Eye safe solid state lasers for remote sensing and coherent laser radar

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1 Eye safe solid state lasers for remote sensing and coherent laser radar Jesper Munch, Matthew Heintze, Murray Hamilton, Sean Manning, Y. Mao, Damien Mudge and Peter Veitch Department of Physics The University of Adelaide Adelaide SA 5005 Australia

2 How to make a laser sensor Eye-safe Keep energy/power low Low power laser, Large transmitted beam Low Duty Cycle Select wavelength for maximum allowed pulse energy

3 Content of talk Coherent eye-safe laser radar: Review of current work in Er:Yb:glass slab lasers Planned work in Er:Yb:YAG New composite slab laser design Eye-safe sensing at low power

4 Our chosen Eyesafe laser species is Erbium Erbium lases at μm, where laser safety allows: 10 the energy per pulse allowed at 2 μm 100x the energy per pulse allowed at 10 μm Allows better spatial resolution (for otherwise similar conditions) Can make use of available telecommunications photonic components: eg Master fiber oscillator BUT: it is a 3-level laser, normally in a phosphate glass host

5 Er:Yb energy level diagram 2 H11/2 4 S3/2 4 F9/2 (1100 nm) 4 I9/2 2 F5/2 2 ms 500 μs 95-99% Energy Transfer (1130 nm) 4 I11/2 976nm PUMP 8 ms 4 I13/2 2 F7/2 1% Thermal Population Laser (1535 nm) Limiting Processes: Ytterbium Bleaching 11/2 Upconversion 13/2 Upconversion 15/2 Depletion 4 I15/2 Ytterbium Erbium

6 Summary of Early work in Adelaide* Demonstrated first injection seeding of single frequency Er:glass laser at 1.5μm Demonstrated successful transform limited coherent Doppler measurement at 1.5μm Initial wind sensing measurements *A. McGrath, J. Munch, G. Smith, P. Veitch, Appl. Opt. 37 (29),

7 Coherent Laser Radar Transmitted Pulse & Reflected Signal SLAVE LASER DETECTOR 2 DETECTOR 1: Transmitted frequency DETECTOR 2: Received power/frequency ADC & Signal Processing

8 First injection seeded Er:glass at 1.5μm Expanded output pulse Tx/Rx telescope Master oscillator AOM Faraday isolators outcoupler Er:glass rod intra-cavity telescope λ/2 plate Littrow grating λ/4 plate PBS λ/2 plate Q-switch Local oscillator beam heterodyne detector

9 The injection seeded, Q-switched laser produced a transform limited linewidth Output monitor signal (arb. units) time (µs) 30 Power (arb. units) FWHM = 1.5 MHz Frequency (MHz)

10 We used the Er:Glass laser to make a Doppler velocity measurement of moving hard-targets 0.14 Power (arb. units) Frequency shift = -4.5MHz Receding velocity 3.5m/s Power (arb. units) Frequency (MHz) Frequency shift = -8.5MHz Receding velocity 6.5m/s Frequency (MHz)

11 Second Generation Er:Yb:Glass Slab Robust laser design Folded, total internal reflection, zig-zag slab Diode laser side-pumping (Q-CW) Injection seeded, Q-switched ring Long output pulse, using new resonator design with efficient out-coupling via throttled Q-switch

12 0 Standing-wave Er:Yb:Glass slab laser Setup Output power Output 1 Output 2 Flat Mirror R = 97.8 % Flat Mirror R = 97.4 % Diode laser 0 Collimating lens 0 Laser slab Total multimode output power (mj) Input pump power (mj)

13 Side-pumped laser head Pumped region of slab Heat Sink Diode Array TEC Collimating lens Slab

14 0 0 0 Injection seeded ring resonator Master laser in Laser output pulse Diode laser Image relay lenses Slab PZT Pockels cell (Q-switch) Max R mirrors

15 Ring Oscillator Q-switch results Gain switched lasing (E=8mJ/pulse) Pump Output Voltage on Photodiode output pulse (V) Voltage on Photodiode pump pulse (V) Voltage on Photodiode output pulse (V) Voltage on Photodiode pump pulse (V) Pump Output Time (sec) Time (sec) Voltage on Photodiode output pulse (V) Voltage on Photodiode pump pulse (V) Time (sec) Pump Output -3-4 Q-switched pulse (E=3mJ/pulse) Q-switch pulse expanded scale: ms

16 Current results with Er:Glass Good long pulse energy in standing-wave oscillator, near TEM oo (50mJ) Q-switched ring oscillator demonstrated Injection seeding demonstrated However :

17 Problems with current Er:Glass slab laser Energy output limited by Er bleaching (measured) High intra-cavity losses in ring oscillator (Pockels cell) Serious thermal lensing limitations Optical damage of glass host Currently max energy per pulse Q-switched is 10mJ/pulse, but need 20-50mJ/pulse raw laser output for scalable systems (eg: larger aperture, system losses) Pulse repetition rate will be limited by thermal effects Pumping limited by frequency chirp in diode-lasers used

18 Continuing effort in Erbium Two parallel approaches: 1. Improve and optimize Er:Yb:glass subject to its inherent thermal limitations. Experiments using different Er, Yb concentrations for optimum pumping Reduce resonator losses Complete injection seeding characterization as laser radar 2. Investigate third generation Er:Yb:YAG

19 Third generation: Er:Yb:YAG at 1.645μm Greatly improved thermal properties of YAG host Better control of thermal lens Better efficiency (lower level has 2% population) Scalable to higher power, rep. rate Manufacture as ceramic YAG material Permits use of our new end-pumped composite slab geometry Experience from our successful Nd:YAG designs directly relevant But requires a new, single frequency master oscillator Recently demonstrated in bulk Er:Yb:YAG* * Georgiou & Kiriakidi, Opt. Eng., 44 Jun mJ output, pumped by 4.7J

20 Scaling to higher power slabs High pump intensities and necessary cooling of the gain medium leads to strong thermal gradients which cause undesirable effects. Issues strong thermal lensing - change from top/bottom cooling to side cooling thermally induced birefringence - use specialized pump distribution

21 Effect of pump profile on depolarization loss in Nd:YAG 4.0 Tophat pump profile Gaussian pump profile 2.0 x [mm] Pump region Intensity Transmission (Birefringence modeling: M. Ostermeyer)

22 Effect of pump profile on depolarization loss in Nd:YAG 4.0 Tophat pump profile Gaussian pump profile 2.0 x [mm] 0.0 Zones of strong depolarization Pump region Intensity Transmission (Birefringence modeling: M. Ostermeyer)

23 Composite end-pumped, side-cooled folded zigzag Nd:YAG slab Pumping Silicon Dioxide Cooling Laser mode AR 0.808μm HR 1.064μm Silicon Dioxide TOP VIEW Brewster angled window SIDE VIEW Glass Undoped:YAG Nd:YAG (0.6 at.%) Undoped:YAG Glass

24 Optical fibres (2D array) Off-axis zigzag pumping rectilinear zigzag duct Optic axis of pump source Optic axis +θ Optic axis -θ Rectilinear zigzag duct allows pumping at normal incidence and mixes pump light prior to slab entry Can pump using fibers by collimated bar-stack-array, and use nonimaging lens duct Scalable by increasing pump power, height of doped and undoped region (mode volume)

25 Tophat pump distribution minimum birefringence Good absorption efficiency due to quasi end-pumping More uniform power loading within slab due to double-clad structure transporting pump light along slab before absorption No hard-edged apertures in vertical direction Large pump input aperture and acceptance angle accommodates real divergent pump sources Insensitive to pump beam-quality due to mixing of pump light in slab Undoped YAG layers produce reduced thermally induced stress Conduction-cooled Composite slab advantages

26 End view of conduction-cooled laser head TEC Heatsink Cu Cu YAG Nd:YAG YAG Indium contact Heatsink Coolant

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28 Initial Laser Performance in Nd:YAG Output Power [W] Slope efficiency 29.5% Pump Power [W] Approximately 90% pump light absorption in end-pumped slab

29 Composite slab design for Er:Yb:YAG Ceramic Doped and undoped Er:Yb:YAG Doping concentrations easily changed Slab configuration based on success with Nd:YAG

30 Er:Yb:YAG laser radar system New master oscillator under development NPRO (non-planar monolithic ring oscillator) Ceramic Er:Yb:YAG To be developed in collaboration with Innolight Injection seeded slave ring oscillator Ceramic composite slab slave as described

31 The DIAL program (DIAL = differential absorption lidar) Aim: Low-Cost profiling of water vapour up to top of boundary layer Provide water vapour concentrations for Quantitative precipitation forecasting, Bushfire danger assessment, fog prediction current technique - radiosondes, high recurrent cost, infrequent data 830nm GaAs diode lasers (mature technology) Single mode limited to ca 0.5W (Average power ca 0.5mW - eyesafe!) Detector technology well developed (low-noise single photon) Wavelength control On-line laser (master oscillator) stabilised to peak of water resonance Off-line/ On-line difference frequency stabilised to 15GHz Water resonance ~ 6GHz sea level ~ 1GHz 4km altitude Freq. stability of ~ 20MHz adequate

32 Setup for DIAL

33 Spectral properties of amplifier

34 Wavelength control of master lasers On-line laser stabilisation BLUE LOOP Wavelength difference stabilisation - GREEN LOOP

35 Water resonances near 829nm accessible for diode lasers appropriate line intensity (10-23 cm -1 ) sufficiently isolated from other resonances other lines at 832nm

36 V Stabilization to water resonance (832nm) - error signal at lock-in output wavelength

37 Conclusion Er:glass at 1.53 μm is a useful approach for a simple, low average power eye-safe coherent laser radar, but is limited by thermal effects and damage in glass. Er:Yb:YAG is a promising new, preferred option at 1.6μm Design experience form Nd:YAG directly transferable Low cost alternatives to eye-safe incoherent sensing for short range (<4km) applications using shorter wavelengths are feasible.

38 Producing a tophat pump distribution How? Use a composite slab (doped & undoped YAG layers) End-pumped for good efficiency Side-cooled zigzag slab Pump absorption is a tophat profile, thus minimizing thermally induced birefringence loss (even though diode-laser pump profiles typically produce Gaussian transverse profiles) Thermal lensing minimized by using a zigzag mode-path in the plane of cooling, and by controlling the heat flow in the orthogonal plane

39 Small-signal gain measurement proves bleaching of Erbium 1.2 Gain Proportional to Pump Energy (mj)