Pump volume of slab.

achieve transparency (Go =1),is∼65 mJ (∼20 Amps diode current). This value is higher than the expected pump energy of ∼50 mJ, calculated using: (Er concentration)Vhνp Epump = , (3.13) 2ηα where V is the volume of the slab pumped (as given by Table 3.2) and ηα is the pump photon absorption eﬃciency (see Section 3.2). However this expected value of ∼50 mJ assumes that the temperature of the diode was optimised for this lower pump energy. In this case the temperature was set for optimum absorption at ∼570 mJ, and consequently the absorption eﬃciency is reduced. Since the voltage across the diode is constant, the temperature required to obtain optimum pump light absorption is proportional to the diode current. The 3.3. SMALL-SIGNAL GAIN 61

trace in Figure 3.3 shows that the optimum operating temperature alters by ∼10◦C for a ∼30 Amp change in diode drive current assuming that the 100 Amp peak occurs at 16◦C. Translating the 70 Amp absorption curve in Figure 3.3 by 16.5◦C is equivalent to a diﬀerence in operating current of 50 Amps. Extrapolating the curve gives the 20 Amp absorption curve shown in Figure 3.8. Thus the absorption eﬃciency at 20 Amps and 16◦Cis∼75 %, which when substituted into Equation 3.13 gives a required incident pump energy of ∼66 mJ.

Figure 3.8: Plot of extrapolated tuning curve for a 20 Amp diode current.

If all of the ground state atoms are excited to the upper state, then N2 = No and the (maximum possible) small-signal gain would be [170,183]:

σeNolg Go,max = e =1.28, (3.14) using values from Table 2.1 and Section 2.8.2. The saturation of the gain at

Go =1.23± 0.04, therefore, is consistent with bleaching of the pump absorption. Bleaching occurs when the rate of excitation of residual population in the lower level is balanced by decay via spontaneous emission and non-radiative decay via upconversion, producing the green ﬂuorescence observed in the Er:Yb:glass gain medium. 62 CHAPTER 3. LASER HEAD CHARACTERISATION

3.4 Upper state lifetime

Efficient lasing performance requires a long fluorescence lifetime of the metastable 4 I13/2 level. The degree of shortening of the fluorescent lifetime is a measure of the amount of concentration quenching and OH quenching in the laser material. To avoid OH quenching effects, the concentration of OH must not be more than 3to5x1018 cm−3 [123,137].

The lifetime of the upper energy level, τEr, can be determined from the decay of the ﬂuorescence emission, Ifl(t) at the end of the pump pulse as [178,182]:

−t/τEr Ifl(t)=Ioe (3.15) and thus

lnIfl(t)=lnIo − (t/τEr). (3.16)

Since this decay is exponential, the analysis is simplified by plotting the decay using a semi-log axis. Analysis of the fluorescence decay begins ∼5msafterthe current to the laser diode is electronically switched off. This is to avoid influences on the fluorescence decay that could arise due to the diode current turn off time and Yb to Er energy transfer that continues for a brief period after the end of the pump pulse. A typical measurement is shown in Figure 3.9, for which the slope is

-0.129 ± 0.001, and thus τEr =7.75± 0.1 ms. This is only slightly less than the value of 7.9 ms quoted in Table 2.1. This measured lifetime indicates that our gain medium has negligible concentration and OH quenching.

3.5 Standing-wave laser tests

A standing-wave resonator was aligned to the optical axis of the slab, to check energy extraction and thermal lensing during lasing. A schematic of the standing- wave laser setup is shown in Figure 3.10. Two ﬂat output couplers with parallel front and back surfaces were used so as to provide two output beams that could be used to position the irises required for alignment of the travelling-wave resonator. The standing-wave resonator alignment technique is described in Appendix E.1.1.

The separation angle, θsep, of the outcoupled standing-wave beams was measured to be ∼45.5◦ rather than 42.5◦, as calculated in Section 2.8.2. The size of the fundamental mode in the standing-wave laser is determined 3.5. STANDING-WAVE LASER TESTS 63

(a)

(b)

Figure 3.9: Determining the Er upper state lifetime. (a) The ﬂuorescence decay after the end of the pump pulse. (b) Examining the vertical axis of Part (a) on a ln scale after removing the zero-oﬀset of 0.28, to determine the Er lifetime. 64 CHAPTER 3. LASER HEAD CHARACTERISATION

Output coupler A Laser diode

Power meter

q sep

Power meter Laser diode Output coupler B

Figure 3.10: Schematic of the setup used to measure the output energy. by thermal lensing in the gain medium and the length of the resonator. When the standing-wave mirrors are positioned as close as possible to the slab, the lowest-order mode does not completely fill the gain and slab apertures, leaving unsaturated gain available for higher order mode oscillations. Positioning the mirrors further back increases the fundamental mode size at the apertures and thus increases diffraction losses for higher order modes. As will be discussed in Section 4.3.2, to obtain a fundamental Gaussian mode output, the aperture radius should be ∼1.8x the mode radius in the horizontal plane and ∼1.6x the mode radius in the vertical plane. Apertures of this size will transmit >99 % of the fundamental mode power [182], but have larger losses for higher order modes. If we assume no diffraction losses for the fundamental mode, the optimum output coupling reflectivity, Ropt, is given by [119,170]: ⎛ ⎞ g l lnR ≈−2δ ⎝ o g − 1⎠ (3.17) opt δ where δ is the slab loss, as was defined in Section 2.8.2. Using the values for the slab loss and gain from Sections 2.8.2 and 3.3.1 respectively, Equation 3.17 gives a predicted optimum output coupling reflectivity of 92.7 ± 0.5 %. Unfortunately only two output coupling mirrors were available.

Their reﬂectivity, ROC, was measured using a CW probe beam at 1535 nm, which 3.5. STANDING-WAVE LASER TESTS 65

gave ROC,1 = 97.8 ± 0.5 %, and ROC,2 = 97.4 ± 0.5 %, giving a combined effective outcoupling mirror reflectivity of ROC,tot ≈ 95.2 ± 1.0 % when used together. A typical gain-switched output pulse at an incident pump energy of ∼570 mJ is shown in Figure 3.112. The continuation of lasing after the end of the pump pulse 4 is consistent with the repopulation of the Er I13/2 lasing level due to the Yb→Er energy transfer time of ∼770 μs (discussed in Section 2.3.2). Laser spiking and relaxation oscillation behaviour occurs at various stages during the output laser pulse. Though these terms are often interchanged in laser literature, here laser spiking will refer to the sharp, large amplitude “spike” that occurs at the start of lasing action. Relaxation oscillations will refer to the smaller amplitude, exponentially damped, oscillations (shown in the insert of Figure 3.113) that the transient spike at the start of the lasing pulse evolves into, or that are created by external disturbances to the system [182]. External disturbances include fluctuations of the pump energy from the laser diodes, leading to a change in gain for the laser medium, and the mechanical vibration of the resonator components which cause variations in the optical cavity losses [119].

5 Input pump pulse Output pulse 4

Amplitude (a.u.) Insert

-202468 Time (ms)

Figure 3.11: A typical standing-wave resonator conﬁguration gain-switched output, for an incident pump energy of ∼570 mJ. The insert shows the relaxation oscillations.

2The plot is limited by digitiser noise due to the low resolution voltage setting used on the oscilloscope when obtaining the results. 3The limited sampling rate limits the resolution of the relaxation oscillations. 66 CHAPTER 3. LASER HEAD CHARACTERISATION

A plot of the output energy as a function of the incident pump energy is shown in Figure 3.12. The output energy increases linearly with the pump energy once the incident pump energy is >∼270 mJ. A slope eﬃciency of 18.3 % was achieved with a maximum output energy of ∼74 mJ from ∼577 mJ of total incident pump energy. Pump absorption bleaching as discussed in Section 3.3.1, will not occur during gain- switched lasing as the upper state population is clamped close to the transparency by the stimulated emission.

Measured output energy Fitted slope efficiency = 18.3 % 80

Total output energy20 (mJ)

0 0 100 200 300 400 500 600 Incident pump energy (mJ)

Figure 3.12: Standing-wave output energy.

During the lasing experiments green upconversion ﬂuorescence was visible in the gain medium. This, as discussed in Section 2.3.1, is a loss mechanism and decreases the slope eﬃciency. A pulsed rod laser system developed by Yanagi- sawa et al [77, 105], which used a similar QX/Er gain material from Kigre and

ROC,tot = 92 %, produced a pulsed laser slope efficiency of 20.3 %. Yanagisawa et al also used a gain material with an Er concentration 4.5x the concentration of that used for this project. Higher Er doping levels should increase the slope efficiency due to better energy transfer, however increasing the Er concentration can lead to greater upconversion losses, consequently decreasing the slope efficiency [138]. The lower slope efficiency achieved in this project is attributed to non-optimum output coupling. 3.6. THERMAL LENSING 67

Figure 3.13: Plot showing the decrease in pulse build-up time of the standing-wave gain-switched laser as a function of incident pump energy.

A plot of the pulse build-up time, ΔTPBT, as a function of the incident pump energy is shown in Figure 3.13. ΔTPBT refers to the time taken from when lasing can begin to occur until the laser pulse ﬁrst emerges from the resonator4.When a laser is gain-switched, lasing will occur once the population inversion reaches the threshold level. As expected, Figure 3.13 shows that the greater the incident pump energy, the shorter the pulse build-up time.

3.6 Thermal lensing

Pump energy not converted into laser pulse energy will produce heating of the gain medium. This heat is removed via conduction cooling of the top and bottom surfaces of the slab, resulting in a thermal gradient within the slab, inducing a thermal lens. Accurate characterisation of this thermal lens is an important step in the design of a laser resonator [119, 170, 182]. The focal length of the lens depends on if it is measured under lasing or non-lasing conditions, as less heat is dissipated in the

4In gain-switched outputs in Chapters 3 and 4 the timing begins from the start of the pump pulse. In Q-switched outputs in Chapter 4 the timing begins after the Q-switch is ﬁred at the end of the pump pulse. 68 CHAPTER 3. LASER HEAD CHARACTERISATION

slab when lasing [165, 170, 184]. Therefore for accurate focal length values, this measurement must be performed while lasing. The majority of the heat is removed through the cooled top and bottom surfaces of the slab, thus the temperature gradient is essentially one-dimensional [170]. Since the central region of the slab is pumped, it is in this region that a parabolic temperature proﬁle is formed [165, 170]. Typically the temperature distribution between the centre and the edge of the pumped region is given by [144]:

2 Qhp 1 2 T (y)= − (2y/hp) + Ta (3.18) 8κtc 3 where y is the cooling direction dimension (= ± hp/2), hp is the height of the pumped region, κtc is the thermal conductivity and Ta is the average temperature of the pumped region of the slab. Q, the heat absorbed per unit volume, is given by: (1 − η )P Q = qd p (3.19) wphplp where ηqd is the quantum defect (or Stokes factor), Pp is the absorbed pump light

[169], wp is the width of the pumped region and lp is the length of the pumped region. Outside the pumped region, the heat ﬂow produces a linear temperature gradient [165]. The focal length of the thermal lens was characterised using two techniques. A Mach-Zehnder (MZ) interferometric approach was initially used, followed by a probe beam displacement technique to verify the interferometric results.

3.6.1 Mach-Zehnder interferometer

The MZ interferometer used to measure the thermal lens is shown in Figure 3.14. Since the thermal lens was measured during lasing, the interferometer was constructed around the standing-wave laser. The π-polarised output of a HeNe laser is expanded and collimated, using two lenses, to fill the aperture formed by the edges of the Brewster angled windows defined as A1 A1* in Figure 2.7, and the blunt end of the slab. Wedge 1 is used to split the HeNe beam into measurement and reference beams. The measurement beam passes through a 33◦ angle of incidence (AoI) mirror that transmits 633 nm but reflects 1535 nm light. This mirror reflects the 1535 nm laser light outcoupled from the slave laser, preventing it from coupling into the HeNe laser. The measurement beam then passes through one of the output coupler mirrors before traversing the 3.6. THERMAL LENSING 69

Slave laser head

Output Output coupler coupler Power Beam meter dump

Imaging o lens 33 AoI D 1.5m m mirrors B l/20 mirror Wedge 2 CCD Laser line Wedge 1 camera filter A HeNe laser Mode matching lenses

Figure 3.14: The Mach-Zehnder interferometer constructed around the standing- wave laser, used to measure the thermal lens.

Er:Yb:glass gain medium, then transmitting through the second output coupling mirror and another 33◦ AoI mirror, to the second beamsplitter (Wedge 2). This second 33◦ AoI mirror prevents the 1.5 μm laser beam from damaging the CCD camera. The reference beam, after reflecting from Wedge 1, is reflected off a λ/20 HeNe mirror before recombining with the measurement beam at Wedge 2. The pathlength of the reference beam was matched to that of the measurement beam. The Mach- Zehnder interferometer is created by recombining at Wedge 2 the transmitted beam (identified by the beam path ACD in Figure 3.14) and reflected beam (identified by the beam path ABD in Figure 3.14) from Wedge 1. The recombined beam is additionally filtered using a HeNe laser line filter and then a lens is used to image the exit Brewster face onto the CCD camera (“COHU”, model 4812-7000/0000 with a Spiricon laser beam analyser, model LBA-100A). The camera is triggered to begin acquiring at the start of the pump pulse. However as the frame grab/integration period of the CCD camera is unknown, the time resolution can not be determined. The MZ interferometer was aligned to produce a zero fringe across the whole slab aperture when the laser head is unpumped. When the gain medium is pumped, horizontal fringes appear above and below the pumped region, as can be seen in 70 CHAPTER 3. LASER HEAD CHARACTERISATION

Figure 3.15(a). This indicates that a vertical refractive index gradient has been introduced in the slab between the centre of the pumped region and the cooled top and bottom surfaces.

(a) (b) (c)

Figure 3.15: Typical interferograms for a Mach-Zehnder interferometer. (a) Pumped zero tilt alignment. (b) Pumped vertical tilt alignment and (c) Pumped horizontal tilt alignment.

The introduction of a tilt in the reference arm of the MZ in either the vertical or horizontal plane, increases the number of fringes in the pumped region. Pumping can then distort these fringes as shown in Figures 3.15(b) and 3.15(c). The thermal lens value can be calculated by comparing the pumped fringe positions to the unpumped fringe positions. The location of the peak of each fringe in the recorded interferograms was analysed using an IDL program written by Mudge [169] and described by Hosken [176]. Figure 3.16 shows a typical vertical proﬁle showing the phase shift of the wavefront when the slab is pumped compared to when the slab is unpumped. The gain medium can be treated as a lens duct where the index of refraction has a quadratic variation in a radial direction, r, to the optical axis, given by [182,185]:

1 2 n(r, d)=n − n2(d)r (3.20) o 2 where the n2 parameter is given by

∂2n(r, d) n2(d) ≡− | =0 (3.21) ∂d2 r 3.6. THERMAL LENSING 71

and n2 γ ≡ , (3.22) no when γ is determined from the wavefront distortion. Comparing the ray transfer matrix for a lens duct with that for a thin lens gives the effective focal length of a stable duct, fduct given by: 1 fduct = (3.23) noγsinγd By fitting a parabolic curve to the pumped region, and using the above equations, the effective focal length of the induced thermal lens in the vertical and horizontal planes can be determined. This process is described in detail by Hosken [176].

1.0

0.5

0.0 Pumped region

-0.5

-1.0 Location in slab (mm) 0.0 0.5 1.0 1.5 2.0 Wavefront distortion in waves at 633 nm

Figure 3.16: Typical vertical plane wavefront distortion measurement in the pumped slab when using a tilted reference wavefront. The thermal lens is formed in the central pumped region.

Vertical lens results

A typical parabolic fit to a vertical lens profile is shown in Figure 3.17. The measured thermal lens for different incident pump energies is shown in Figure 3.18. As expected, increased pump energy results in stronger thermal lensing in the vertical plane. For typical incident pump energies, the vertical thermal lens value ranged from 30 - 60 cm with errors ranging from ± 1to2.5cm.

Horizontal lens results

Ideally, in the plane of the zigzag path there should be no wavefront distortion due to the change in refractive index with temperature [119]. In practice however, tem- 72 CHAPTER 3. LASER HEAD CHARACTERISATION

Measured wavefront 0.6 Fitted parabola 0.3

0.0

-0.3

Locationinslab(mm) -0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Wavefront distortion in waves at 633nm

Figure 3.17: A typical parabolic ﬁt to the measured vertical proﬁle data in the pumped region.

0.6

0.5

0.4

0.3 Measured focal length (m) 200 300 400 500 600 Incident pump energy (mJ)

Figure 3.18: Measured vertical thermal lens focal lengths for various incident pump energies. 3.6. THERMAL LENSING 73

perature non-uniformities across the slab width lead to weak wavefront distortions. Thus a weak thermal lens in the horizontal plane is expected. The limited number of fringes obtainable for analysis, combined with the small change in the pumped fringe position, produces reduced conﬁdence in the parabolic ﬁt, as a typical result in Figure 3.19 shows.

0.20 Measured wavefront 0.15 Fitted parabola

0.10

0.05

0.00 Waves at 633 nm -0.05 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Locationinslab(mm)

Figure 3.19: A typical parabolic ﬁt to the measured horizontal fringe position in the pumped region.

Figure 3.20 shows that the thermal lens is essentially independent of the pump energy, with an average value of ∼3m.

0 Measured focal length200 (m) 300 400 500 Incident pump energy (mJ)

Figure 3.20: Measured horizontal thermal lens focal lengths for various incident pump energies. 74 CHAPTER 3. LASER HEAD CHARACTERISATION

3.6.2 Probe beam displacement technique

The purpose of this measurement is to observe the behaviour of the thermal lens as a function of time. In this measurement technique, a collimated HeNe beam is passed though the gain medium and changes to the beam waist position caused by the thermal lens are analysed [186]. A schematic of this technique is shown in Figure 3.21.

Lens1&2, Lens 3,

focal length, f1 focal length, f2 HeNe Slave laser laser CCD D camera 4f1 z’+ z’

Figure 3.21: Block diagram of the beam displacement technique for measuring the thermal lens.

A collimated HeNe probe beam is incident on the gain medium. The beam at the output Brewster face of the slab is imaged onto the principal plane of lens 3 using lens 1 and 2. The beam is focussed to a waist by lens 3 and the unpumped position of the waist, z , which is approximately a distance f2 from lens 3, is located using a CCD camera. The change in longitudinal position Δz of the waist when the gain medium is pumped is then measured. The induced thermal lens focal length, fth, can be found for weak focusing (Δz z ) using [186]:

−z2 f (3.24) th Δz for πR 2 1 (3.25) λωm

where R and ωm are the wavefront curvature and beam radius of the probe beam at lens 2. The setup used to measure the thermal lensing in the CPFS using this technique is shown in Figure 3.22. Measurements were performed with the laser lasing in a standing-wave resonator conﬁguration. A collimated HeNe beam was shaped so that it ﬁlled the region of the gain medium where the laser mode oscillates [165]. The same CCD camera as that used in the MZ technique determined the waist 3.6. THERMAL LENSING 75

Slave laser head

Beam Beam dump dump

o Mode matching 33 AoI lenses 1.5m m mirrors Lens focal length f HeNe 1 laser

Laser line filter

Lens Lens CCD focal length f focal length f camera 2 1

Figure 3.22: The beam displacement setup used to measure the thermal lens. 76 CHAPTER 3. LASER HEAD CHARACTERISATION

position. The temporal behaviour of the induced thermal lens can be investigated by triggering the CCD camera to acquire data with a variable time delay after the start of the pump pulse.

Results

The measured vertical and horizontal thermallensfocallengthsfordiﬀerentinci- dent pump energies and diﬀerent time delays using the probe beam displacement technique are plotted in Figures 3.23 and 3.24.

Pump pulse 1.4 Incident energy ~265mJ Incident energy ~415mJ 1.2 Incident energy ~577mJ 1.0 0.8 0.6 0.4

Vertical thermal0.2 lens (m) 0 50 100 150 200 Time (ms)

Figure 3.23: Measured vertical thermal lens focal lengths at diﬀerent time delays after the start of the pump pulse, and for diﬀerent incident pump energies. Lines are drawn between the measured values to help guide the eye.

Figure 3.23 shows a shorter thermal lens focal length during, and shortly after, the time the gain medium is pumped. However this is still in reasonable agreement with results obtained using the MZ interferometer in Section 3.6.1, since the MZ technique records an average thermal lens value after the start of the pump pulse.

For a comparable incident pump energy of ∼577 mJ, fth ≈ 0.28 ± 0.01 m was measured at the end of the pump pulse. This is compared to 0.32 ± 0.01 m using the MZ technique. The measured horizontal thermal lens is plotted in Figure 3.24. Similar to that for the vertical lens, the focal length at the end of the pump pulse is stronger than the average focal length measured using the MZ interferometer. However, fth is still ∼3 m and thus agreeable with the results obtained in Section 3.6.1. 3.7. CONCLUSION 77

8 Pump pulse Incident energy ~415mJ Incident energy ~577mJ 6

2 Horizontal thermal lens (m) 0 50 100 150 200 Time (ms)

Figure 3.24: Measured horizontal thermal lens focal lengths at diﬀerent time delays after the start of the pump pulse, and for diﬀerent incident pump energies. Lines are drawn between the measured values to help guide the eye.

3.6.3 Summary

The thermal lens during lasing conditions was measured using an interferometric Mach-Zehnder technique and a probe beam displacement technique and was found to be essentially one-dimensional. Both techniques showed that there was a strong vertical thermal lens with the focal length dependent on the incident pump energy. The greater the incident pump energy, the shorter the focal length of the lens that is formed. In the horizontal plane, the MZ technique showed that the thermal lens was weak and essentially independent of the incident pump energy, whereas the probe beam displacement technique indicated a small pump energy dependence. In both the vertical and horizontal planes, the thermal lens focal length values obtained using the MZ technique agreed with those measured between 0 secs and ∼20 ms after the start of the pump pulse using the probe beam displacement technique.

3.7 Conclusion

The measurements reported in this chapter demonstrate that the laser head can be used to produce an eﬃcient and reliable laser. Temperature tuning of the pump diodes results in a maximum pump absorption of 93.4 %. By probing the gain region of the slab, the small-signal gain factor and the upper state lifetime were measured to be 1.23 and 7.75 ms respectively. Evidence of bleaching of the Er ions was also observed. 78 CHAPTER 3. LASER HEAD CHARACTERISATION

Integrating the laser head into a standing-wave resonator was investigated and discussed. The maximum output energy achieved was 74 mJ from an incident pump energy of 577 mJ, with a slope efficiency of 18.3 %. This is similar to the slope efficiency published on the Mitsubishi system [77, 105]. Thermal lensing was characterised while lasing in the standing-wave resonator configuration. Focal lengths of 0.28 - 0.60 m and 2.8 - 4.1 m were measured in the vertical and horizontal planes respectively. A stronger dependence of the thermal lens focal length on incident pump energy was observed in the vertical plane than for the horizontal plane. The thermal lens values measured in this chapter will be used in the following chapter where the design and performance of the travelling-wave resonator will be discussed. Chapter 4

Travelling-wave slave laser

4.1 Introduction

While a standing-wave resonator is simple to align, efficient and useful for initial characterisation of laser properties including gain, optimum outcoupling, and thermal lensing, it is not ideal as an injection-seeded single frequency source. Lasing in a standing-wave resonator usually results in spatial hole burning in the gain medium, leading to the oscillation of multiple longitudinal modes [170]. Therefore to obtain single longitudinal mode operation, spatial hole burning needs to be prevented, as is achieved with a travelling-wave (or ring) resonator where the eigenmode is a travelling-wave. A travelling-wave resonator is also ideal for injection-seeding and this is required in a CLR system in order that the output pulses are at a frequency close to that of the CW master laser (also known as the local oscillator). In a standing-wave resonator configuration, laser light from the slave resonator can propagate back to the master laser, leading to difficulties in providing sufficient isolation for the master laser from this optical feedback. Travelling-wave resonators naturally provide separation of the injected and outcoupled beams, significantly reducing the amount of isolation needed to protect the master laser. One disadvantage of a travelling-wave resonator is that the alignment is more complex, especially when additional intracavity components are required. Travelling- wave lasers are also more sensitive to resonator losses as the gain medium is only traversed once per round-trip, reducing the available gain and requiring the use of lower outcoupling fractions [182,187]. However, these disadvantages are outweighed by the advantages provided by using a travelling-wave resonator.

79 80 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

4.2 Objective

The goal of this chapter is to describe the development of a injection-seeded slave laser and report on its performance. This chapter will begin by describing the design of the travelling-wave resonator in Section 4.3, which uses many of the results reported in Chapter 3. Gain-switched and unseeded Q-switched operation will be discussed in Sections 4.4 and 4.5. The chapter will conclude with a description of the injection-seeding in Section 4.6.

4.3 Travelling-wave resonator

4.3.1 Overall resonator layout

The slave laser used in this project uses a Q-switched travelling-wave resonator. In a Q-switched laser, lasing is prevented by lowering the cavity Q during pumping, allowing the population inversion to reach a value larger than the threshold for gain- switched lasing. When the stored energy reaches the desired level, the resonator is suddenly switched to a high Q, for which the loss is less than the round-trip gain, and thus lasing occurs. The intensity in the resonator builds up quickly, producing a very short, intense pulse of light. To achieve precise single-shot wind velocity measurements with a CLR, the transmitted pulse width needs to be relatively long for transform-limited velocity resolution. A long resonator will aid in obtaining long output pulse widths, but long resonators are more sensitive to vibrations and misalignment. The travelling-wave resonator used has a “bowtie” shape as shown in Fig- ure 4.1. This allows for a long, yet compact, resonator to be constructed, and allows coarse adjustments of the resonator length to be made without changing the position of resonator optics, except for the mirrors. Fine adjustment of the resonator length, which is needed when tuning the frequency of the longitudinal modes, is achieved by using a piezoelectric transducer (PZT) glued to one of the resonator mirrors. This resonator design is an even mirror cavity. Misalignment of one mirror will not modify the direction of the outcoupled light, however the outcoupled laser energy will be reduced [188]. Maintaining the outcoupled beam direction is important as it will remain aligned to the CLR receiver even if a slight misalignment of the resonator occurs. Due to the low gain of Er:Yb:glass, it is especially important that the number of components in the resonator be minimised. The travelling-wave resonator shown 4.3. TRAVELLING-WAVE RESONATOR 81

PZT RW 33o AoI Image-relay mirror 1 lens 2 PBSC 33o AoI mirror 2 FW Image-relay HWP Legend lens 1 Pockels cell (Q-switch) FW = Forward-wave FW RW = Reverse-wave o AoI = Angle of incidence 0.5 wedge HWP = Half-wave plate at Brewsters RW PZT = Piezoelectric angle transducer PBSC = Polarising beam Laser splitter cube head

Figure 4.1: The slave laser conﬁguration. in Figure 4.1 contains a HWP to rotate the polarisation of light by 90◦, a relay imaging telescope [182] to control the mode size inside the resonator, and an intracavity wedge which outcouples a beam that is used to produce an error signal for a control servo when injection-seeding. An Inrad PBC06-DC04/1535 Pockels cell, which contains 2 beta barium borate (BBO) electro-optic crystals, is used as the Q-switch. BBO was chosen as it has low insertion loss, a high damage threshold and exhibits no piezoelectric ringing [77, 189, 190]. The losses for the resonator components are summarised in Table 4.1. The Pockels cell controls the Q of the cavity by rotating the polarisation of light. This occurs when an external voltage is applied to the Pockels cell, producing a change in the refractive index of the crystals and a resultant polarisation rotation [119]. The control circuitry for the Pockels cell was designed and built with the help of Mr Neville Wild and Mr Bob Nation at The University of Adelaide. A description of this circuitry is found in Appendix F.

4.3.2 Modelling the travelling-wave resonator

The travelling-wave resonator design was modelled using the Paraxia 2.0 ABCD program [191] which is a laser resonator and beam propagation analysis software package. The ABCD matrix analysis technique describes the propagation of a Gaus- sian beam through an optical system [170,182] and allows the horizontal and vertical mode sizes and the resonator round-trip stability to be ascertained. 82 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

Resonator component Loss % per pass Pockels cell (measured) 4.0 ± 1% Polarising beam splitting cube (PBSC) 0.5 % Half-wave plate (HWP) 0.5 % Uncoated wedge (@ Brewsters angle) 0.5 % Laser mirror 1 0.5 % Laser mirror 2 0.5 % Image-relay lens 1 0.1 % Image-relay lens 2 0.1 % Total expected loss 7 ± 1%

Table 4.1: Component losses for the travelling-wave resonator. Except for the measured loss of the Pockels cell, the component losses are obtained from the manufac- turers speciﬁcations.

Mode size requirements in the slab

To obtain a TEM00 mode, intracavity apertures can be used to block or attenuate higher order mode oscillations. However, there can be a substantial decrease in output energy, compared with multimode operation [119,182].

To obtain minimal TEM00 mode loss when using a hard aperture of radius ar, the mode size should be appropriately matched to the aperture. For low gain laser systems, such as Er:Yb:glass, the optimum Gaussian beam radius, wm,isgiven by [182]:

2ar ≈ (3.5 − 4.0)wm (4.1) where ar is the radius of the mode control aperture. For the gain medium, the hard aperture at the Brewster windows in the horizontal plane has diameter Aeff = 1.63 mm,

(Section 2.8.2). Substituting Aeff =2ar into Equation 4.1 gives:

A w ≈ eff ≈ 435 μm (4.2) m 3.75

In the vertical plane, the gain distribution creates a soft aperture with a full 2 width at 1/e = hp ≈ 1040 μm, as discussed in Section 2.9.2. For soft apertures in four-level laser systems, wm ≈ ar [119]. Since the mode discrimination provided using an aperture of this size is less than that for a hard aperture in a low gain system, an increased discrimination with only small reabsorption losses for the fundamental mode is required. An aperture size at the Brewster windows that allows ∼99 % of the Gaussian beam energy to pass is given by [182]:

2ar = πwm (4.3) 4.3. TRAVELLING-WAVE RESONATOR 83

and thus required that

wm ≈ hp/π ≈ 330 μm, (4.4) where hp =2ar, giving an aperture diameter of ∼1 mm.

4.3.3 Paraxia resonator design

The resonator design needs to satisfy the mode size requirements described in the previous section with minimal sensitivity to changes in the thermal lens focal lengths. The range of thermal lens focal lengths considered are 0.28 m - 0.45 m in the vertical and 2.5 m - 4.1 m in the horizontal planes, as determined from Section 3.6. The resonator must be optically stable over these ranges of thermal lens focal length.

L2 L5 L4 D3 D2

D1 L1 D5

L3 c c*

Figure 4.2: Deﬁnition of path lengths for the resonator design.

For a multielement stable laser resonator design, the Gaussian mode size and resonator stability can be investigated by multiplying the ABCD matrices for the individual elements, to obtain an overall round-trip ray matrix. The resonator stability can be found using the ABCD matrix half trace. The half trace of the overall ABCD matrix can be deﬁned as [182]:

A + D m = (4.5) 2

Asystemwith-1

and be maintained within the system. To obtain the least sensitivity of mode size A+D ≈ change to variations in thermal focusing, 2 0.5 is required [182]. The half trace stability can be deﬁned in terms of the resonator g parameters g1 and g2. These unitless parameters are given by [182,192]:

L g1 ≡ 1 − (4.6) R1

L g2 ≡ 1 − (4.7) R2 where R1 and R2 are mirror curvatures and L is the distance traversed. In terms of the g parameters, to meet the stability condition of Equation 4.5 results in the relation:

The Gaussian mode size in the travelling-wave resonator is not only controlled by thermal lensing, but by a combination of additional factors that include the resonator apertures, the overall resonator length, and the image-relay magniﬁcation. As a long resonator aids in producing the required long duration output pulses, having a “bowtie” design, as discussed in Section 4.3.1, allows a compact design, and adjustments to be made to the overall length with minimal disturbance of intracavity optics.

The “bowtie” resonator can be thought of as two triangles, as shown in Figure 4.2. The triangle of sides L1, L2 and L3 contains all the intracavity optics (excluding the resonator mirrors), while the smaller triangle of sides L4, L5 and L6 is used for resonator length adjustment.

◦ The length L1, combined with the slab separation angle, (θsep), of 45.5 ,sets the lengths of L2 and L3. L1 needs to be long enough to allow all the intracavity optics to ﬁt in the main triangle, but as short as possible to maintain a compact resonator. The image-relay system controls the mode around the resonator by imaging the mode waist formed by the thermal lens at χ, (a distance L3-(D5+D4) from the Brewster window), to the corresponding waist at χ*, (a distance L1-D1 from the other Brewster window). Since the lengths D5 and D1+D2 equals the focal length of the image-relay lenses, then L4, L5 and L6 are adjusted (whilst maintaining the desired AoI’s) such that D4+L4+L5+L6+D3 gives the required magniﬁcation. 4.3. TRAVELLING-WAVE RESONATOR 85

Results

The parameters that result in a solution with the required mode sizes and best stability for the desired range of thermal lens values are listed in Table 4.2. The length D5 is an optical length rather than the geometric length as it includes ∼6.8 cm of travel through the BBO Pockels cell. The focal length of the image-relay lenses was chosen so that the overall resonator length was as short as possible, but long enough to form the smaller triangle required for the “bowtie” resonator.

Component Value Laser resonator mirror curvatures Flat Focal length of image-relay lenses 0.3085 m Imaged waist distance from Brewster windows 0.058 m D1 0.240 m D2 0.069 m D3 0.235 m D4 0.036 m D5 (optical length) 0.3085 m L4 0.125 m L5 0.096 m L6 0.125 m

Table 4.2: Resonator model parameters. The lengths correspond to those in Fig- ure 4.2.

The mode size at the Brewster windows and the overall stability over the range of thermal lens effective focal lengths (EFL) are shown in Table 4.3. The mode sizes satisfy Equations 4.2 and 4.4 at the mean thermal lens values. Increased thermal lens focal lengths, which occur for decreased incident pump energies, lead to increased mode sizes at the apertures which may lead to a decrease in output energy as some of the fundamental mode might be clipped. At decreased thermal lens focal lengths the mode sizes are slightly smaller than the requirements, leading to the potential for higher order mode oscillations. The design configuration in the horizontal plane is near the optically stable region limit, and therefore it can be very sensitive to optical and mechanical fluctuations [119]. Though the stability could be improved by adjusting the resonator length, imaging the mode further from the Brewster windows results in an increase in mode size in one plane (leading to clipping of the fundamental mode), and a decrease in mode size in the other (resulting in multimode operation in that plane). Therefore a compromise is made between mode size and stability. The resonators overall half-trace stability is as far from the stability limit as possible, while still 86 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

Plane Thermal lens Predicted mode size Predicted overall EFL (m) at Brewster face (μm) half trace stability Horizontal 3.3 447 0.9601 Vertical 0.365 326 0.8305 Horizontal 3.3 447 0.9601 Vertical 0.28 306 0.7793 Horizontal 3.3 447 0.9601 Vertical 0.45 342 0.8624 Horizontal 2.5 417 0.9469 Vertical 0.365 326 0.8305 Horizontal 4.1 470 0.9675 Vertical 0.365 326 0.8305

Table 4.3: Modelled mode size and stability results. satisfying the mode size requirements to achieve a Gaussian mode output in both planes. The length of the resonator also determines the separation of the slave lasers longitudinal modes (free spectral range, FSR). The FSR is given by [185]:

c FSR = , (4.9) P where P is the round-trip optical length. Combining the lengths in Table 4.2 with the distance the mode travels in the slab, 24.1 mm (from Section 2.8.2), gives P =1.37m and therefore a FSR = 219 MHz.

4.4 Gain-switched output

When gain-switching the travelling-wave resonator the Pockels cell serves no function, however the resonator requires some polarisation control to set an appropriate outcoupling fraction. This was achieved by inserting a HWP between the Pockels cell and the intracavity wedge, as shown in Figure 4.3. The energy outcoupled in the forward-wave (FW) direction was observed on a power meter and the wave plate adjusted to maximise this energy. Replacing the power meter with a photodiode allows the pulse shape of the gain-switched output in the forward-wave direction to be observed. Figure 4.4 shows a typical output for an incident pump energy of ∼577 mJ. By comparing the pulse build-up time with that for the standing-wave resonator results in Section 3.5, it is clear that the pulse build-up time for the travelling- wave is longer for the equivalent pump energy. This is expected as the travelling- 4.4. GAIN-SWITCHED OUTPUT 87

PZT 33o AoI RW mirrors Image-relay lens 2 PBSC

FW Image-relay HWP Legend lens 1

FW = Forward-wave Pockels cell HWP RW = Reverse-wave (Q-switch) FW HWP = Half-wave plate AoI = Angle of incidence BAW BAW = Brewster-angled RW wedge PBSC = Polarising beam Laser splitter cube head

Figure 4.3: Schematic of the slave laser travelling-wave resonator for gain-switched operation.

2.5 Pump pulse Gain switched output 2.0

1.5

1.0

0.5 Amplitude (a.u.) 0.0 02468 Time (ms)

Figure 4.4: A typical gain-switched output pulse for the travelling-wave resonator. 88 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

wave resonator has additional components, increasing the overall round-trip loss, and the gain is traversed only once per round-trip. Consequently it takes longer to reach the lasing threshold. The sum of the forward and reverse-wave energies is shown in Figure 4.5. The maximum combined energy outcoupled by the PBSC is 22.0 ± 0.4 mJ for an incident pump energy of 577 mJ1, with a slope efficiency of ∼5.6 %. This slope efficiency is higher than the ∼2 % reported by Wu et al [190] for a gain medium with an erbium doping similar to that used here. Yanagisawa et al [77,105] and Wu et al [141] reported higher gain-switched travelling-wave resonator slope efficiencies of 7 % and 9.7 % respectively, however the erbium dopings of the gain media used in these experiments were 4.6x and 5.0x higher than the doping levels used in this project. They did not report on how intracavity resonator components may affect their slope efficiencies. As discussed previously in Section 3.5, higher dopings should lead to a better transfer efficiency of energy from Yb to Er. However higher doping concentrations can also lead to a potential increase in upconversion losses and raises the lasing threshold. The results are thus not directly comparable, and insufficient details are published to allow a detailed evaluation of the differences. The intracavity wedge in the resonator also outcouples light in both the forward and reverse-wave directions. The total energy outcoupled by the wedge is not insignificant, and when combined with the energy outcoupled by the PBSC gives an overall outcoupled energy of 25.6 ± 0.4 mJ for 577 mJ of pump energy, with a slope efficiency of 6.5 %. As was shown in Table 4.1, the Pockels cell had the largest loss of all the components in the resonator. When lasing, beams are reflected by the Pockels cell which reduces the energy outcoupled by the PBSC. As expected, removal of the Pockels cell from the resonator leads to a significant increase in the gain-switched energy, as shown in Figure 4.6. By combining the forward and reverse-wave outputs from the PBSC, a total of 37.9 ± 0.8 mJ from 577 mJ of pump energy was achieved, with a slope efficiency of 8.9 %. This represents ∼50 % of the energy outcoupled in the standing-wave (Section 3.5). It was subsequently discovered that the Pockels cell introduced a slight wedge. Since the travelling-wave resonator output energy was not optimised after the Pockels cell was removed, potentially energies greater than 37.9 mJ could be achieved. By including the energy reflected by the intra-cavity wedge, the total

1This pump energy is the maximum possible due to no recalibration being performed between when the laser diodes were temperature tuned to when this measurement was taken. 4.4. GAIN-SWITCHED OUTPUT 89

5 Total energy outcoupled (mJ)

0 Measured gain-switched output energy (mJ) Fitted slope efficiency = 6.5 %

100 200 300 400 500 600 Incident pump energy (mJ)

Figure 4.5: Total combined forward and reverse-wave gain-switched energy outcoupled by the PBSC and wedge when using the travelling-wave resonator.

10 Total energy outcoupled (mJ)

0 Measured gain-switched output energy (mJ) Fitted slope efficiency = 10.2 %

100 200 300 400 500 600 Incident pump energy (mJ)

Figure 4.6: Total combined forward and reverse-wave gain-switched energy outcoupled by the PBSC and wedge when using the travelling-wave resonator but with the Pockels cell removed. 90 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

energy from the travelling-wave resonator was 43.6 ± 0.8 mJ, when pumped with 577 mJ, with a slope eﬃciency of 10.2 %.

4.5 Unseeded Q-switched output results

The Q-switch driver is triggered by the falling edge of the pump pulse and applies a voltage, Vapplied, to the Pockels cell for a period of ∼25 μs. The outcoupling fraction is given by [119]:

π V outcoupling fraction = 1 − sin2( applied ) (4.10) 2 V1/2 where V1/2 is the half wave voltage of the Pockels cell. For the BBO Pockels cell used in this project, V1/2 = -6.88 kV [193].

0.4 End of pump pulse Unseeded Q-switched output pulse

0.3

0.2

Amplitude (a.u.) 0.1

0.0

01234 Time (ms)

Figure 4.7: Unseeded Q-switched output pulse.

The optimum output coupling fraction was determined by adjusting Vapplied until the maximum energy was outcoupled by the PBSC in the forward-wave direction. This occurred at a voltage of -6.44 kV. A typical unseeded Q-switched output pulse is shown in Figure 4.7. However in practice, it was diﬃcult to determine accurately the actual output coupling fraction as the voltage applied to the Pockels cell decreased slowly over 4.5. UNSEEDED Q-SWITCHED OUTPUT RESULTS 91

time (∼800 V over the ∼25 μs the Q-switch was open), as the capacitor that held the voltage discharged slowly. Figure 4.8 gives an example of the time dependence of the voltage applied to the Pockels cell. Thus, the optimum output coupling fraction for maximised outcoupled energy was 2 ± 1%.

1 Voltage applied to Pockels cell 0 Unseeded Q-switched output pulse

-1

-2

-3

-4 Voltage (kV) -5

-6

-7

0 5 10 15 20 25 30 35 Time (ms)

Figure 4.8: An example of the voltage applied to the Pockels cell by the Q-switch driver for a -6.6 kV setting, showing the slowly varying voltage over time. An unseeded output pulse has been placed in the graph to aid in determining the ap- proximate Vapplied at the start and end of the output pulse.

The total energy outcoupled by the PBSC as a function of incident pump energy for optimum output coupling is plotted in Figure 4.9. More energy was outcoupled through the PBSC in the forward-wave direction than in the reverse- wave direction. This result is expected due to the extra losses the reverse-wave experiences in comparison to the forward-wave when passing through the Brewster faces of the gain medium after Q-switching. This is due to the light in the forward- wave direction being π-polarised when passing through the Brewster windows of the gain medium. However in the reverse-wave direction, the light is a mixture of π- and σ-polarised light at the Brewster windows and some of the σ-polarised light is reﬂected by each Brewster face. The observed rollover in the output energy as the incident pump energy increases in Figure 4.9 is consistent with bleaching of the pump absorption, as was discussed in Section 3.3.1. The apparent increase in pump energy at which the 92 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

2.5

2.0

PBSC (mJ) 1.5

1.0

0.5

Total energy outcoupled0.0 by

100 200 300 400 500 600 Nominal incident pump energy (mJ)

Figure 4.9: Typical unseeded Q-switched output energies. These relative energies are credible, however the absolute calibration of the incident pump energy at this time in the project is unknown due to laser diode degradation.

bleaching rollover occurs is attributed to degradation of the laser diodes (discussed in Appendix D), leading to a signiﬁcant reduction in the pump energy absorbed in the gain medium for a given diode drive current. The incident pump energy axis is therefore no longer calibrated and thus the slope eﬃciency cannot be accurately determined. The percentage the incident pump energy has reduced from the original nominal incident pump energy is not known. However comparing the pump energies in Figure 3.7 and Figure 4.9 where rollover due to bleaching occurs, the decrease could be as much as 50 %. Since there appeared to be pump bleaching, and since 2 mJ/pulse was adequate for the work described later in this thesis, we did not replace the diodes but continued on to demonstrate injection-seeding and obtain proof of principle atmospheric results.

Although the conversion of gain-switched pulse energy to Q-switched pulse energy obtained is low, Yanagisawa et al [77] have shown that ∼50 % of the gain- switched pulse energy can be extracted in a Q-switched pulse, if the pump absorption does not bleach. This even applies when using Er dopings of 6 x 1019 ions/cm3,for which losses due to upconversion should be larger. 4.5. UNSEEDED Q-SWITCHED OUTPUT RESULTS 93

Beam quality

The outcoupled beam is elliptical in shape due to the stronger vertical thermal lens in the gain medium. To study the transverse mode characteristics of the laser output, the beam quality of the output was measured, as represented by the beam quality factor, M 2 [119,170,194]. The beam quality in the forward-wave direction was measured as this is the outcoupled beam direction when the slave laser is injection seeded. M 2 measurements were taken by focusing the forward-wave outcoupled beam with a lens to create a new beam waist. If the lens is diffraction limited, the new waist and the original laser beam have the same M 2 [170], and thus the diameter of the beam can be measured at various places at and around the new beam waist. The fitting function described by Hodgson et al [170] was used to determine the M 2 value. The measured beam diameter data and the fit from which the M 2 value was calculated are shown in Figure 4.10. The outcoupled laser beam was determined to have an 2 ± 2 ± Mh =1.38 0.14 and Mv =1.56 0.03.

Measured beam radius (mm) 2 Measured beam radius (mm) M fit 2 M fit

Beam radius (mm)

Distance from waist (mm) Distance from waist (mm) (a) (b)

Figure 4.10: M 2 of slave laser beam showing the beam waist data and the line of best ﬁt for (a) the horizontal and (b) the vertical plane.

Even though the output of the laser is not diffraction limited, it is still compat- ible with coherent detection, as an M 2≤1.73 will give a loss in SNR of ≤3 dB [106]. The measured beam quality is also comparable to other lidar transmitters used for coherent detection. The 1 μmsystemofHawleyet al [11] had an M 2 of 1.0 to 1.5, while the 1.5 μm fibre systems of Bouteyre et al [22] and Pearson et al [81] used laser transmitters with M 2 =1.4andM 2 = 1.5 respectively. The solid-state system of Yanagisawa et al [77] used a transmitter with an M 2<1.4. Diffraction-limited Gaussian beams have an M 2 = 1.0. A possible explanation 94 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

for the reduction in beam quality from that of a perfect Gaussian beam, evident from the measured M 2 values being greater than 1, is the mode size sensitivity to the resonator length. Modelling shows that if the image-relay magnification is kept as 1:1, and the overall length of the resonator is increased by ∼1 % (i.e. 14 mm, by moving the conjugate planes of the image-relay system 7 mm further from the Brewster windows), then the mode size at the aperture formed by the Brewster angled windows (Section 2.8.2) would increase by 14 % in the horizontal plane and 22 % in the vertical plane. Decreasing the length by ∼1 % results in a similar decrease in mode size, so less mode discrimination occurs at the apertures, leading to degradation in beam quality. Another possible explanation is that the effectiveness of the apertures sizes in Section 4.3.2 by themselves are not enough to obtain a diffraction-limited output and additional intracavity apertures are required.

4.6 Injection-seeding

For a CLR, it is essential that the pulsed oscillator lases in a single longitudinal mode and with a frequency that is close to that of the reference oscillator. Both of these requirements can be satisfied by injection-seeding the pulse by injecting a low power CW beam from a stable, single frequency master laser (local oscillator) into the slave laser. In injection-seeding [119, 182, 195–197], the injected power provides a photon number in the closest longitudinal mode (illustrated in Figure 4.11) that is larger than that due to spontaneous emission prior to Q-switching. This mode quickly saturates the gain after Q-switching, which prevents lasing in other longitudinal modes and suppresses reverse-wave oscillation, thereby producing a single frequency output pulse at the frequency closest to that of the injected master laser light. Unlike injection-locking of CW lasers [171,176,182,198–200], the central frequency of the slave mode can be different to that of the injected power [195,196], with larger injected powers able to support larger frequency differences. While seeding does not require precise matching of the master and slave frequencies, some controlled feedback to the slave laser resonator is needed to keep them sufficiently close to each other. The two most popular control techniques are “pulse build-up time” [201–203] and “ramp and fire” [204–206].

There is a decrease in the Q-switched ΔTPBT when injection-seeding occurs.

The “pulse build-up time” technique monitors and attempts to minimise ΔTPBT by using a control servo. The length of the slave resonator is dithered by adjusting the 4.6. INJECTION-SEEDING 95

FSR

n Injected Axial power mode

Figure 4.11: Illustration of axial mode selection when injection-seeding. voltage to a PZT on which the resonator mirror is mounted. The dither amplitude should be small enough to prevent a subsequent dither step from disrupting successful injection-seeding, but large enough to allow the control servo to achieve seeding if laser instabilities are present. This trade-off becomes less problematic in systems that have larger pulse repetition frequencies (PRF’s), as the frequency matching is sampled faster. The slowest repetition rate of a slave laser in a travelling-wave configuration for which successful injection-seeding has been demonstrated using the pulse build-up technique is 10 Hz [203]. In situations where there is higher noise and vibrations, larger shot to shot variations in the frequency of the master laser or low repetition rates, the “ramp and fire” technique is more suitable. In “ramp and fire”, ramping the voltage to a piezoelectrically mounted resonator mirror alters the position of the mirror, thereby adjusting the length of the resonator. When resonant build-up of the injected power is observed, the Q-switch is triggered. A disadvantage of this technique is that the resonator length continues to be ramped after detection of the resonance peak, thus by the time the Q-switch pulse appears, the slave laser may have moved off resonance. A technique similar to “ramp and fire” is “ramp-hold-fire”. In this technique, when the resonant build-up reaches some preset level, the ramp is stopped and the resonator length is held constant for somepredefinedtimeaftertheQ-switchis triggered. However, to suppress the mechanical ringing of the PZT that occurs when the ramp is abruptly stopped, the ramp speed needs to be reduced. While “ramp and fire” servo’s have used ramp times of ∼30 μs [204], ramp times for “ramp-hold-fire” servo’s are slowed to ∼300 μs [206] or longer.

The purpose of this section is to demonstrate that the slave laser can be injection-seeded. Section 4.6.1 describes the alignment of the master laser to the 96 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

slave laser and the control servo techniques that were explored. This is followed by an examination of the intensity and frequency stability of the master laser used as the seed laser, in Section 4.6.2. The eﬀectiveness of the diﬀerent control servo systems and injection-seeding performance are presented in Section 4.6.3.

4.6.1 Approach

A schematic of the setup used to injection-seed the slave laser is shown in Figure 4.12. The seed beam originates from a commercial, single frequency, CW, erbium-doped fibre master laser. The beam is collimated and then passed through two free-space isolators, each with ∼40 dB of reverse mode attenuation. These provide overall isolation of the fibre laser from backward propagating energy caused by stray reflections from the slave laser and unseeded reverse-wave output pulses from the slave.

Free space f=125mm isolators HWP PBSC Master AOM laser f=125mm

Mode matching lens (f=300mm)

Legend PZT Image-relay lens 2 PBSC HWP = Half-wave plate AOM = Acousto-optic HWP modulator Image-relay PZT = Piezoelectric lens 1 Photodiode transducer Pockels cell PBSC = Polarising beam (Q-switch) splitter cube BAW BAW = Brewster-angled wedge Slave laser

Figure 4.12: Schematic of the injection-seeding layout.

As disscussed in Section 1.1 an AOM is used to provide a frequency offset of the transmitted beam from that of the master laser. In this project a Brimrose, AMP-40-10-1535 AOM, which produces a 40 MHz offset, is used. This frequency offset when used in the receiver discussed in the next chapter will allow the system to measure a maximum wind speed of ± 30 m/s. 4.6. INJECTION-SEEDING 97

Optimum seeding eﬃciency occurs when the modes of the injected light and the resonator are matched. Consequently the beam from the master laser needs to be transformed to match that of the slave. This was accomplished using the mode-matching equations described by Kogelnik et al [185]. The waist located at the AOM is spherical and needs to be matched to the asymmetric waist inside the resonator (caused by the astigmatic thermal lensing, as discussed in Chapter 3). Though mode-matching is helpful it is not critical. Non-optimum matching simply increases the number of photons that need to be injected into the system to achieve seeding.

A spherical f = 300 mm lens, located ∼0.45 m and ∼0.85 m from the waists at the AOM and inside the resonator respectively, was used to match the vertical size of the AOM waist to that formed in the resonator when the slave laser was pumped with ∼577 mJ of incident pump energy. No attempt was made to adjust the distances or the focal length of the lens when lower incident pump energies were used. Two steering mirrors between the mode-matching lens and the PBSC are used to align the seed beam to the slave laser optical axis.

The π-polarised seed beam is injected into the slave laser resonator through the PBSC. On the first round-trip the HWP converts the seed light to σ-polarisation and with no voltage applied to the Pockels cell, the seed light continues around the resonator and is completely reflected (Rσ>99.5 %) by the PBSC. On the second pass through the HWP the seed light is converted back to π-polarised light, which is then transmitted through the PBSC (Tπ>95 %). Therefore the gain medium initially has both σ-andπ-polarised light within it. When the Q-switch is triggered, it rotates the π-polarisation back to mostly σ-polarisation, confining this polarisation to the resonator and seeding the pulse.

The length of the slave laser is controlled using two diﬀerent PZT’s. Initially one of the intracavity resonator mirrors was mounted (using Loctite, E-30CL, Hysol, epoxy adhesive) on a PZT5A tube (with an outer diameter and length of 1”, and an inner diameter of 0.75”) from Morgan Matroc, Inc., Vernitron Division [207]. Subsequently, the second intracavity mirror was aﬃxed to a Piezomechanik, HPSt 1000/25-15/5 actuator, with a maximum stroke of 7 μm for 1 kV of applied voltage and a resonant frequency at 40 kHz [208]. This PZT has a higher bandwidth than the PZT5A, enabling the resonator length to be changed at a faster rate. 98 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

4.6.2 The master laser

The master laser is a “commercial” erbium doped fibre laser (EDFL) from Redfern optical components Pty Ltd, model number DFB-S5.35-T, which produces a linearly polarised, single frequency output. It is fibre coupled using polarisation maintaining fibre with an inline polarisation controller, and once passed through two isolators, there is 6 mW of power.

Intensity stability

Fluctuations in the power from the master laser can cause spurious changes in the pulse build-up time and the amplitude of the resonant build-up signal, unrelated to changes in the master or slave frequencies. They can also degrade the performance of the CLR receiver as it produces shot to shot variability in the heterodyne beat amplitude. The intensity noise was measured by focusing the master beam onto a photodiode and monitoring the voltage change over time using an oscilloscope. Typical results are shown in Figure 4.13, and indicate an unexpectedly large level of ﬂuctu- ations.

Figure 4.13: Typical ± 20 % power ﬂuctuations of the output from the master laser. Note: zero power onto photodiode results in 0 V on oscilloscope.

Frequency stability

Even though the output from the master laser is single frequency, the frequency at any point in time can vary. These ﬂuctuations can prevent seeding of the Q-switched pulse, particularly for low PRF lasers such as that used here. 4.6. INJECTION-SEEDING 99

The frequency stability was investigated by measuring the power transmitted through a high-stability Fabry-Perot cavity. As no appropriate Fabry-Perot was available, a half-concentric Fabry-Perot cavity [182] was constructed (shown in Fig- ure 4.14) using two 99 % output couplers, one flat and the other with a radius of curvature of 30 cm, to give a cavity finesse of 156. The flat output coupler was mounted on a PZT (model PZT5A) and the two mirrors were separated by a distance of 25 cm using rigid invar rods. This resulted in a FSR = 600 MHz and a transverse mode frequency separation of 220 MHz [209].

Figure 4.14: The Fabry-Perot.

Frequency ﬂuctuations of the master laser were determined by monitoring the calibrated change in voltage applied to the PZT to maintain a resonance inside the Fabry-Perot. A periodic voltage ramp was applied to the PZT and the power transmitted through the Fabry-Perot was measured using a photodiode. When the resonance peak reached a predetermined value (the threshold trigger value, Pt), the ramp voltage applied at that time is recorded. The voltage ramp ﬁnishes its sweep and then begins again. The change in voltage, ΔV , required for the resonance peak to reach Pt for each sweep, was converted to a change in frequency, Δν,usingthe 100 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

calibration factor: FSR Δν = ΔV (4.11) ΔVFSR where ΔVFSR is the voltage change required to ramp the cavity over one FSR. A typical example of the change in frequency over time can be seen in Fig- ure 4.15. The top trace in Figure 4.15 shows that the frequency changes by ∼100 MHz over long periods of time. The bottom trace shows that this change is not always slow and that the frequency can change by up to 21 MHz/s, which would require the length of the slave laser resonator to be adjusted by ∼150 nm/s to maintain resonance.

80 60 40 20 0 -20

Frequency shift-40 (MHz)

0 100 200 300 400 500 600 Time (s)

21 MHz -20 Frequency shift (MHz) -40 25 26 27 28 29 30 31 32 33 34 35 Time (s)

Figure 4.15: Typical frequency variation of the master laser.

This frequency variation is much larger than the maximum apparent variation shown in Figure 4.16, due to the ﬂuctuating master laser power, ΔνP,givenby: FSR ΔP ΔνP ≈ ≈ 200 kHz (4.12) 4(ﬁnesse) Po 4.6. INJECTION-SEEDING 101

where ΔP/Po is the fractional change in the resonance peak due to the ﬂuctuating power from the master laser.

Du p u

Figure 4.16: Schematic showing how a change in the master power, ΔP, will produce an apparent change in the frequency, ΔνP, at which the Fabry-Perot resonance crosses a threshold trigger point, Pt.

The amount of variation in the master laser power and frequency is some- what surprising and the cause of these ﬂuctuations was never understood. No re- engineering of the master laser was performed by Redfern as it was an out-of-date system. As a more suitable master laser was not available, it was decided to proceed and attempt to seed the slave laser.

4.6.3 Control systems

Three control techniques were attempted in an eﬀort to automate the seeding of the slave laser. Even though the repetition rate of the slave laser was only 2 Hz, the “pulse build-up time” technique was attempted by using a wedge to pick-oﬀ a small portion of the outcoupled pulse. A feedback control system built by a previous student at The University of Adelaide, which had been used to stabilise the injection-seeding of a 50 Hz, 20 mJ Nd:YAG ring laser, was usedwiththePZT5Aactuatortomonitor and minimise the pulse build-up time. 102 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

It was quickly established that the combination of a low slave laser repetition rate and large frequency changes in the master laser would prevent the control system from minimising the pulse build-up time, even when set to the largest dither range. For this technique to work, it would require either a much faster slave laser repetition rate, a master laser with much better frequency stability, or a combination of both. The second technique tried was “ramp-hold-fire”. To the best of the authors knowledge, there have been no reported examples of successfully using a “ramp and fire” or “ramp-hold-fire” technique with a travelling-wave resonator that uses a polarisation controlled output. Henderson et al [204] and Fry et al [205] both used standing-wave resonators, while Walther et al [206] used a ring laser with a fixed output coupler and a Q-switched pump pulse to obtain a short duration gain- switched output pulse. Our “ramp-hold-fire”technique is similar to that used by Walther et al [206]. The error signal was observed using the photodiode in Figure 4.12. It was produced by interference between the part of the injected beam that is reflected by the intracavity Brewster-angled wedge (BAW) during the first pass, with beams that have traversed multiple round-trips in the resonator. A voltage ramp was applied to the HPSt 1000/25-15/5 PZT to which one of the resonator mirrors was secured. Once a resonance peak was detected and the signal crossed a pre-determined trigger level the voltage ramp on the PZT was stopped and held at a constant value. To limit ringing of the PZT, the time taken to ramp the voltage was trialed between ∼550 μs and ∼2 ms. It was found that the intensity variation of the master laser did not allow a consistent resonant build-up and thus, the preset trigger level would often stop the ramp before sufficient build-up occurred, or the build-up would never reach the trigger level. Finally, manual control of the seeding was attempted. It was observed that seeding occurred when the voltage on the photodiode had the shape shown in Fig- ure 4.17, in which the dip at ∼60 ms after the previous Q-switched pulse was the important marker. The voltage applied to the PZT was manually adjusted to maintain this shape. The photodiode voltage should be constant during the time period between the Q-switched laser pulse and the next pump pulse. However in practice, the thermal gradient and stresses in the gain medium are changing during this time, and these changes may account for the strange shape of Figure 4.17. As the frequency change of the master laser output was very erratic, trying to 4.6. INJECTION-SEEDING 103

-180

-200

-220

-240 ~60 ms Voltage (mV) -260

-280 Q-switched output pulses -300

-320 0 100 200 300 400 500 600 Time (ms)

Figure 4.17: The photodiode voltage shape required for successful seeding of the slave laser. Increased negative voltages correspond to larger powers. track the frequency change by hand was diﬃcult. Therefore while injection-seeding was accomplished, it was inconsistent and reliable seeding proved to be impossible. Because of the limitations of the master laser and the success, albeit limited, of the seeding, it was decided to continue using manual control to obtain proof-of-principle results.

4.6.4 Injection-seeded output

To test that successful seeding of the slave occurred, a measurement of the outcoupled frequency was undertaken using the receiver in Chapter 5. Results presented in Section 5.3.1 show that the output is a transform limited pulse that is oﬀset from the master laser frequency by ∼40 MHz, as expected. A typical injection-seeded Q-switched pulse is shown in Figure 4.18(a). As expected, when the slave laser is optimally seeded, the reverse-wave is completely extinguished, as shown in Figure 4.18(b), and the energy that was outcoupled in the reverse-wave direction is now outcoupled in the forward-wave direction. The measured FWHM of the injection-seeded pulse is ∼360 ns. This value 104 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

(a)

(b)

Figure 4.18: Comparison of unseeded and seeded Q-switched output pulses for 1 mW injected master power. (a) Shows the pulse build-up time and power of the unseeded and seeded Q-switch pulses. (b) Demonstrates the extinction of the reverse-wave when the slave laser is seeded. 4.6. INJECTION-SEEDING 105

agrees with that expected, which is determined from [119]: t lnz τ = r , (4.13) δrt z[1 − a(1 − lna)] where tr is the cavity round-trip time, δrt is the round-trip loss due to the output coupling fraction and the cavity losses, z = golg/δrt,anda =(z-1)/(zlnz). Using the 2 % Q-switching output coupling fraction estimated in Section 4.5, the loss of the components in the resonator as listed in Table 4.1, and the loss due to the slab as discussed in Section 2.8.2, results in δrt ≈ 10 %. Substituting this value into Equation 4.13 gives an expected FWHM pulse width of 350 ns, in good agreement with the observed value.

The eﬀect of the injected master power on the seeded Q-switched pulse build- up time and peak power are shown in Figure 4.19. As expected, increasing the injected power decreases the pulse build-up time. Normally, increasing the injected master power into the slave would improve the seeding reliability, however the poor frequency stability of the master laser used in this project made any improvements in reliability that an increase in injected signal power might provide not apparent.

Determining the energy in the injection-seeded pulse using a power or energy meter was not possible, as the slave laser could not be reliably seeded. Thus, photodiode 2 was calibrated using the setup shown in Figure 4.20. A power meter was used to measure the average power of the unseeded Q-switched pulses, which yielded the average pulse energy. The area under the voltage curve produced by photodiode 2 was also calculated. Combining these values gave a calibration factor that could be used to convert the average area for an injection-seeded pulse to the pulse energy.

The total unseeded Q-switched energy out of the PBSC at this stage of the project was 2.2 ± 0.2 mJ, with 1.3 ± 0.2 mJ outcoupled in the forward-wave direction, when pumped with an incident pump energy of 577 mJ. The area under the curve for the seeded forward-wave output was found to be ∼2.15x the unseeded forward-wave output curve, indicating a Q-switched forward-wave output of 2.8 ± 0.4 mJ when seeded. The pulse energy increased by more than a factor of two because the extra losses experienced by the reverse-wave (as discussed in Section 4.5) are not present to dissipate the intracavity energy. Additionally, uni-directional operation does not allow the reverse-wave to rob gain from the forward-wave energy build-up. 106 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER

2.7

2.6 Seeded Q-switched output Unseeded Q-switched output 2.5

s) 2.4 m

2.3

2.2

2.1

Pulse build-up time ( 2.0

1.9

1.8 012345 Local oscillator power injected into slave (mW)

(a)

500

450 Seeded Q-switched output Unseeded Q-switched output 400

350

300 Peak power (a.u.)

250

200

012345 Local oscillator power injected into slave (mW)

(b)

Figure 4.19: Injected master laser power dependence of (a) the seeded Q-switched pulse build-up time, and (b) the peak power of the seeded Q-switch pulse. 4.7. CONCLUSION 107

Free space f=125mm isolators HWP PBSC Master AOM laser f=125mm

Mode matching lens (f=300mm)

Photodiode 2

Legend PZT Image-relay Power lens 2 PBSC meter HWP = Half-wave plate AOM = Acousto-optic Wedge HWP modulator Image-relay PZT = Piezoelectric lens 1 Photodiode transducer Pockels cell PBSC = Polarising beam (Q-switch) splitter cube BAW BAW = Brewster-angled wedge Slave laser

Figure 4.20: Setup to calibrate photodiode 2 to determine the injection-seeded pulse energy.

4.7 Conclusion

In this chapter, the slave laser travelling-wave resonator design was described and measurements of its performance reported. The maximum combined forward and reverse-wave gain-switched energy outcoupled via the PBSC was 22.0 mJ from 577 mJ of incident pump energy. Q-switching was demonstrated but bleaching of the pump absorption limited the maximum pulse energy that could be obtained. The rollover due to bleaching occurred at an incident pump energy that was higher than that expected from the small-signal gain measurement in Chapter 3. This paradox is attributed to the degradation of the laser diodes. The performance of the slave laser when injection-seeded was also described. Successful seeding was obtained, despite the poor stability of the master laser, with a forward-wave outcoupled energy of 2.8 mJ from 577 mJ of pump energy. The 2 2 beam quality was measured to have an Mh =1.38andMv = 1.55. Successful seeding of the slave laser results in a single frequency output with a frequency close to that of the injected beam. This is demonstrated in the next chapter when the slave laser is used as a laser transmitter in a CLR system. Hetero- dyne/FFT analysis of returns from hard and atmospheric targets are also reported. 108 CHAPTER 4. TRAVELLING-WAVE SLAVE LASER Chapter 5

Laser radar measurements

5.1 Introduction

The aim of this chapter is to describe the development of the CLR receiver and to demonstrate single-shot atmospheric wind profiling. The receiver used in this project is monostatic receiver, where a Transmit/Receive (T/R) switch separates the transmitted light from the return signal. As was discussed in Chapter 1, the receiver uses heterodyne detection to measure the line of sight wind speed. Balanced photoreceivers are employed to detect the backscattered return and to remove intensity noise contributions. Unlike the Mitsubishi system which uses a single balanced heterodyne detector to monitor both the frequency of the transmitted light (using light back-reflected by the transceiver optics) and the return signal [17], our system, like Koch et al [19], uses two photoreceivers. This is because the intensity of the light reflected by the transmit optics is significantly larger than that backscattered by the atmosphere. The high gain required to analyse the atmospheric backscatter would saturate and potentially damage the analog-to-digital (A/D) card when the light reflected by the transceiver is initially detected. A two detector arrangement allows the greatest flexibility, where one detector is used to measure the transmitted frequency (transmit) and the other detector measures the frequency of the return signal (return). This optimises the receiver by allowing gain to be added to the output signal from the photoreceiver monitoring the return signal. Acquisition of the data is accomplished using a high sampling rate A/D card interfaced to a Matlab graphical user interface (GUI), which allows parameters such as sampling rate and sample size to be adjusted. Another Matlab GUI that incor- porates windowing and zero-padding functions is used to process the data.

109 110 CHAPTER 5. LASER RADAR MEASUREMENTS

In this chapter, Section 5.2 will describe the design of the receiver hardware, including the telescope that transmits light into, and collects light from, the atmosphere, and the data acquisition and processing software used. This is followed by a description of the testing of the system, ﬁrst using a stationary reﬂective hard target in Section 5.3.1 and then a moving hard target in Section 5.3.2. This chapter concludes with a discussion of successful atmospheric results in Section 5.4.

5.2 Receiver system

5.2.1 Receiver overview

The CLR receiver is shown in Figure 5.1. Light from the master laser is split into two beams, one of which is frequency shifted (using the AOM described in Section 4.6.1) and used to injection-seed the slave laser. The other is sent to the receiver where it provides the local oscillator (LO) fields for the detectors. It is π-polarised using HWP 3 to match the polarisation of the transmitted light and the atmospheric backscattered radiation incident on the detector. A beam splitter cube is then used to allow a portion of the light to go to the “transmit” detector and the rest to the “return” signal detector. The output from the slave laser, which is asymmetric due to the thermal lens, is divided into two beams using a wedge. The beam reflected from the wedge, which contains only a small fraction of the total energy, is split to provide equal power on each of the photodiodes of the free-space balanced photoreceiver. Here it mixes with the light from the local oscillator to generate a heterodyne beat signal from which the frequency of the transmitted light can be measured, as discussed in Section 5.2.3. The beam transmitted through the wedge is shaped to make it spherical and collimated before reaching the T/R switch, which is created by the combination of PBSC 1 and the QWP in Figure 5.1. Using HWP 4 in combination with PBSC 1 provides a convenient way of controlling the amount of energy sent out to the telescope. This is useful when aligning and calibrating the system using a highly reflective target, such as the mirror in Section 5.3.1, which could damage the detectors. After passing through the QWP, the circularly polarised radiation is expanded using a telescope, and transmitted into the atmosphere using a large flat mirror. A small percentage of the backscattered radiation is collected by the telescope and converted to π-polarisation as it passes back through the QWP. This light is coupled into the polarisation-maintaining fibre splitter (FS), where it mixes with light from 5.2. RECEIVER SYSTEM 111

Figure 5.1: Schematic of the receiver system. 112 CHAPTER 5. LASER RADAR MEASUREMENTS

the local oscillator. This creates a heterodyne beat which is measured with the fibre-coupled balanced photoreceiver. To maximise the heterodyne efficiency, the local oscillator and signal beams must have their wave front curvatures matched with negligible tilt or offset, and have spot sizes slightly smaller than the detector so as to collect all the light. Additionally, for balanced detection, the optical distances between the beam splitter and the detectors need to be as closely matched as possible [210–217]. The use of an optical fibre splitter and a fibre-coupled balanced photoreceiver, instead of free-space components, allows for the above conditions to be achieved more conveniently as the positioning of components is more flexible. If the fibres after the FS are the same length the optical pathlengths are matched, and coupling into fibres inherently eliminates mismatches in tilt and offset of the two beams [17,19,61]. However, the fibres need to be single mode, polarisation-maintaining (PM), and have high coupling efficiency. To minimise the coupling loss, the polarisation of the light needs to align to the stress direction in the PM fibre core and the beams from the transmitter and the local oscillator need to be matched to the numerical aperture (NA) of the PM fibre. However, even perfect matching will result in some coupling loss due to Fresnel reflections [218] and problems with fabricating microlenses for PM fibre [219]. In practice, the improvement in heterodyne efficiency by having no tilt and/or offset misalignment more than negates any decrease in signal due to coupling loss. Unfortunately, only one of the two photoreceivers was a fibre-coupled system. Since the return from the atmosphere is less energetic than that picked off from the transmitted beam, it requires the detector that will provide the optimal heterodyne efficiency. Therefore the fibre-coupled photoreceiver is used to monitor the light returned from the atmosphere.

5.2.2 Telescope

The 20x Galilean telescope used in the receiver, shown in Figure 5.2, is a homemade construction. The “objective” lens consists of two lenses that contact at the edge. They are held together within a single mount that has tilt adjustment in the horizontal and vertical directions. It has an f/3.5. This composite lens is used to reduce spherical aberration. The “eyepiece” lens has an f/3, and could also be a composite lens, though it is hard to determine. It is mounted to enable maximum alignment ﬂexibility, and has x, y and z translation and horizontal and vertical tilt. Both lenses 5.2. RECEIVER SYSTEM 113

(a) (b)

Figure 5.2: Telescope used in the receiver. (a) A schematic of the lenses in the telescope, and (b) a photo of the telescope setup. The eyepiece lens is in the foreground and the objective lens is in the background. are mounted on the same base so that any vibrations are common to both lenses. The degradation of a beam passing through the telescope was characterised in the near field by passing a collimated HeNe beam through the telescope and retro-reflecting the beam using a flat mirror. By analysing the transverse profile of the reflected beam, it was determined that it remained close to diffraction limited. Since atmospheric effects will distort the wavefront of the transmitted and returned light, the telescope performance was deemed suitable.

5.2.3 Balanced photoreceivers

The photoreceivers used are two Newfocus model 1817 80 MHz detectors. A balanced photoreceiver consists of two matched photodiodes to produce a signal that is proportional to the diﬀerence in the individual photodiode currents. A schematic of a ﬁbre splitter coupling light onto a balanced photoreceiver is shown in Figure 5.3.

I1 Esig (t)

D 50/50 I=I12 -I splitter I2 ELO (t) -V

Figure 5.3: Schematic of a ﬁbre splitter and a ﬁbre coupled balanced photoreceiver.

The electric ﬁeld of the local oscillator, ELO, and the signal, Esig, coupled into 114 CHAPTER 5. LASER RADAR MEASUREMENTS

the ﬁbre splitter can be written as [220]: (jνLOt) (jφLO(t)) ELO(t)=k PLO(t)exp exp (5.1) (jνsigt) (jφsig(t)) Esig(t)=k Psig(t)exp exp (5.2) where k is a constant, PLO(t)andPsig(t) are the local oscillator and signal powers,

νLO and νsig the frequencies, and φLO(t)andφsig(t) the phases of each optical field. The fibre splitter (and the free space PBSC) acts as an optical mixer where a 90◦ phase shift is generated in the reflected beam [221]. Assuming both photodiodes in the balanced photoreceiver have an identical responsivity, Rpd, and the fibre splitter has perfect 50/50 splitting of power, the photocurrents in Figure 5.3 are given by [220]:

Rpd I1(t)= P (t)+P (t)+2 P (t)P (t)sin((ν − ν )t + φ (t) − φ (t)) 2 sig LO sig LO sig LO sig LO (5.3) Rpd I2(t)= P (t)+P (t) − 2 P (t)P (t)sin((ν − ν )t + φ (t) − φ (t)) 2 sig LO sig LO sig LO sig LO (5.4) The third terms in these equations are the heterodyne mixing terms, with a beat frequency of (νsig − νLO). The beat signals on the two photodiodes are exactly 180◦ out of phase [222], as indicated by the negative sign in Equation 5.4. As the two photodiodes are connected together, the diﬀerential current output from the balanced photoreceiver is [220]: ΔI(t)=I1(t)−I2(t)=2Rpd Psig(t)PLO(t)sin((νsig −νLO)t+φsig(t)−φLO(t)) (5.5)

Thus, intensity noise at the beat frequency on Psig and PLO is cancelled, while the heterodyne beat is doubled. Equation 5.5 shows that the amplitude of the heterodyne beat signal and thus the photocurrent, can be enhanced by increasing the local oscillator power as it √ scales as P . Since shot noise associated with the detection of the photocurrent LO√ also scales as PLO when PLO>Psig, the signal to noise in a heterodyne detection system is maximised by increasing the local oscillator power until its shot noise dominates all other noises [18,223]. To ensure that the “return” detector is limited by the shot noise of the local oscillator, each photodiode has ∼0.88 mW of local oscillator power incident, which is ∼1/5th the damage threshold of the photodiodes. The local oscillator power on the 5.2. RECEIVER SYSTEM 115

“transmit” detector was reduced to ∼10 μW per diode using OD ﬁlters to prevent saturation of the detector due to the much larger power of the light picked oﬀ from the slave laser. However this still gives a satisfactory heterodyne beat for analysis.

5.2.4 Return signal ampliﬁcation

To utilise the full dynamic range of the A/D card, the output of the balanced photoreceiver is amplified by 40 dB. This amplification is what prohibits using only one photoreceiver to monitor both the frequency of the transmitted light and the backscattered return from the atmosphere. The relatively large reflection from the transceiver optics would create a large photocurrent that when amplified would saturate and possible damage the A/D card. However the amplified photoreceiver will detect backscatter from the tran- sciever optics. To prevent damage to the A/D card, the signals from the photoreceiver need to be swapped to the alternate output of the switch (which is 50 Ω terminated) during the time it takes for the output pulse to leave the transceiver. A schematic of the system is shown in Figure 5.4 and the timing diagram is shown in Figure 5.5.

Triggering control electronics Switch DC block 40dB Amp GaGe (Mini-circuits: (Mini-circuits: (Mini-circuits: A/D ZYSWA-2-50DR) BLK-18+) ZKL-1R5) Balanced card photoreceiver

Figure 5.4: Block diagram of the ampliﬁer setup on the signal return arm of the receiver, between the balanced photoreceiver and the A/D card.

It was also found that if the AOM was left on continuously, then the light used to seed the slave laser would leak through to the receiver, generating a constant heterodyne beat signal at the AOM modulation frequency. This masks the beat signals generated from the light backscattered from the atmosphere. Therefore the AOM is turned off after an injection-seeded laser pulse is formed. The timing of this is also shown in Figure 5.5. If a single detector was used to monitor both the transmitted and returned beams, then the gain of the amplifier would need to be switched during the data acquisition. If this was done using a pre-defined timing sequence then it would result in an increase in the minimum range to allow for fluctuations in the transmitted pulse 116 CHAPTER 5. LASER RADAR MEASUREMENTS

ON Pulse generator OFF

ON Pump pulse OFF 5ms

~33m s Q-switch OFF ON

Q-switch output pulse OFF ~2m s

~3.5m s AOM ON OFF 2ms ON Amplifier OFF

Figure 5.5: Timing diagram of the “return” arm of the receiver system. timing. Therefore to maintain a short minimum range, the ampliﬁcation would need to be switched according to the amplitude of the beat from the photoreceiver. Additionally, if the reﬂection by the transceiver optics resulted in saturation of the detector at the LO power level required for the backscattered return signals it would be necessary to switch the LO level.

5.2.5 Data acquisition/processing

The analog output of the photoreceivers is converted to a digital signal using an A/D card mounted in a PC. When initial testing of the CLR system was performed, the data acquisition system used a GaGe CompuScope 14105 Comm Analyser which is a 14 bit, 105 MS/s dual channel card, set to sample at 100 MS/s. This was upgraded to a faster sampling, GaGe “Cobra” Compuscope CS22GB high speed digitiser card after modelling by Dr David Ottaway at The University of Adelaide indicated that increasing the sampling rate would improve the SNR of the system. This card has 8 bits of resolution, can sample at 2 GS/s for single channel use (or 1 GS/s for dual channel operation), and has 256 MS of on-board memory. The interface with the card is via a Matlab GUI designed and written by Dr Francois Jeanneret at The University of Adelaide. The steps involved in processing the data to produce atmospheric wind pro- ﬁles are similar to that used by Koch et al [19]. An outline of the steps involved is displayed in the ﬂow diagram in Figure 5.6 and described in greater detail below. 5.2. RECEIVER SYSTEM 117

Figure 5.6: Flow diagram of data processing. 118 CHAPTER 5. LASER RADAR MEASUREMENTS

Step (1): Acquisition. The signals from the “transmit” and “return” photoreceivers are recorded for about 66 μs, which corresponds to a range of 10 km, and generates 66 x 103 samples per channel.

Step (2a): Fourier spectrum of the transmitted pulse. A gate typically 2to3μs in length, centered on the heterodyne beat of the transmitted pulse, is used to calculate the Fourier spectrum of the transmitted pulse using an FFT. As the gate is much longer than the duration of the beat, the amplitude of the beat is close to zero at the edges of the gate. Therefore, no weighting factor is needed to remove sampling artifacts. Zero-padding is used to reduce the size of the Fourier bins as a means for interpolation when estimating the frequency of the peaks.

Step (2b): Fourier spectrum of the return signal. The continuous return data sequence is divided into multiple range gates. For atmospheric measurements in this project, each gate contains 500 samples, which corresponds to a duration of 500 ns, and a range confusion of 75 m. The choice of the number of samples in a range gate depends on the desired range and velocity resolutions. Shortening the gate length improves the range resolution but degrades the velocity resolution, and vice versa. A Hanning weighting (shown in Figure 5.7) is applied to each gate. As it attenuates the signal to zero at the gate edges, the gates are overlapped so that no loss of information occurs and no discontinuities form at the regions near the gate edges [224]. As continuous signals are to be analysed, an overlap of 75 % on two consecutive gates is used [225]. A drawback of weighting is that it widens the linewidth of FFT peaks. In the case of Hanning weighting, the FFT linewidth will double [226]. Using a 500 sample range gate with a Hanning weighting results in a minimum linewidth of 4 MHz, giving a frequency resolution of ∼2 MHz and a velocity resolution of ∼1.5 m/s. The number of samples in the FFT of each gate is increased by zero-padding after the Hanning window has been applied. Typically 3500 zeros are added to the 500 samples giving a total length of 4000 samples in each gate, which decreases the bin-width, but does not change the shape of the spectrum.

For the hard targets used initially, the analysis of the return signal used the process in Step (2a) rather than that described in Step (2b). 5.2. RECEIVER SYSTEM 119

1.2

0.8

0.6 Weighting 0.4

0.2

0 0 N/2 N Samples

Figure 5.7: Shape of the Hanning weighting function.

Step (3): Frequency check of transmitted beam. The frequency of the peak in the transmit spectrum is compared to a user defined frequency range, typically 39.0 to 40.5 MHz (see Section 5.3.1 for justification of this range). This checks the injection-seeding of the laser pulse. If the frequency of the transmitted pulse lies within this range it will be classified as being a “good seed”. Any data sets that are outside this range are discarded.

Step (4): Frequency shift. To remove the eﬀect of pulse to pulse frequency jitter when averaging, the frequency of the ﬁrst transmitted spectrum to pass Step (3) is assigned as the reference transmit frequency. Each subsequent spectrum that passes Step (3) is frequency shifted so that the transmitted frequency matches the reference transmit frequency.

Step (5): Averaging. Spectra for each range gate are averaged over a user- speciﬁed number of pulses.

Step (6): Wind profiling. For each range gate, the frequency of the largest peak of the spectrum is determined. The Doppler frequency shift is then calculated by subtracting the transmit frequency. The LOS velocity is calculated using Equa- tion 1.1 and the range is determined from the time of flight of the backscattered light with the transmitted pulse defining zero time. 120 CHAPTER 5. LASER RADAR MEASUREMENTS

5.3 Testing of the system using a hard target

Before undertaking atmospheric measurements, the system underwent “shakedown tests” using a hard target. As discussed in the previous section, these tests used the 105 MS/s A/D card. The transmit and return gates consisted of 200 samples. The return heterodyne beats were not ampliﬁed as the backscattered returns were much larger than for atmospheric tests.

5.3.1 Stationary highly reﬂective hard target

Initially a stationary hard target was used to check that the analysis of the transmit and return heterodyne beats showed the same pulse build-up time and produced the same Fourier spectrum. The results were also used to determine if there was a frequency chirp in the Q-switched pulse. The setup used a stationary mirror, located ∼0.3 m from the telescope objective to retro-reflect light, as shown in Figure 5.8. Since the mirror was highly reflective, the pulse energy sent to the telescope was reduced by adjusting HWP 4. Typical heterodyne beats obtained from the “transmit” and “return” photoreceivers are shown in Figure 5.9. As expected, the pulse build-up times of the two beat signals agree. No heterodyne beat occurs >3.5 μs after the end of the pump pulse. This determined the switching of the amplifier timing in Figure 5.5. The FWHM of the spectrum is 450 ± 10 ns. This value is larger than that measured using a photodiode (∼360 ns, see Figure 4.18(a)) because the amplitude of the heterodyne beat is ∝ Psig, while the amplitude of the pulse detected by the photodiode is ∝ Psig. The Fourier spectra are shown in Figure 5.10. As expected both the “transmit” and “return” spectra have their maximum amplitude at the same frequency. The FWHM of a typical “return” Fourier spectrum (as shown in Figure 5.11) is 0.9 ± 0.05 MHz. This results in a time-bandwidth product for the pulse of 0.41 ± 0.03, which agrees with the value (0.44) expected for a Gaussian pulse [182]. Thus, the pulse appears transform limited and the chirp is negligible. The layout in Figure 5.8 was also used to investigate whether the frequency of the transmit pulse could be used as an indicator of the quality of the injection- seeding. This was accomplished by using the “transmit” photoreceiver such that it functioned as a simple reverse-biased photodiode to monitor the power as a function of time, while simultaneously measuring the heterodyne beat frequency on the “return” photoreceiver. The transmit pulse frequency, build-up time and peak power 5.3. TESTING OF THE SYSTEM USING A HARD TARGET 121

Figure 5.8: Schematic of the setup to take results from a stationary highly reﬂective target. 122 CHAPTER 5. LASER RADAR MEASUREMENTS

Figure 5.9: Heterodyne beats on the “transmit” and “return” photoreceivers.

−3 −3 x 10 x 10 4 4

X: 40 Y: 0.003707 X: 40 3 Y: 0.00332 3

2 2

Amplitude (a.u.) 1 Amplitude (a.u.) 1

0 0 0 10 20 30 40 50 0 10 20 30 40 50 Frequency (MHz) Frequency (MHz) (a) Transmit (b) Return

Figure 5.10: Fourier spectra when using a high reﬂectivity stationary hard target. The markers on each graph indicate the frequency and the amplitude of each FFT peak. 5.3. TESTING OF THE SYSTEM USING A HARD TARGET 123

Figure 5.11: Bandwidth measurement of Fourier spectrum.

3.6 0.38

0.37 s) 3.4 m 0.36 3.2

0.35 3.0 0.34 2.8 Pulsebuild-uptime( 0.33 2.6 0.32

34 35 36 37 38 39 40 41 42 43 44 Peak power of seeded output pulse (a.u.) 34 35 36 37 38 39 40 41 42 43 44 Frequency (MHz) Frequency (MHz)

(a) (b)

Figure 5.12: (a) The relationship between the pulse build-up time and the frequency of the output. (b) The relationship between the pulse peak power and the frequency of the output. 124 CHAPTER 5. LASER RADAR MEASUREMENTS

were recorded for many pulses, and are plotted in Figure 5.12. As discussed in Section 4.6, when injection-seeded, the oscillation occurs at the axial mode closest to the injected frequency. If there is no frequency diﬀerence then the transmit heterodyne beat will be at the AOM frequency. Figure 5.12 shows that the closer the longitudinal mode of the slave laser is to the injected frequency, the shorter the pulse build-up time and the greater the peak power. One interesting observation is that the shortest pulse build-up time and highest peak power occur at a transmit frequency (39.75 MHz), slightly less that that of the AOM. This is presumably due to a change in the optical length of the resonator as the pulse extracts the gain. Figure 5.12 indicated that frequencies 39.75 ± 0.75 MHz would constitute a “good seed” in the processing software in Section 5.2.5, as the pulse build-up times relating to this frequency range are within 5 % of the shortest pulse build-up time.

5.3.2 Diﬀusely-reﬂecting moving hard target

A belt sander that had a belt made from Solas reﬂective tape was placed ∼6mfrom the telescope objective [61, 100, 227], as shown in Figure 5.13. The belt was large enough to ensure that it provided a uniform velocity target over the entire diameter of the beam. The nominal belt speed is 6.6 m/s. Typical results obtained for the belt approaching or receding from the laser beam are shown in Figure 5.14. For this system, the movement of the belt towards the laser beam gives a heterodyne beat at a lower frequency, and conversely for movement of the belt away from the laser beam, gives a beat at a higher frequency. This is because in this setup the frequency of the light injected into the slave laser was 40 MHz below that of the master laser. Therefore, the Doppler shift produced by an approaching object shifts the frequency of the backscattered light towards that of the LO, decreasing the frequency of the heterodyne beat.

The expected LOS velocities are given by 6.6cosθi where θi is the angle between the surface of the belt and the incident laser beam. For the belt moving towards ◦ the incident laser beam, θi ≈ 42 ± 2 . This results in an expected LOS velocity ◦ of 4.9 m/s. For the belt moving away from the incident laser beam, θi ≈ 35 ± 2 , giving an expected LOS velocity of 5.4 m/s. The measured LOS wind speeds can be calculated using Equation 1.1 and the results from Figure 5.14. For the belt approaching the beam this resulted in a LOS wind speed of 5.0 ± 0.7 m/s and for the belt receding from the beam 5.4 ± 0.7 m/s. This is in good agreement with the predicted velocities as shown in Table 5.1. 5.3. TESTING OF THE SYSTEM USING A HARD TARGET 125

Figure 5.13: Schematic of the receiver system to detect Doppler shifts from the belt sander. 126 CHAPTER 5. LASER RADAR MEASUREMENTS

0.016 Return 0.016 33.8 MHz

Return 0.012 0.012 47.4 MHz

0.008 0.008 Transmit 40.3 MHz Amplitude (a.u.) Amplitude (a.u.) Transmit 0.004 40.4 MHz 0.004

0 0 0 10 20 30 40 50 0 10 20 30 40 50 Frequency (MHz) Frequency (MHz) (a) (b)

Figure 5.14: Doppler shifts from a moving hard target. (a) Typical LOS Doppler shift of the system with the belt approaching the laser beam at an angle of ∼42◦ to the laser beam. (b) Typical LOS Doppler shift of the system with the belt receding from the laser beam at an angle of ∼35◦ to the laser beam.

Belt direction Predicted LOS velocity Measured LOS velocity Approaching incident beam 4.9 m/s 5.0 ± 0.7 m/s Receding from incident beam 5.4 m/s 5.4 ± 0.7 m/s Table 5.1: Comparison between predicted and measured velocities using a moving hard target. 5.4. ATMOSPHERIC TEST OF CLR 127

5.4 Atmospheric test of CLR

A schematic of the layout used to test the CLR with an atmospheric target is shown in Figure 5.15. The beam from the telescope was transmitted through the laboratory window and then reflected from a large flat mirror into the atmosphere. The proximity of the buildings defines the laser beam projection to obtain a clear field of view. This resulted in the beam being directed at an angle 13 ± 2◦ from the zenith, and 21◦S from magnetic east (29◦ from true east). A major drawback with this projection is that the measured Doppler shift will be small compared to the actual horizontal wind velocity. During these atmospheric tests, the maximum seeded energy from the slave laser was 1.1 ± 0.3 mJ. The decrease in energy is attributed to a variety of reasons. These included the contamination of the optical surfaces of the slave laser when the window was opened to allow the laser beam to be transmitted into the atmosphere and continued ageing of the pump diodes. The telescope was adjusted to transmit a collimated beam into the atmo- sphere1. Simulations show that focusing the CLR beam at a range of ∼1kmmay slightly increase the SNR at that range in typical daytime turbulence. However, the SNR for the focussed beam is similar to that for the collimated beam at longer ranges due to the effect of turbulence [228]. Figure 5.16 shows an example of a Fourier spectrum of atmospheric backscattered light and a spectrum of the transmitted light. It is clear that the spectrum of the backscattered light has been Doppler shifted. A typical observed backscattered return Fourier spectrum from different ranges for the same output pulse is shown in Figure 5.17. It can be seen that for this particular output, atmospheric returns at ranges of 215 m, 605 m, and 2065 m are observed. Doppler shifted returns at ranges less than ∼1000 m are due to winds in the aerosol rich planetary boundary layer (PBL) [229]. Above the PBL is the “free troposphere”, where the aerosol backscatter is up to 1 or 2 orders of magnitude lower [19]. Based upon the amplitudes of the backscattered spectra observed with our system, it would not be surprising if no backscattered returns were observed at ranges >∼1000 m. However, inversion layers can occur in the free atmosphere above the PBL [230]. These layers are typically temperature inversion layers which

1Before attempting atmospheric tests, laser safety calculations and measurements were performed to ensure that the MPE of the transmitted beam satisﬁed the class 1 laser classiﬁcation. 128 CHAPTER 5. LASER RADAR MEASUREMENTS

Figure 5.15: Schematic of the setup to take atmospheric results. 5.4. ATMOSPHERIC TEST OF CLR 129

Range = 1735 m 250 0.4

200 0.3

150 0.2 100 Amplitude (a.u.) Amplitude (a.u.) 0.1 50

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz) (a) (b)

Figure 5.16: Comparison of the Fourier spectrum of the transmitted pulse and the backscattered return from the atmosphere. (a) The spectrum of the transmitted pulse. (b) A spectrum of a Doppler shifted backscattered return from the atmosphere. trap aerosols underneath [231]. It is from one of these layers that the backscattered return from a range of ∼2000 m occurs.

At the same time (04:55:00 UTC time) as the measurement in Figure 5.17 was recorded, measurements with a VHF boundary layer radar installed at the Adelaide airport2, ∼5 km west of the laboratory location, were also taken. These results are shown in Figure 5.18 and display the bottom of the inversion layer at ∼2km.

The ∼4 MHz linewidth of the peaks in the “return” spectra are determined from the resolution of a FFT performed on a 500 ns range gate with a Hanning weighting. This was conﬁrmed by recording the noise on the return photoreceiver by blocking the output of the slave laser directly after the telescope objective (and will be discussed in further detail later). A single frequency tone that had an amplitude required to reproduce the observed peak height is added to the noise. The Fourier spectrum of the synthesized heterodyne beat is plotted in Figure 5.19(a), and the measured spectrum is shown in Figure 5.19(b). Both have a FWHM ≈ 4MHz.To check that the 4 MHz linewidth was not due to measurement noise, the amplitude of the tone added to the noise was increased. The resultant Fourier spectrum is shown in Figure 5.19(c), which also has a 4 MHz FWHM.

2The Adelaide airport boundary layer radar is a 1 kW Very High Frequency (VHF) Spaced Antenna, operating at 54.1 MHz. It uses Full Correction Analysis (FCA) to obtain measurements of 3-D wind ﬁelds and precipitation from 400 m up to 8 km every minute [232]. 130 CHAPTER 5. LASER RADAR MEASUREMENTS

Transmitted signal Range = 605 m 300 X: 39.69 0.2 250 Y: 288.1

200 0.15

150 X: 35.5 0.1 Y: 0.08479 100 Amplitude (a.u.) Amplitude (a.u.) 0.05 50

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz)

Range = 175 m Range = 1280 m

0.2 0.2

0.15 0.15

0.1 0.1 Amplitude (a.u.) Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz)

Range = 215 m Range = 2065 m

0.2 0.2

X: 38.25 0.15 0.15 Y: 0.185

X: 40 0.1 Y: 0.08348 0.1 Amplitude (a.u.) Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz)

Range = 490 m Range = 2255 m

0.2 0.2

0.15 0.15

0.1 0.1 Amplitude (a.u.) Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz)

Figure 5.17: A typical set of spectra at diﬀerent ranges for a single output pulse. Only those Fourier spectra which have peaks of statistical signiﬁcance are marked with their peak frequency X in (MHz), and amplitude Y in (a.u.). 5.4. ATMOSPHERIC TEST OF CLR 131

Figure 5.18: Atmosphere proﬁle on the 10-02-2009 using the VHF boundary layer radar at the Adelaide airport. Plot courtesy of the Atmospheric group at The University of Adelaide. 132 CHAPTER 5. LASER RADAR MEASUREMENTS

Range = 2065m

0.2 0.2

0.15 0.15

0.1 0.1 4 MHz 4 MHz

Amplitude (a.u.)

Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz) (a) (b)

3 4 MHz 2

Amplitude (a.u.) 1

0 20 30 40 50 60 Frequency (MHz) (c)

Figure 5.19: Fourier spectra FWHM linewidth analysis. (a) Modelled spectrum with target scatterers having uniform velocity. (b) Typical measured spectrum. (c) Modelled spectrum with a higher density of target scatterers, resulting in a Fourier spectrum peak with a larger SNR. 5.4. ATMOSPHERIC TEST OF CLR 133

Histogram analysis of the backscattered spectra

A histogram analysis was performed to confirm that the peaks in the spectra of the backscattered returns were actual signals. The background noise was obtained by blocking the transmitted beam ∼0.30 m after the telescope with an opaque material and recording a data set 300 range gates in length (equivalent to looking to a range of 5770 ± 30 m). Since the signals from the “return” photoreceiver are switched from the A/D card for a duration that ensures all the pulse has left the system, no backscatter from this opaque material is recorded. One hundred and thirteen single- shot background noise files, each 300 range gates in length were recorded. Thirty four “good” injection-seeded atmospheric returns were also recorded over a short time period. A flow diagram of the analysis technique to produce the histograms is shown in Figure 5.20 and described below.

Binary data files Binary data files (atmospheric return blocked) (atmospheric return)

Break up into range Break up into range gates, zero-pad gates, zero-pad and FFT and FFT

Measure largest amplitude Measure largest amplitude in spectrum between in spectrum between 35 MHz and 45 MHz 35 MHz and 45 MHz

Histogram Histogram

Figure 5.20: Flow diagram of process to obtain histograms.

Each background noise data set was divided into 300 individual range gates, with each FFT’d. The resultant spectra were analysed in the frequency range 35 MHz to 45 MHz, and the amplitude of the largest peak recorded. This was carried out for all 33900 background noise range gates. Figure 5.21(a) displays the resulting distribution of the maximum amplitude of the background noise spectra peaks. An amplitude of 0.04 a.u. is estimated to be the amplitude noise ﬂoor level as <3 % of amplitudes exceed this level, and there are no amplitudes above 0.05 a.u. 134 CHAPTER 5. LASER RADAR MEASUREMENTS

Background noise data 0.25 Background noise distribution envelope

0.2

0.15

Probability 0.1

0.05

0 0 0.05 0.1 0.15 0.2 0.25 0.3 Maximum spectrum amplitude (a.u.)

(a)

Return data 0.25 Background noise distribution envelope

0.2

0.15

Probability 0.1

0.05

0 0 0.05 0.1 0.15 0.2 0.25 0.3 Maximum spectrum amplitude (a.u)

(b)

Figure 5.21: Histogram analysis of peaks in the backscatter spectra. (a) Background noise spectra peak amplitude probability distribution. (b) Backscattered spectra peak amplitude probability distribution. 5.4. ATMOSPHERIC TEST OF CLR 135

Using a noise floor of <0.04 a.u., the peak amplitude of the Fourier spectrum in Figure 5.16(b) is approximately a factor of 9 above the noise. However, return spectra amplitudes are typically a factor of 5 to 6 above the noise floor. The distribution of the maximum amplitudes in the backscattered spectra for the 25 range gates between 1775 ± 30 m to 2225 ± 30 m is shown in Figure 5.21(b). Since the distribution is distinctly different to that for the background noise, the peaks in those range gates are clearly due to real signals.

Atmosphere proﬁling

Spectra at various ranges for a typical single-shot backscattered return were shown in Figure 5.17. Figure 5.22 shows the results of analysing all the range gates in a single-shot backscattered return to a distance of 2500 m. Only those gates for which amplitudes of the largest peak was greater than the estimated amplitude noise ﬂoor were considered This proﬁle indicates that in the PBL there are wind directions both receding from (Region 3) and approaching (Region 2) the laser beam transmitted into the sky. The returns from the temperature inversion layer (Region 1) also have wind directions approaching the transmitted beam, but at a lower LOS velocity than Region 2. The wind speed of ∼1 m/s recorded in Region 3 equates to a horizontal wind speed of ∼16 km/h. This result is consistent with measurements performed at the same time by the Australian Bureau of Meteorololgy (BOM) at its Adelaide Kent Town site. The BOM recorded an average horizontal wind speed of ∼19 km/h from the SSE at a height of 48 m. Though the CLR in this project measures a wind velocity component rather than the exact wind direction, a wind from the SSE would result in a component of the wind receding from the transmitted beam as was observed in Region 3.

Shot to shot repeatability

Measuring reproducible shot to shot Doppler shifts in the return Fourier spectra peaks over a short time period gives an indication of the durability of the system and gives further conﬁdence in the measured single-shot wind speeds. Figure 5.23 shows the reproducibility in the Fourier spectrum peaks for 3 consecutive “good” seeded data sets. The transmitted pulse peak power and frequency for these pulses were not identical, and thus a frequency shift of the data, as discussed previously in Section 5.2.5, was performed. Although the amplitudes of the peaks in the return 136 CHAPTER 5. LASER RADAR MEASUREMENTS

Region 1 2500 2200

Region 1 2100

Range (m) 2000 2000

1900 -5 -4 -3 -2 -1 0 1 2 3 Wind speed (m/s) Region 2 800 1500

700

Range (m) 600 Range (m)

1000

500 Region 2 -5 -4 -3 -2 -1 0 1 2 3 Wind speed (m/s) Region 3 400

500 Region 3 300

Range (m) 200

0 100 -5 -4 -3 -2 -1 0 1 2 3 -5 -4 -3 -2 -1 0 1 2 3 Wind speed (m/s) Wind speed (m/s)

Figure 5.22: Atmospheric wind speed profile. The plot on the LHS is the LOS wind profile to 2500m for a single-shot. The plots on the RHS are looking in more detail at certain regions of the atmosphere. Only gates for which the largest peak was greater than the background noise floor are included in this plot. Thus, gates with no significant peak presumably had low aerosol density. Winds receding from the transmitted beam are plotted with positive wind speeds while those approaching are plotted with negative wind speeds. 5.4. ATMOSPHERIC TEST OF CLR 137

Range = 1975 m Range = 2035 m 0.25 0.25 Return 1 Return 1 Return 2 Return 2 0.2 Return 3 0.2 Return 3

0.15 0.15

0.1 0.1 Amplitude (a.u.) Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz)

Range = 1995 m Range = 2055 m 0.25 0.25 Return 1 Return 1 Return 2 Return 2 0.2 Return 3 0.2 Return 3

0.15 0.15

0.1 0.1 Amplitude (a.u.) Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz)

Range = 2015 m Range = 2070 m 0.25 0.25 Return 1 Return 1 Return 2 Return 2 0.2 Return 3 0.2 Return 3

0.15 0.15

0.1 0.1 Amplitude (a.u.) Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz)

Figure 5.23: Reproducibility of spectra. Returns 1 and 2 are separated by 1 s. Returns 2 and 3 are separated by a further 4 s. 138 CHAPTER 5. LASER RADAR MEASUREMENTS

spectra for each range gate vary (discussed below), the peak frequencies agree well. These results indicate that averaging the results of multiple pulses should improve the SNR of the resulting spectrum.

Shot to shot Fourier spectra peak amplitude ﬂuctuations

The density of the atmospheric backscattering particles at a specific range is usually determined from the Fourier spectra peak amplitude, or the area under the peak. In order to measure accurately the density on a single-shot, the amplitude of the Fourier spectra peaks should agree shot to shot. While the frequency of the peaks in the Fourier spectra of the backscattered light in Figure 5.23 appear reproducible, their amplitudes exhibit large variability. This variability has also been observed in other coherent lidar systems [62, 233– 235]. These publications attributed the variability to the loss in coherence due to atmospheric turbulence acting on the laser beam as it propagated to and from the target, and target induced speckle giving fluctuations in the backscattered power and coherence. It is difficult to separate the relative contributions of the speckle induced fluctuations or the atmospheric turbulence effects to the variability in the SNR amplitude. Other possible explanations include:

• ﬂuctuations in the density of the scatterers,

• ﬂuctuations in the LO power, described in Section 4.6.2,

• ﬂuctuations in the energy of the transmitted pulse,

• ﬂuctuations in the coupling and overlap of the transmitted, returned and LO beams due to vibrations of the optical breadboard on which the CLR was mounted.

However, Figure 5.24 shows that there is still signiﬁcant variability in the shot to shot backscattered spectra peak amplitudes, even when the LO power and the transmitted pulse energy and frequency are identical.

5.4.1 Summary

Atmospheric experiments were undertaken using ∼1.1 mJ pulses. Single-shot returns from a range ≥2 km were observed in agreement with results obtained simultaneously with a boundary layer radar. To show that the measured atmospheric returns were 5.4. ATMOSPHERIC TEST OF CLR 139

Transmitted signal Range = 1665 m 250 0.25 Transmit 1 Return 1 Transmit 2 Return 2 200 0.2

150 0.15

100 0.1 Amplitude (a.u.) Amplitude (a.u.) 50 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz) (a) (b)

Range = 1700 m Range = 1760 m 0.25 0.25 Return 1 Return 1 Return 2 Return 2 0.2 0.2

0.15 0.15

0.1 0.1 Amplitude (a.u.) Amplitude (a.u.) 0.05 0.05

0 0 20 30 40 50 60 20 30 40 50 60 Frequency (MHz) Frequency (MHz) (c) (d)

Figure 5.24: Variation in backscattered spectra peak amplitudes. Returns 1 and 2 are separated by 1 second. (a) Transmitted spectra of two consecutive injection- seeded outcoupled pulses with the same transmitted frequency and peak power. (b) (c) (d) Backscattered spectra from 3 diﬀerent ranges, showing the shot to shot variation in the peak amplitude. 140 CHAPTER 5. LASER RADAR MEASUREMENTS

real, a histogram analysis was performed on the return signal spectra. Peak spectral amplitudes were measured to be a factor of 9 above the noise, however amplitudes of a factor of 5 or 6 above the noise are typical. A LOS velocity proﬁle of the atmospheric wind velocity with range was presented and shot to shot reproducibility of the peak in the Fourier spectra was investigated.

5.5 Conclusion

This chapter has described the design of the receiver system that when used in combination with the laser transmitter described in the previous chapter, created a CLR system. The data processing software was also described. Results characterising the system with a stationary target confirmed that both the “transmit” and “return” sections of the receiver produced identical results as required. The spectral content of the output from the transmitter was also characterised using the receiver, and it was found that the pulses are transform limited at a frequency close to that of the injected LO. Further characterisation using a moving hard target was reported with measured Doppler shifts agreeing with expected values. Returns from the atmosphere were confirmed using a histogram analysis and single-shot wind profiles to ranges ≥2kmpresented. Chapter 6

Conclusion

The aim of the research presented in this thesis was the development and characterisation of a new laboratory based laser transmitter and receiver for a CLR system at 1.53 μm. The system was required to be capable of measuring range-resolved atmospheric wind speeds from a single pulse, with range and velocity resolutions of ∼75 m and ∼1.5 m/s respectively. Such a system would allow for high temporal resolution scanning of the sky for applications such as real time wind ﬁeld mapping and pollution dispersion monitoring. Developing such a CLR relies on:

• Choosing the optimum glass gain host, doping concentrations, and gain medium geometry.

• Optimising the design and eﬃciency of the slave laser head to provide thermal control and make maximum use of the available pump energy.

• Optimising the design of the Q-switched resonator to produce an output that is close to diﬀraction limited and maximises the extracted energy.

• Optimising the injection-seeding of the slave laser.

• Designing a receiver that provides eﬃcient transmission and collection of the output light, optimises the sensitivity of the detectors and is unaﬀected by spurious signals.

6.1 Summary of results

This thesis has described the successful development and characterisation of a laser transmitter and receiver, and their combination to yield a CLR. It also describes

141 142 CHAPTER 6. CONCLUSION

the measurement of single-shot, range resolved, Doppler shifted returns from the atmosphere. Chapter 2 discussed the choice of the gain host material, the doping concentrations, optimum gain medium architecture and the design of the laser head for the slave laser. It was shown that Er:Yb co-doped phosphate glass was the best choice forlasingat1.53μm. The pump absorption bands have an excellent frequency match with the pump diodes, allowing the full exploitation of the available pump energy. The optimisation of the doping densities, resulting in the choice of an Er doping of 1.28 x 1019 ions/cm3 and Yb doping of 1.69 x 1021 ions/cm3 was discussed, including how upconversion limits the Er doping density and therefore the amount of energy that can be stored in the gain medium. Investigation of the optimum gain medium geometry for the low gain Er system led to the CPFS architecture being chosen. This provided the longest mode pathlength inside the gain material. Additionally, when the CPFS is side-pumped using fast-axis collimated laser diodes, good pump absorption and overlap of the pumped volume and the laser mode results. Using a thin slab and cooling through the top and bottom faces provides better heat removal than cooling through the pump faces when using a gain medium that has a low pump absorption coefficient. The chapter concluded with a description of the mounting of the laser diodes and gain medium in the slave laser head. Chapter 3 described the optimisation of the laser head and its characterisation using a standing-wave resonator. Tuning of the laser diode output wavelength by adjusting the temperature of the diode package yielded a pump absorption of 93.4 %. Bleaching of the Er ions at high incident pump energies limited the small-signal gain to a maximum of 1.23 at a pump energy of ∼250 mJ. A gain-switched pulse energy of 74 mJ from ∼577 mJ of pump energy with a slope efficiency of 18 % was obtained, which compares very well with similar systems. The thermal lens was measured using two techniques, a Mach-Zehnder interferometer and a probe- beam displacement method. Good agreements of thermal focal lengths for both the vertical and horizontal planes were shown, with measured values of 0.28 - 0.60 m and 2.8 - 4.1 m respectively. The design of the Q-switched resonator was described in Chapter 4. This novel travelling-wave resonator was used as it is ideal for injection-seeding, providing natural isolation between injected and outcoupled beams. The outcoupling is controlled electronically, providing a convenient means of adjusting the pulse length. It was shown that the resonator produced a near diffraction-limited output 6.1. SUMMARY OF RESULTS 143

2 ± 2 ± with Mh =1.38 0.14 and Mv =1.55 0.03. Measurements of the unseeded Q- switched output energy showed the expected pump bleaching. While it was apparent that the performance of the laser diodes had degraded signiﬁcantly, the observed onset of bleaching made it clear that replacing them would not signiﬁcantly increase the Q-switched pulse energy.

Injection-seeding of the slave laser was also discussed in Chapter 4. Unex- pectedly large intensity and frequency ﬂuctuations of the output of the master laser were measured, which prevented active feedback control of the seeding. Despite these limitations, successful seeding using manual control was demonstrated. A forward- wave pulse energy of 2.8 mJ was achieved, which is suﬃcient for atmospheric CLR measurements.

In Chapter 5, a monostatic, heterodyne detection, receiver system was discussed. Using separate photoreceivers to monitor the frequency of the transmitted light and the frequency of the backscattered light enabled the gain of each detector to be optimised. Fibre coupling provided the best overlap of the LO and backscattered return at the detector, and along with optimised detectors, maximised the sensitivity of the receiver. Measurements of the heterodyne beat produced using a hard target showed that the injection-seeded pulses were transform limited as required, and good agreement between measured and predicted Doppler shifts was demonstrated.

The use of the CLR system to measure atmospheric wind velocities was also demonstrated in Chapter 5. Reproducible single-shot measurement of range resolved wind velocities with a range resolution of 75 m and a velocity resolution of 1.5 m/s for 1 mJ pulses was described. The LOS wind velocity at ∼2 km was measured with a maximum SNR of 9, and the range of this temperature inversion layer agreed with independent measurement from a VHF boundary layer radar.

Significant fluctuations in the amplitude of the Fourier peaks of the backscattered light, due to effects such as speckle, was observed. However, it was shown that the measured Doppler shift was insensitive to these fluctuations, and to small changes in the frequency and the peak power of the transmitted pulse.

The CLR system described in this thesis provides a proof-of-principle demon- stration of a CLR that can be used to measure wind ﬁeld velocities. The development has highlighted important issues that will need to be addressed when developing the next generation, high temporal resolution CLR, and these are discussed in the next section. 144 CHAPTER 6. CONCLUSION

6.2 Future directions

While the CLR was capable of measuring atmospheric velocities from a laboratory environment, the end application requires a ﬁeld deployable unit that has a range of many kms and can scan the atmosphere at a rapid rate. The following improvements should be incorporated to achieve this goal. To improve the range and temporal resolution, it is recommended that the pulsed laser should have a higher Q-switched output pulse energy and run at higher PRF’s. Though the current laser diodes have a maximum PRF of 20 Hz, they are at present limited to a PRF of 2 Hz due to thermal issues. To operate at higher PRF’s using the current laser diodes, the design of the heat sink used to cool the hot side of the TEC should be improved to allow the extra heat to be removed more easily. To operate at PRF’s >20 Hz, new laser diodes would be required. In the current gain medium design, the pulse energy is limited by bleaching of the pump absorption. Increasing the pump height would allow more energy storage but the TEM00 mode size would have to be increased to extract the stored energy. The thermal properties of glass limits the ability to scale glass host gain medium lasers to high average powers [128]. Therefore to achieve higher PRF’s and average powers than presently used, I would suggest investigating materials with better thermal properties. One possibility is to use a crystalline host such as YAG which has ∼15x higher thermal conductivity than glass, a better quantum defect when resonantly pumped at either 1.530 μm or 1.470 μm, and is now a more mature technology than it was at the start of this project. The injection-seeding also needs to be improved. I would suggest trialing the use of a master laser with better frequency and intensity stability than that currently used. This should allow an active feedback control servo to be employed, which would improve the data collection rate. Appendix A

Publications and presentations

This appendix contains lists of publications and presentations that resulted from and are associated with, work undertaken in this thesis.

A.1 Publications resulting from this work

A paper relating to work presented in Chapters 4 and 5 on the slave laser performance and atmospheric results is currently being prepared. The paper has a working title of, “An Er:Yb:glass coherent lidar and single-pulse measurements of wind speeds”, and the targeted journal is Applied Optics.

A.2 Presentations resulting from this work

1. D. Ottaway, M. Heintze, A. MacKinnon, P. Veitch and J. Munch, “Coherent Laser Radar for Air Pollution Transport Studies”, ACOLS/ACOFT, Nov. 29- Dec. 3 2009, Adelaide, Australia.

2. M. Heintze, F. Jeanneret, J. Munch, D. Ottaway and P. Veitch, “Develop- ment of an Er:Yb:glass Coherent Laser Radar”, AIP 18th National Congress, Nov. 30-Dec. 5 2008, Adelaide, Australia.

3. M. Heintze, J. Munch and P. Veitch, “Er:Yb:glass Laser for Coherent Lidar”, ACOLS, Dec. 5-9 2005, Rotorua, New Zealand.

4. M. Heintze, J. Munch and P. Veitch, “Er:Yb:glass Coherent Laser Radar”, AIP 16th National Congress, Jan. 31-Feb. 4 2005, Canberra, Australia.

145 146 APPENDIX A. PUBLICATIONS AND PRESENTATIONS

A.3 Presentations associated with this work

1. N. Chang, M. Heintze, D. J. Hosken, J. Munch, D. Ottaway and P. Veitch, “Er:YAG Lasers for Coherent Remote Sensing”, (presented by N. Chang), ACOLS/ACOFT, Nov. 29-Dec. 3 2009, Adelaide, Australia.

2. J. Munch, M. Heintze, M. Hamilton, S. Manning, Y. Mao, D. Mudge and P. Veitch, “Eye safe solid state lasers for remote sensing and coherent laser radar”, (presented by J. Munch), IEEE LEOS 18th Annual Meeting, Oct. 23-27 2005, Sydney, Australia.

3. M. Heintze, D. Hosken, D. Mudge, J. Munch and P. Veitch, “High power cw and pulsed lasers for remote sensing”, (presented by J. Munch), Laser Tech- nology for Military Applications colloquium, D.S.T.O., May 4 2005, Adelaide, Australia. Appendix B

Laser crystal schematics

This appendix contains the schematic sent to Jung Precision Optics Pty. Ltd. for the manufacture of the laser slabs used in this project.

147 148 APPENDIX B. LASER CRYSTAL SCHEMATICS

Figure B.1: CPFS design speciﬁcations. Appendix C

Laser diode driver

A schematic of the diode driver constructed by a previous student is shown in Figure C.1

Pulse generator

0-5ms TTL pulse

Diode driver

Schmidt Voltage Battery Comparator FET’s Batteries trigger divider “trickle charger”

Current sensor

Laser diodes

Figure C.1: Block diagram of the diode driver.

A low inductance design is necessary because of the high current being switched. To minimise mains power glitches and surges, batteries were used to store the power for the diodes. Batteries have the advantage over capacitors in that they have a smaller internal resistance. Also as a constant current to the diode is required over the 5 ms long pump pulse duration and only a low voltage is dropped across the diodes, we would require any capacitor to store a large charge, which would mean using a potentially prohibitive large capacitor. Therefore each diode uses a parallel- connected bank of 6 Hawker technology, 2 Volt 8.0 A.H. E-cell, CYCLON sealed lead acid rechargeable batteries, which have an extremely low internal resistance. The

149 150 APPENDIX C. LASER DIODE DRIVER

advantages of these batteries is reliability, low cost, voltage stability and low main- tenance. The switching of the high current is achieved by using three IRFP048N MOSFETs per laser diode. These are connected in parallel to obtain the drive currents required. The batteries are charged when the laser is not in operation, using trickle chargers that are on a separate circuit. To protect the laser diodes they are shorted during the battery charging process. Appendix D

Laser diode characteristics

D.1 Laser diode performance speciﬁcations

The laser diode packages used in this work were purchased from Thomson-CSF (now known as Thales Laser Diodes). The speciﬁcations of the package dimensions is shown in Figure D.1. The two packages supplied and used to pump the gain medium had serial numbers 10854 and 10855. The measured performance speciﬁcations (provided by Thomson-CSF) for these laser diodes are shown in Figures D.2 to D.5.

D.2 Conversion of laser diode drive current to incident pump energy

During the course of this thesis the terminology will switch between talking about the laser diode current set point and the incident energy. At the commencement of this project the laser diodes had a ﬂat top output as shown in Figure D.6(a). By measuring the voltage across the current sensor resistor in the diode driver, the drive current applied to the laser diodes can be determined. This is converted to a total combined incident pump energy incident on the laser slab (assuming both laser diodes are set to the same drive current) by using the laser diode performance speciﬁcations in Figures D.2 and D.4. A summary of the results is listed in Table D.1

D.3 Diode degradation

The pump laser diodes used to pump the gain medium were not new at the commencement of the project, and they had operated for an unknown number of hours.

151 152 APPENDIX D. LASER DIODE CHARACTERISTICS

Figure D.1: Speciﬁcations of the Thomson-CSF laser diode. D.3. DIODE DEGRADATION 153

Figure D.2: Performance of laser diode package, serial # 10854, 1 of 2. 154 APPENDIX D. LASER DIODE CHARACTERISTICS

Figure D.3: Performance of laser diode package, serial # 10854, 2 of 2. D.3. DIODE DEGRADATION 155

Figure D.4: Performance of laser diode package, serial # 10855, 1 of 2. 156 APPENDIX D. LASER DIODE CHARACTERISTICS

Figure D.5: Performance of laser diode package, serial # 10855, 2 of 2. D.3. DIODE DEGRADATION 157

Laser diode drive current (Amps) Total incident pump energy (mJ) 0 0 5.5 5 13.0 12 19.5 63 25.0 114 32.0 164 38.5 215 45.0 266 51.0 316 57.0 367 63.5 418 70.5 467 76.5 519 83.5 577

Table D.1: Conversion of the laser diode drive current settings to the total incident energy on the slab.

However, when the output was directed onto a photodiode the output pulse, shown in Figure D.6(a), typically had a ﬂat, top hat shape. When undertaking Q-switched output energy measurements, the laser diodes are estimated to have operated for many 1000’s of hours. Since the laser head would have to be completely dismantled in order to fully characterise the laser diode output and due to time constraints, measurements of the present laser diode output where performed by analysing the pump light that bypassed the nose of the CPFS gain medium. Preliminary investigations indicated that the laser diodes had degraded and the output pulse shape was no longer top hat in shape. This is shown in Figure D.6(b). For higher laser diode drive currents the worse the apparent degradation in output pulse shape. A nominal drive current of ∼83.5 amps was used for the laser diodes during the course of this project, and Figure D.6(b) indicates a ∼30 % reduction in performance between the beginning and end of the pump pulse. 158 APPENDIX D. LASER DIODE CHARACTERISTICS

20 Amplitude (a.u.)

10 ~25.0 Amps ~56.5 Amps 0 ~70.0 Amps

0246 Time (ms)

(a)

20 Amplitude (a.u.)

10 ~25.0 Amps ~56.5 Amps ~70.0 Amps 0 ~83.5 Amps

0246 Time (ms)

(b)

Figure D.6: Degradation of the laser diodes. (a) Typical output pulse shape of the laser diode at the commencement of the project. (b) Typical output pulse shape of the laser diode at the time of undertaking Q-switched output results. D.3. DIODE DEGRADATION 159

The consequence of laser diode degradation is that for a given current, the outcoupled energy is decreased1, and the wavelength at the diode temperature setting may also have changed2. The diodes output might also experience an increased frequency chirp. This would cause a significant reduction in the pump energy absorbed in the gain medium by the end of the project compared to the start. Not only will this affect the amount of gain available to the laser mode, leading to a decrease in performance, it may also affect the focal length of the thermal lens, which in turn will alter the structure of the resonator mode.

1It is unclear at what stage during the project diode degradation started occurring, resulting in uncertainity in the incident pump energy. A timeline of when various project measurements were taken in relation to the small-signal gain measurement are listed in the below table:

Project measurement Date reported result taken Small-signal gain April 2005 Standing-wave output March 2006 Travelling-wave gain-switched output March 2006 Travelling-wave Q-switched output June 2007 Atmospheric February 2009 2No measurement of the exact spectral content of the laser diodes were recorded. The data from the manufacturer was relied upon and transmission measurements through the slab were used to determine the eﬀective absorption. While this method is practical, it does not help to explain any changes due to possible changes in the spectral content of the diodes as they age. In hindsight, not measuring the spectral characteristics of the diodes at the beginning of the project to compare against was a mistake. 160 APPENDIX D. LASER DIODE CHARACTERISTICS Appendix E

Laser resonator alignment

Optimum laser head performance requires the resonator optical axis to be carefully aligned to the gain region. A probe beam of either a HeNe laser or the ﬁbre master laser is used to achieve this. To measure beam separation angles, four thin, sharp, alignment spikes, S1, S2, S3 and S4 are used. These are positioned with two before and two after the beam angle to be measured. The distances between the spikes are measured as shown in Figure E.1. Using Equations E.1, E.2 and E.3 (which are derived from the cosine rule), the beam angle can be determined.

2 2 2 −1 a + b − c Φ1 = cos (E.1) 2ab

2 2 2 −1 a + e − d Φ2 = cos (E.2) 2ae

θangle = 180 − Φ1 − Φ2 (E.3)

E.1 Standing-wave resonator alignment

E.1.1 HeNe alignment

A collimated HeNe probe beam is used where the beam size is small enough such that no clipping of the beam by the slab or gain apertures will occur. The following procedure is then used to align the HeNe probe beam to the optical axis of the gain

161 162 APPENDIX E. LASER RESONATOR ALIGNMENT

S2 b

f1 d

q a angle

f2 S3 e

Figure E.1: Alignment spike positioning and distances to be measured to calculate beam angles. medium.

Horizontal

1. Adjust the beam separation angle using steering mirrors, to obtain a 5 bounce solution inside the slab, with clean transmission of the beam through the slab.

2. Measure the separation angle of the entry and exit beam by positioning the alignment spikes as shown in Figure E.2.

3. Repeat steps 2 and 3 until the desired θsep angle is achieved.

4. Conﬁrm that the bounce solution has two TIR bounces equally and oppositely spaced along the slab side and that the probe beam is not clipped as it traverses the slab.

Vertical

1. After obtaining correct horizontal alignment and before the slab is pumped, direct the exiting probe beam onto an IR camera and mark the beams position as shown in Figure E.2. E.1. STANDING-WAVE RESONATOR ALIGNMENT 163

2. During the pump pulse use the IR camera to observe the position and shape of the beam. The beam should become elliptical in shape due to the induced vertical thermal lens. If the pumped beam translates so that the central region of the unpumped beam and pumped beam no longer coincide and/or if the beam does not stretch evenly about the unpumped beam position, there is incorrect vertical alignment. Correct vertical alignment will result in the beam shape and position elongating as shown in the insert of Figure E.2.

3. Adjust the vertical alignment and repeat steps 1 and 2 as necessary.

4. Check that the horizontal alignment of the probe beam is still correct.

Insert

Laser diode S1 Before After S2 Camera pumping pumping

q sep HeNe laser

S3 Laser diode S4

Figure E.2: Layout used to align the HeNe probe beam to the gain region of the slab. The insert shows how the pumped HeNe beam should elongate symmetrically about the unpumped centre point on the IR camera when correctly aligned to the middle of the thermal lens.

E.1.2 Standing-wave mirror positioning

As shown in Figure E.3 the HeNe probe beam is used to align the two ﬂat standing- wave resonator mirrors to the gain region. An iris is positioned so that the input beam to the slab passes through the centre of the iris. Output couplers A and B are positioned to be equal distances from the slab Brewster faces and aligned perpendicular to the optical axis. This is achieved when the mirror backreﬂections pass back through the centre of the iris. Once coarse alignment of the resonator mirrors is achieved, the HeNe is blocked and more precise alignment is achieved by optimising the outcoupled energy. 164 APPENDIX E. LASER RESONATOR ALIGNMENT

Output coupler A Laser diode

HeNe laser

Laser Iris diode Output coupler B

Figure E.3: Alignment of the standing-wave mirrors using a HeNe probe beam.

E.2 Travelling-wave resonator alignment

The travelling-wave resonator alignment procedure uses the outcoupled beam of the standing-wave resonator. It is therefore only valid if the standing-wave mirrors have no wedge, which was measured to be the case for the mirrors used in this project. Note: All irises and optics are positioned so that the probe beam passes through, or reﬂects from, the centre of the component.

1. With the slab pumped at the maximum incident pump energy, place irises 1, 2, 3 and 4 in both outcoupled beams from the standing-wave resonator as shown in Figure E.4.

2. Cease pumping the slab and remove the standing-wave resonator mirrors. Col- limate the output from the master laser and adjust the beam size to be smaller than the gain region apertures. Direct the beam to pass through the centre of the four irises as shown in Figure E.5.

3. Use a wedge to split the master laser probe beam exiting the slab into two beams as shown in Figure E.6. Focus one beam onto a photodiode and direct the other onto an IR camera. While the laser head is pumped, simultaneously measure the single-pass gain with the photodiode and the beam position with the IR camera. The small-signal gain factor obtained in Chapter 3 is thus attained, and the master laser position should behave like that observed with the standing-wave resonator alignment, splitting equally about, and having no translation from, the unpumped beam location. If this is not the case, adjust the alignment of the master laser traversing the gain region. E.2. TRAVELLING-WAVE RESONATOR ALIGNMENT 165

4. Position the PBSC in the probe beam a distance L1 (see Section 4.3.3) from the Brewster window, as shown in Figure E.7. Use the alignment spikes to measure the angle between the incident beam and that reﬂected from the PBSC. This angle should be ∼90◦. Rotate the PBSC until this separation angle is achieved. Probe beam backreﬂections from the PBSC should also pass back through the centres of irises 1 and 2 when the correct alignment of the PBSC is achieved. To obtain the correct PBSC tilt, the unpumped beam position on the IR camera should be the same as in step 3, and the beam should still pass through the centre of the irises positioned on the beam exiting the slab. At this point HWP 1 can also be positioned.

5. Position the 0.5◦ intracavity wedge at Brewsters angle, close to the laser slab. Use the alignment spikes to obtain the correct angle. Due to the limited space available, position the spikes on the beams reﬂected and transmitted by the wedge as shown in Figure E.8 and adjust the tilt of the wedge to obtain the ◦ beam angle θangle of ∼66 . Adjust the vertical tilt of the wedge until the reﬂected and transmitted beams travel parallel to the optical table surface.

6. As the wedge will translate the master beam passing through it, reposition irises 3 and 4. HWP 2 can also be aligned perpendicular to the probe beam as shown in Figure E.9. HWP 2 is only used to assist in aligning the resonator.

7. The Pockels cell alignment uses a HeNe probe beam. The master laser is blocked and the HeNe probe beam is directed through irises 1, 2, 3 and 4. The Pockels cell is then placed in the beam as shown in Figure E.10, and aligned using the Inrad alignment technique [236]. Correct alignment is obtained when a Maltese cross is observed on a screen placed after the Pockels cell. The Pockels cell used in this project slightly translates a beam passing through it, thus requiring iris 3 to be repositioned.

8. Position the ﬁrst of the intracavity resonator mirrors (mirror 1) a distance

L3+L4 (see Section 4.3.3) from the Brewster window. The angle θangle is set to ∼67◦ using the alignment spikes as shown in Figure E.11.

9. Position the second intracavity resonator mirror (mirror 2) a distance L5 (see Section 4.3.3) from mirror 1 as shown in Figure E.12. Coarse alignment of the resonator is obtained by setting the angle of mirror 2 so that the probe beam retraces the same path around the resonator on the second round trip. 166 APPENDIX E. LASER RESONATOR ALIGNMENT

10. To obtain precise alignment, the interference between the probe beam that travels halfway around the resonator (shown in Figure E.13) and that which travels one and a half round trips of the resonator, before outcoupled by the wedge (shown in Figure E.14) is observed on an IR camera. Mirrors 1 and 2 are adjusted until the whole beam on the camera constructively and destructively interferes when the mount of either mirror 1 or 2 is tapped lightly, which adjusts the round-trip length of the resonator very slightly. This is repeated for various camera positions from the wedge to achieve good beam overlap.

11. To align the image-relay lenses, the HWP’s are adjusted so that upon the probe beam completing a round-trip of the resonator, it is transmitted through the PBSC. This beam is directed onto the IR camera, as shown in Figure E.15, and the beam position marked. Two additional irises (irises 5 and 6) are also located on the probe beam.

12. To align the image-relay lenses, lens 1 is aligned ﬁrst. As shown in Figure E.16, it is positioned a distance D4+L4 (see Section 4.3.3) from mirror 1. The tilt and X,Y translation is adjusted until the probe beam transmitted through lens 1 still passes through the centre of irises 3, 5 and 6 and is incident on the IR camera in the same position as in Step 11.

13. The second image-relay lens (lens 2) is positioned a distance D2 (see Sec- tion 4.3.3) from the PBSC, as shown in Figure E.17. Like the previous step, the tilt and X, Y translation is adjusted until the transmitted probe beam passes through the centre of irises 5 and 6 and is incident on the IR camera in the same position as in Step 11.

14. The final fine adjustment is undertaken while the slave laser is lasing. The probe beam is blocked and HWP 2 removed. As shown in Figure E.18, the output energy and beam profile are analysed simultaneously on a power meter and IR camera respectively. While still maintaining a Gaussian beam profile, the outcoupled energy is maximised by tilting the alignment of the Pockels cell and mirrors, and slight X, Y and Z translation of the image-relay lenses. E.2. TRAVELLING-WAVE RESONATOR ALIGNMENT 167

Iris 1

Iris 2

Output coupler Iris 3 Iris 4 Output coupler

Figure E.4: Travelling-wave resonator alignment 1 of 15.

Master laser

Iris 1

Iris 2

Iris 3 Iris 4

Figure E.5: Travelling-wave resonator alignment 2 of 15. 168 APPENDIX E. LASER RESONATOR ALIGNMENT

Master laser

Iris 1 I.R. Photodiode camera Iris 2

Wedge Iris 3 Iris 4

Figure E.6: Travelling-wave resonator alignment 3 of 15.

Master laser

S1 Iris 1 S2 I.R. Photodiode camera Iris 2 S4 PBSC S3 HWP 1

Wedge L1 Iris 3 Iris 4

Figure E.7: Travelling-wave resonator alignment 4 of 15. E.2. TRAVELLING-WAVE RESONATOR ALIGNMENT 169

Figure E.8: Travelling-wave resonator alignment 5 of 15.

Figure E.9: Travelling-wave resonator alignment 6 of 15. 170 APPENDIX E. LASER RESONATOR ALIGNMENT

Master laser HeNe laser Iris 1

Iris 2

Iris 3

Pockels cell Iris 4

Figure E.10: Travelling-wave resonator alignment 7 of 15.

Figure E.11: Travelling-wave resonator alignment 8 of 15. E.2. TRAVELLING-WAVE RESONATOR ALIGNMENT 171

Figure E.12: Travelling-wave resonator alignment 9 of 15.

Figure E.13: Travelling-wave resonator alignment 10 of 15. 172 APPENDIX E. LASER RESONATOR ALIGNMENT

Figure E.14: Travelling-wave resonator alignment 11 of 15.

Figure E.15: Travelling-wave resonator alignment 12 of 15. E.2. TRAVELLING-WAVE RESONATOR ALIGNMENT 173

Figure E.16: Travelling-wave resonator alignment 13 of 15.

Figure E.17: Travelling-wave resonator alignment 14 of 15. 174 APPENDIX E. LASER RESONATOR ALIGNMENT

Figure E.18: Travelling-wave resonator alignment 15 of 15. Appendix F

Circuit diagrams

Circuit diagrams of the signiﬁcant non-commercially available electronics used during the course of this project are shown in this Appendix. Note: Resistor values are in ohms unless otherwise stated.

F.1 Temperature control of diodes

The laser diode temperature control and stabilisation feedback servo circuit diagram is shown in Figure F.1. This PID control circuit is essentially the same as that designed and implemented by Mudge [169]. A Melcor CP 1.4-127-06L TEC is used in conjunction with the PID circuit to control the laser diode temperature and the error signal stage is adjusted to be able to tune the set point temperature range between 15◦Cto35◦C. The proportional, integration and diﬀerentiation stages were adjusted to ﬁnd the optimum tuning response for the cooling system.

175 176 APPENDIX F. CIRCUIT DIAGRAMS #

) # ) # # # ,

& !!"# '()'*+ (,,(-')./ 0, ' 1# 2 $####%# !,#$ .0#+>/#

6 & /- /6

0: ): # >. 3 6 7!!& 7!!& 3

;

! 6 !6

+ :-. +0#-)

7 4 6 34 ;#<#",6$=8> 9 0##>#<#8",6$=;9#3#

7! :--+>#-) & &

7 !6 !& 4 34 !!6 !6 ). ++0#.+ 0 !6 !6 !6 !!6 '.-+ 0#8 9 7! + .> +0 7! +&&.. + +0 7! )0)0 +0/ -

!!6 &

,"&

374 6 !6 !!6 7 7 7 4 4 4 34 34 34 - 7!6 !!6 7!6 76 56 ,- 5!& 4 7!6 34 - ' '3 76 0 ' ) #%, ) /-""

Figure F.1: Laser diode temperature control feedback servo schematic. F.2. POCKELS CELL CONTROL ELECTRONICS 177

F.2 Pockels cell control electronics

To operate the Pockels cell to obtain Q-switched pulse outputs a custom driver was designed and built. It uses the same signal from a pulse generator that controls the diode driver as its trigger. Schematics of the driver are shown in Figures F.2- F.6. It was designed to work in two diﬀerent operating modes. The “Long Timing” approach uses the rising edge of the input pulse as its trigger to enable the voltage to be applied to the Pockels cell during the pump pulse after some preset delay. The “Short Timing” operating approach is designed to apply a voltage to the Pockels cell immediately after the end of the pump pulse. Throughout the duration of this project the “Short Timing” operational mode is used. The timing of these two methodsisshowninFigureF.7. 178 APPENDIX F. CIRCUIT DIAGRAMS # !# " " (( ! ( $ ! !, ( $ * + ) " & ! ! ' " # ! "!# %! % ! ( " , !" - . !$//, 0 ! $

Figure F.2: Pockels cell driver schematic 1 of 5. F.2. POCKELS CELL CONTROL ELECTRONICS 179 # # # - "' . ,' 10 . ,' / ! / !$ &!, &!, " * + ) ) ) % "' & & &!, !/!! ! ( ) ! " " $ ( , /

+ +

$ ! " " ,$ ,$ ) ) !

( ( " !/!! /

+ +

! !( & ! "& ,$ ,$ ! !! $,' $,' " " !/ $ #

Figure F.3: Pockels cell driver schematic 2 of 5. 180 APPENDIX F. CIRCUIT DIAGRAMS # / ))

+ + /

! !( " " , " !" & ,$ ,$

$ ,$ (

( ,$ !(

! / ! !!

( ! ( ' &!, ' " " ' 5 (& ,$ !( !"2 $ " $& ,$ ( $, $ " ( )) ) * + ) )) ) (# $ ,$ ( ! "

+ +

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# !$ + + & , ! !

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!! !( !

" + + !

! & ! ,$ $"(/ $ " ( .! 34 " -40 ( " ( $,' $,' ( ,$ !( !! " " ! " $ ,$ ( !! ! ! !( ) '

Figure F.4: Pockels cell driver schematic 3 of 5. F.2. POCKELS CELL CONTROL ELECTRONICS 181 # $# & * + ) ) ) ! ! ! ' ! ' ! ' ! ! / ( ) !' ) " )) $ 1 2 #,"' ( $ ! !3 ! 3 " ((

))

Figure F.5: Pockels cell driver schematic 4 of 5. 182 APPENDIX F. CIRCUIT DIAGRAMS

" "!# .670 ! " .670

) "!# .89:'0 ($ ) "!# .89:'0

) .89:'0 " " .A99?;0

.;<=>0 ,/ % .@6A0

.?6:2@0 ! ) .89:'0

.8930 !! ! ) .89:'0

!( !$ # .86?;20

.;<=>0 !" ! .@6A0

.A99?;0 !, !/ # .?6:2@0

" .8930 ! # .86?;20

.@6A0 ! ) .89:'0

) .86?;20 ( $ # .;<=>0

.A99?;0 " )# .8930

" .670 , / " # .670

" "!# .670 (

# )

(

) * + "# #

Figure F.6: Pockels cell driver schematic 5 of 5. F.2. POCKELS CELL CONTROL ELECTRONICS 183

B" -4

.! E=2 ,0

! 34 " -4

E=2 "

2C> D >6:24=>=?2 34

.( E=2 0 .=64> D !# A9 F! 340

E=2

))

B(( 34

B" -4

))

B(( 34

) * + ) ) # ) #

Figure F.7: Pockels cell driver timing diagram. 184 APPENDIX F. CIRCUIT DIAGRAMS

F.3 Interlock

An interlock system used to control the operation of solid-state lasers was jointly designed by Dr. D. Mudge, Dr. C. Hollitt, Dr. D. Hosken, N. Wild and the author. It was designed to monitor a door controlled interlock and the laser diode, slab and laser head base temperatures, as well as the mains power and the water flow (if applicable). It switch’s the laser diodes off if unauthorised entry to the laser area has occurred, or no cooling water is flowing to the laser system. If the diode, slab or base temperature exceeds a user defined operating range, the controller determines which module needs to be shut down. This means that only the affected system will be stopped. To prevent the laser switching off and on if the mains power turns off and on, the interlock will latch the laser system off if the mains power switches off unexpectedly. An emergency stop is also integrated into the system to turn off the laser diodes. A user reset is used to reactivate any of the systems once shut down. The interlock circuitry is shown in Figures F.8 - F.16. F.3. INTERLOCK 185 # .0 .0 " ( $ ! . & 0 .& 0 . ! 0 . ! 0 . " ! " ! "& ! ' ) G $ !# ( " ! '

& " & & " " " " / ! , ! ! ! ! ! ! ! ! ! ! ( $ " ! ! !! ! , ! !( !$ % . &0 & ! ( / " $ , $ $ & $ ! ( ! .0 .0 (" . & ("0

Figure F.8: Interlock schematic 1 of 9. 186 APPENDIX F. CIRCUIT DIAGRAMS # #/ ! % % ) ) ! !H / , ( ' ' ( $ E=2 !! I ) E=2 $ I / / , ! -#7@ ! -#7@ ) G ) ! H $ !, ( $ !,& ( $

" ! !$ ! ! 3 ! 3 !/ ' !/ ' !, ( $ !, ( $ ( ! !( ' ' I ! ! ! I ! ! !,/ !,/ ! $ ' ( !$, ' ! $ ' ( !$, ' " ' !$/ ' " ' !$/ ' I ! " " I "$ /, % (" (" ! ! ' J " 7@ . !"! $(0 . & ("0

Figure F.9: Interlock schematic 2 of 9. F.3. INTERLOCK 187 # $ ( % % ) ( ) $ (H$ !/ !, ! !" ' ' ( $ E=2 !! I ) E=2 $ I / / , ! -#7@ ! -#7@ ) G ) ( H $$ (# ! ( $ ! & ( $

" ! ! !$ ! 3 ! 3 !/ ' !/ ' ! ( $ ! ( $ ( ! !( ' ' ! ! I ! ! I ! ! !,/ !,/ $ ' ( !$, ' ! $ ' ( !$, ' ! " ' !$/ ' " ' !$/ ' " " I "$ /, I "$ /, ( $ ! ' J " 7@ . !"! $(0 . & ("0

Figure F.10: Interlock schematic 3 of 9. 188 APPENDIX F. CIRCUIT DIAGRAMS # $# ! !H ! !( ) G ! H $ !( ,$ !( ,$

! ' ! '

$ ! " ! / ! !! !( ! ! ! ( $" ! ,$ ! ,$ ( & & ! !! ! " " ! $ ! ,$ ! & ,$ / " (( E=2 ! I ) E=2 ( I /

, ! ' ,& ! '

! $ (

" " !

, ! ' , ! '

! " ! " " !$ !( !! ! ! ! !

! " " " " ! ! !" (( ! ) ! !" (( ! ! !" (( ! ! !"& (( " $ , / " ' " ' / " , !! ! $ " ' / " ' ! ! ! ! ! ! ! !$ !' !' & !' !' !' !' ) !' !' !" !!

!( , ! " ( ) ! )

Figure F.11: Interlock schematic 4 of 9. F.3. INTERLOCK 189 # "# ( $ (H$ ! $ ) G ( H $ $ !( ,$ !(& ,$

& ! ' ! '

( ! ! ( " ! !( !& ! , !/ !$ ,$ !$ ,$ ( & ( & $ !! ! " " $ ! !$& ,$ !$ ,$ " / (( E=2 ! I ) E=2 ( I /

/& ! ' / ! '

! ( "

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! $ ! " " !$

!( !( ! ! $ $ ( ( !$ !" " " " " ! ! ! (( ! ! ! (( ! ! ! (( ! & ! !& (( $ / ! " $ " ' !( " ' ( , !! ! !" " ' !! " ' ! ! , ! ! ! " ( !' !' !! !! !' !' !' !!& !! / $ !!) !! !' !' !'

!! !! ! ! ! !$ ) ( ) (

Figure F.12: Interlock schematic 5 of 9. 190 APPENDIX F. CIRCUIT DIAGRAMS # #

,$ "$ !

& ! & & (

! +!

( !$ !/

$ !( ( !,

( +(

" ! $ !

$ +$

!! !"

! " ! !( !

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, +,

! + + / + + + +

! (

+/ / (

$ $ / !

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" ! ) " ( " " ( ( !! %! " " "

& ( $ ! ' $& ! ' & $ ! '

! ! ! & !

) ) G $ & ! %! / !! ! " $ ,$( & ,$( ,$( ! ,$ "$

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+ + + + + + + + + + + +

$ / ! $ / ! $ / ! $ !( & $ !( , $ !( ,& $ !( / $ !( /& $ !( " " " ( ( ( ( $ " !! !! !! $ !( ! $& ,$ " + +

( ! !( !

%! ! / ! !!& $ !( !" / !! " ,$( ,$ ,$ $ ,$ " " $

! / ( !! ! !! ( ! ' $ ! ' ! " ! !/!(

Figure F.13: Interlock schematic 6 of 9. F.3. INTERLOCK 191 # ,# & ! ' " . 0 . & 0 ! ) G $ & ! & % / $ ( ! ! ! ' " ! ! ! ! " $ " ./! 0 ! $ $, ($ ($ ($ " " " & & ( & !

Figure F.14: Interlock schematic 7 of 9. 192 APPENDIX F. CIRCUIT DIAGRAMS # /# / " ! 3 ! 3 ! ' '

% % ) G $ (!, (!, ! ! ( $ E=2 !! I ) E=2 $ I / !,/ ! ' , / !$ %( . &0 ! !/& ( $ !/ ( $ !/ ( $ " ! ! !( !/ ( $ ( " !" ' / ' " (/"! !!

Figure F.15: Interlock schematic 8 of 9. F.3. INTERLOCK 193 # # %!H% ) ) ! " " ! " $ " ( ) ( ) $ ) ) " " ) . 0 & ) G ) %! % $ ( , / " ( $ ! ! !,!/ !"! !(!$ !!! ,/ " ($ ! % ! . 0 . !0 . 0 . !0 .&0 . &0 . &0 .&0 .0 .0 ! ! $ ( ( $ " & ) ) ) . 0 ) . 0 ) " " " & ) . 0 ) & & ( ( $ ! ! !,!/ !"! !(!$ !!! ,/ " ($ ! %! ! " ( $ ! & & ! & ( & & ! .0 0 . # . & 0 .& 0 . ! 0 . 0 . 0 . 0 . & 0 . 0 . 0 . & 0

Figure F.16: Interlock schematic 9 of 9. 194 APPENDIX F. CIRCUIT DIAGRAMS

F.4 Manual seeding electronics

To monitor the interference error signal outcoupled by the intracavity wedge, a purpose made detector was built. The circuit schematic is shown in Figure F.17. Originally the circuit was designed for use with a photodiode with an active area of 0.3 mm (model # G8376). It was later modified for use with a photodiode with an active area of 3 mm (model # G8370-03) and it is with the latter photodiode that results presented in this thesis were obtained. Note: The greater the incident power on the photodiode the more negative the output signal. A schematic of the feedback system used to injection-seed the laser is shown in Figure F.18. The 0 to -9 V DC output is amplified by a high voltage amplifier with a gain of 100, which is then connected to the PZT. F.4. MANUAL SEEDING ELECTRONICS 195 # #" B(" - ) ! ! , / H ) * H (!) $,3 $,3 ! ! ! ! ' $', ! ' ! E $!! ?7=?7 :27 :-E9=K=6 :6 7=:4> 8?C $!! $( ( ( ! 2 B("3 B("3 ( ! !

( )/(, ( )/(, ! ! ( ( & & & &

Figure F.17: Schematic of the photodiode used to acquire the error signal for the seeding feedback servo. 196 APPENDIX F. CIRCUIT DIAGRAMS

100 k

+15 V

AC input

100 k 741

Output

-15 V

50 k -9 V

Figure F.18: DC oﬀset circuit used to adjust the PZT voltage. Bibliography

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