スライド 1 - University of Colorado Boulder

スライド 1 - University of Colorado Boulder

High-average-power, conductively cooled Tm,Ho:YLF laser for Doppler wind lidar Atsushi Sato1,2, Makoto Aoki2, Shoken Ishii2, Kohei Mizutani2, Satoshi Ochiai2, and Katsuhiro Nakagawa2 1 Tohoku Institute of Technology, Japan National Institute of Information and Communications Technology, Japan 2 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Outline Introduction Laser design Experimental setup Laser experiments for high-average-power operation Investigation of a laser transmitter operating at 40 Summary 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Introduction Space-borne Doppler wind lidar (DWL) Improvements in the weather prediction accuracy and the climate model. W. E. Baker et al., Bull. Amer. Meteor. Soc. 95, 543 (2014). Development of a laser transmitter for DWL Tm,Ho-codoped lasers are suitable for use as transmitters. Eye safety (Lasing at 2 mm) High-energy storage capability Narrow linewidth in Fourier-transform-limited operation Super-Low-Altitude Satellite (JAXA) 220 km Target resolution Horizontal: < 100 km Vertical: < 0.5 km (Altitude 0-3 km) <1 km (Altitude 3-8 km) < 2 km (Altitude 8-20 km) Transmitter requirements 2-mm laser pulse Wavelength 2.05 mm Pulse energy 125 mJ Pulse repetition frequency 30 Hz Pulse width 200 ns Cooling

Conductiv e 3.75W S. Ishii et al., J. Meteor. Soc. Japan 95, 301 (2017). 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Figure of Merit (FOM) FOM = Pulse energy Pulse repetition frequency (Telescope diameter)2 = 0.11 125 mJ 30 Hz 0.4 m 200 20 PRF * Ave. power 150 15 100 10 50 5 FOM=0.11 0 0 25 50 75 100 125 150 175 200 Pulse energy (mJ) Average output power (W) Pulse repetition frequency (Hz) High average power is needed. The same FOM can be obtained even at low pulse energies. However, low energy lasers require higher average power, leading to a large heat load. High energy lasers are advantageous for the

decrease of the heat load. *PRF: Pulse repetition frequency 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Objective Development of a 100-mJ-class, conductively cooled 2-mm laser. Demonstration of high-average-power operation (> 3.75 W). Optimization of a laser transmitter operating at 40 . Diode-pumped Tm,Ho:LLF laser at 6 1) M. Petros et al., Proc. SPIE 5653, 158 (2005). Diode-pumped Tm,Ho:YLF laser at 80 2) K. Mizutani et al., Appl. Opt. 54, 7865 (2015). 3) S. Ishii et al., Appl. Opt. 49, 1809 (2010). 4) A. Sato et al., CLEO-PR 2015, paper 25F3-4. Tm-fiber-laser-pumped Ho:YLF laser at 196 5) H. Fonnum et al., Opt. Lett. 38, 1884 (2013). 100 Pulse repetition frequency (Hz) High-energy, conductively cooled, Q-switched 2-mm laser oscillators Our goal > 30 Hz 50 125 mJ 30Hz at 40 3,4 - 80C 20 10 2 5 5 1 - 196C 5 - 6C 2 1 > 125 mJ 1 10

20 50 100 200 Pulse energy (mJ) 500 1000 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Rate-equation model used in simulations 3H4 Pumping p41 3H5 Cross relaxation F4 3H6 I5 5 p27 HoTm 3 Pump 792 nm Upconversion 5 5 p41 p28 p 71 p27 p (T ) 71 p28 (~ 100 ms) I6 TmHo Laser 2050 nm p28 p

71 Ho Equilibrium constant p very small variation with (T ) 71 dopant concentrations p28 Tm 3F4 and Ho 5I7 population densities in quasi-thermal equilibrium N7 N 1 2 N Ho N Tm (T ) 1 (T ) N 2 N Tm B. M. Walsh et al., J. Lumin. 75, 89 (1997). Lasing I7 5 Tm Upconversion effect is taken into account by assuming a shorter lifetime. Reabsorption I8 Rate equation dN 7 N cf f 7 N 7 f 8 N 8 7 dt t7 N Ho N 7 N 8 df l f cf f 7 N 7 f 8 N 8 dt lr tc f7, f8: thermal occupation factors NTm, NHo: dopant concentrations N2, N7, N8: population densities of the manifolds f: photon density l: laser-rod length lr: resonator length t7: Ho upper manifold lifetime tc: photon decay time 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Calculation of pump intensity distribution Heat sink Pump f433mm 4%Tm,0.4%Ho:YLF Laser rod

Pump 1 mm Pump Pump f2mm 0-0.2 Pump Pump 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1 1-1.2 1.2-1.4 1.4-1.6 Pump absorption efficiency 84% in the whole rod 44% in the central region within a diameter of 2 mm Light guide Ray-tracing software TracePro (Lambda Research Corp.) 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Resonator design Output mirror In the case of fc=1.5m, a stable resonator and a good overlapping between the cavity mode and the central pumped region can be realized. For compensation of thermal lensing Cavity mode radius (mm) 3000 f=-1m f=-4m 2500 f=-2m f=-6m f=-2.5m f=-8m Output mirror

2000 1500 1000 f=1.5m lens 500 Tm,Ho:YLF rod Thermal lens f=-1 -10m 3000 f=-3m f=-10m Cavity mode radius (mm) AO Q-switch 0 Cavity mode radius (mm) f=-3m f=-10m f=-4m Output mirror 2000 1500 1000 f=2m lens 500 Tm,Ho:YLF rod Lens fc=2m 1000 2000 3000 0 4000 1000 2000 Distance (mm) 3000 f=-2m f=-6m 2500

f=-2.5m f=-8m f=-3m f=-10m 3000 f=-4m Output mirror 2000 1500 f=2.5m lens 1000 Tm,Ho:YLF rod 500 3000 4000 Distance (mm) Output mirror 2500 2000 1500 f=3m lens 1000 Tm,Ho:YLF rod 500 f=-2.5m f=-6m Lens fc=2.5m fc=1.5 3m f=-2.5m f=-8m 0 0 Output mirror f=-2m f=-6m 2500 Lens fc=1.5m Cavity mode radius (mm)

Tm,Ho:YLF head 0 f=-3m f=-8m f=-4m f=-10m Lens fc=3m 0 0 1000 2000 Distance (mm) 3000 4000 0 1000 2000 3000 4000 Distance (mm) 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Geometry of the Tm,Ho:YLF laser head Laser head Vacuum container 135 mm 220 mm 300 mm Weight: 18kg Tm,Ho:YLF rod Laser rod 4%Tm,0.4%Ho:YLF diameter: 4 mm, length: 33 mm Cooling of the laser rod Conductive cooling Cu heat sinks were cooled by fluorinert coolant Pump source 3 sets of 3 quasi-CW laser diodes 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato

Resonator configuration Laser output Laser head M1 OC This mirror was used to achieve unidirectional lasing in the ring resonator. M4 AO Q-switch M3 Operating conditions Pump pulse length: 0.50.8 ms Pulse repetition frequency: 5080 Hz L1 L2 M2 2-mm laser output HR Laser head Output mirror HR HR Q-switch fc=1.5m (3m & 3m in the experiment) HR HR mirror High reflection at 2051 nm, flat Output mirror R=6174%, flat Resonator length 3.86 m Q-switch Crystal quartz RF signal: 27.12MHz (Gooch & Housego I-QS027-5C10V10-X5-OS17) 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Dependence on pumping condition Highest energy Average output power (W) 8 50 Hz, 0.8 ms

60 Hz, 0.7 ms 6 70 Hz, 0.6 ms 80 Hz, 0.5 ms 4 2 PRF* (Hz) Pump pulse length (ms) Q-sw. output energy (mJ) Ave. output power (W) 50 0.8 125 6.26 60 0.7 118 7.06 70 0.6 104 7.28 80 0.5 86 6.88 Duty4.2%4.2% 0 0 20 40 60 80 100 Average pump power (W)

120 Highest power *PRF: Pulse repetition frequency 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Lasing characteristics at high PRFs 300 500 250 200 400 150 300 100 200 50 100 50Hz, (a) 0.8ms 0 0 0 0.5 1 1.5 Pump energy (J) 2 Pulse energy (normal) Pulse energy (Q-sw) Pulse width (Q-sw) 600 500 200 400 150 300 100 200

50 100 70Hz, (b) 0.6ms 0 0 0.5 1 1.5 Pump energy (J) Pulse width (ns) 250 600 Pulse energy (mJ) Pulse energy (normal) Pulse energy (Q-sw) Pulse width (Q-sw) Pulse width (ns) Pulse energy (mJ) 300 0 2 A. Sato et al., IEEE Photon. Technol. Lett. 29, 134 (2017). 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato 2.5 1.5 Q-switched 1 pulse 0.5 Condition of simulations 0 0 1000 2000 3000 Time (ns) Pump energy = 1.45 J Crystal temperature = 40 4000 Residual population of the Ho

upper manifold after lasing N7 ~ 0.2 NHo 300 0.5 Pulse interval 250 Stored efficiency 0.4 200 0.3 150 0.2 100 0.1 50 0 0 0 (Typical value for high energy operation) Stored efficiency 2 I7 population density, N7 Pulse interval (ms) 5 Photon density (a. u.) Population of Ho upper manifold ( 1019 cm-3) Contribution of the residual population 10 20 30 40 50 60 70

80 PRF (Hz) 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Dependence of output energy on PRF Output energy (mJ) 80 70 Pump pulse (ms) Pump energy (J) 60 50 Normal mode, 40 1ms 0.9ms 0.8ms 0.7ms 0.6ms 1 0.9 0.8 0.7 0.6 1.47 1.44 1.43 1.47 1.45 40 0 10 20 30 40 Pulse repetition frequency (Hz) 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Dependence of output energy on PRF Output energy (mJ) 120 100 80 Normal (-80 ) Q-sw (-80 ) 60 Normal (-40 ) Q-sw (-40 )

40 20 0 0 10 20 30 40 50 Pulse repetition frequency (Hz) 60 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Comparison between model and experiments 140 400 Pump = 1.45 J R=74%, -40 (Calc.) R=74%, -40 (Exp.) 100 80 60 R=74%, R=74%, R=61%, R=61%, R=74%, R=74%, 40 20 0 0 20 40 -80 (Calc.) -80 (Exp.) -80 (Calc.) -80 (Exp.) -40 (Calc.) -40 (Exp.) 60 Pulse repetition frequency (Hz) 0.30 R=74%, R=74%, R=61%, R=61%, R=74%, R=74%, 0.25

FOM 0.20 0.15 -80 (Calc.) -80 (Exp.) -80 (Calc.) -80 (Exp.) -40 (Calc.) -40 (Exp.) 0.10 0.05 Pump = 1.45 J 0.00 0 80 Pulse width (ns) Pulse energy (mJ) 120 R=61%, -80 (Calc.) 300 R=61%, -80 (Exp.) R=74%, -80 (Calc.) R=74%, -80 (Exp.) 200 100 Pump = 1.45 J 0 0 20 40 60 80 Pulse repetition frequency (Hz) The laser can operate at 40 . Operation at 40 is suitable for generating a long Q-switched pulse. Pulse-energy enhancement is needed at 40 to fulfill our DWL requirement. 20 40 60 80 Pulse repetition frequency (Hz) 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato

Candidates of a laser transmitter operating at 40 Laser oscillator with two laser heads Laser heads Output Master oscillator and power amplifier (MOPA) Laser head (oscillator) Q-switch Q-switch Laser head (2-pass amplifier) Output 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Comparison between two candidates 200 400 300 R=60% (Energy) 125 mJ R=75% (Width) 100 200 R=60% (Width) 50 100 81 ns 0 0 0 Amp.=1.9J Amp.=1.8J Amp.=1.7J Amp.=1.6J Osc. Pulse width 2.9 J 1 2 3 Pump energy (J)

4 Output energy (mJ) 150 200 Pulse width (ns) R=75% (Energy) Output energy (mJ) Master oscillator and power amplifier (MOPA) 150 100 3.1 J (1.5 J+1.6 J) 400 300 125 mJ 200 166 ns 50 100 0 Pulse width (ns) Laser oscillator with two laser heads 0 0.5 1 1.5 Oscillator pump energy (J) 2 PRF = 30 Hz, Crystal temp. = 40 The pump energy for 125-mJ output is somewhat lower than the MOPA. The pulse width is too short. The pump energy for 125-mJ output is somewhat higher than the oscillator. The pulse width is longer than 160 ns. 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Summary An average output power higher than 7 W was demonstrated

in a 100-mJ-class Tm,Ho:YLF laser operating at -80C. In this laser, a Q-switched pulse energy of 45 mJ was obtained even at -40C. Contribution of the residual population of the Ho upper manifold to the next pumping was investigated. A PRF of 30 50 Hz is a promising target for our system. The requirement of our DWL system will be fulfilled by using a MOPA configuration. Please also refer to M. Aoki et al., paper P9 S. Ishii et al., paper 19 We10 Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato th Previous model of the 2-mm laser at NICT Laser head Vacuum container 135 mm Crystal temp. 80 Tm,Ho:YLF rod 220 mm 300 mm Average power 2.4 W (< 3.75 W) 2-mm laser output Q-switch energy, 198K 60 energy, 218K 600 width, 193K width, 198K 400 width, 209K width, 218K 20 200 Q-sw, 30Hz 0.5 HR 800 energy, 209K

40 0 f=3m HR HR 80 Pulse width (ns) Laser head Output mirror 1000 energy, 193K Output energy (mJ) HR 100 0 1 1.5 Pump energy (J) 2 A. Sato et al., CLEO-PR 2015, paper 25F3-4. 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato Beam quality Beam quality factor M2 2.0 70 Hz, 0.6 ms (Horizontal) 70 Hz, 0.6 ms (Vertical) 50 Hz, 0.8 ms (Horizontal) 50 Hz, 0.8 ms (Vertical) 1.5 1.0 0.5 40 60 80 100 Average pump power (W) 120 19th Coherent Laser Radar Conference, Okinawa, Japan, June 18, 2018, Mo6, A. Sato

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