High-average-power, conductively cooled Tm,Ho:YLF laser for Doppler wind lidar

1. High-average-power, conductively cooled Tm,Ho:YLF laser for Doppler wind lidar Atsushi Sato1,2, Makoto Aoki2, ShokenIshii2, Kohei Mizutani2, Satoshi Ochiai2, and Katsuhiro Nakagawa2 1Tohoku Institute of Technology, Japan 2National Institute of Information and Communications Technology, Japan

2. Outline • Introduction • Laser design • Experimental setup • Laser experiments for high-average-power operation • Investigation of a laser transmitter operating at −40℃ • Summary

3. Introduction • Space-borne Doppler wind lidar (DWL) • Improvements in the weather prediction accuracy and the climate model. • Developmentof a laser transmitter for DWL • Tm,Ho-codoped lasers are suitable for use as transmitters. • Eyesafety (Lasing at 2 mm) • High-energy storage capability • Narrow linewidth in Fourier-transform-limited operation W. E. Baker et al., Bull. Amer. Meteor. Soc. 95, 543 (2014). Super-Low-Altitude Satellite (JAXA) Transmitter requirements 2-mm laser pulse 3.75W 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) S. Ishii et al., J. Meteor. Soc. Japan 95,301 (2017).

4. Figure of Merit (FOM) FOM = Pulse energy × Pulse repetition frequency × (Telescope diameter)2 = 0.11 125 mJ 30 Hz 0.4 m 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. * FOM=0.11 High energy lasers are advantageous for the decrease of the heat load. *PRF:Pulserepetitionfrequency

5. 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℃. High-energy, conductively cooled, Q-switched 2-mm laser oscillators Our goal 125 mJ ×30Hz at −40℃ • Diode-pumped Tm,Ho:LLF laser at −6℃ • Diode-pumped Tm,Ho:YLF laser at −80℃ • Tm-fiber-laser-pumped Ho:YLF laser at −196℃ 1) M. Petros et al., Proc. SPIE 5653, 158 (2005). 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. 5) H. Fonnum et al., Opt. Lett. 38, 1884 (2013).

6. Rate-equation model used in simulations Pumping ④3H4 Upconversion effect is taken into account by assuming a shorter lifetime. 5I5 ⑤ Crossrelaxation p41 Upconversion 5I6 ⑥ ③3H5 p27 Ho→Tm Lasing ②3F4 5I7 ⑦ Pump 792 nm Laser p28 p71 p28 p71 Reabsorption p27 p41 2050 nm (~ 100 ms) ①3H6 5I8 ⑧ Tm Ho Tm→Ho Rate equation Equilibrium constant very small variation with dopant concentrations Tm3F4 and Ho5I7population densities in quasi-thermal equilibrium 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 B. M. Walsh et al.,J. Lumin. 75, 89(1997).

7. Calculation of pump intensity distribution f4×33mm 4%Tm,0.4%Ho:YLF Heat sink Pump Laser rod f2mm • Pump absorption efficiency • 84% in the whole rod • 44% in the central region within a diameter of 2 mm Pump Pump Light guide Ray-tracing software： TracePro(Lambda Research Corp.)

8. Resonator design Outputmirror Tm,Ho:YLFhead 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. AO Q-switch For compensation of thermal lensing Thermallens f=-1～-10m Outputmirror fc=1.5～3m

9. Geometry of the Tm,Ho:YLF laser head Laser head Vacuum container 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 135 mm 220 mm 300 mm Weight: 18kg

10. Resonator configuration This mirror was used to achieve unidirectional lasing in the ring resonator. • Operating conditions • Pump pulse length: 0.5–0.8 ms • Pulse repetition frequency: 50–80 Hz • HR mirror • High reflection at 2051 nm, flat • Output mirror • R=61–74%, flat • Resonator length • 3.86 m • Q-switch • Crystal quartz • RF signal: 27.12MHz • (Gooch & Housego • I-QS027-5C10V10-X5-OS17) 2-mm laser output HR HR Output mirror Laser head HR fc=1.5m (3m & 3m in theexperiment) Q-switch HR

11. Dependence on pumping condition Highest energy Duty≦4.2% Highest power *PRF:Pulserepetitionfrequency

12. Lasing characteristics at high PRFs 50Hz, 0.8ms 70Hz, 0.6ms A. Sato et al., IEEE Photon. Technol. Lett. 29, 134 (2017).

13. Contribution of the residual population Condition of simulations Pump energy = 1.45 J Crystal temperature = −40℃ 5I7 population density, N7 Q-switched pulse Residual population of the Ho upper manifold after lasing N7 ~ 0.2 NHo (Typical value for high energy operation)

14. Dependence of output energy on PRF Normal mode, −40℃ Pump pulse (ms) 1 0.9 0.8 0.7 0.6 Pump energy (J) 1.47 1.44 1.43 1.47 1.45

15. Dependence of output energy on PRF

16. Comparison between model and experiments • 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.

17. Candidates of a laser transmitter operating at −40℃ Laser oscillator with two laser heads Master oscillator and power amplifier (MOPA) Laser head (oscillator) Laser heads Output Q-switch Q-switch Laser head (2-pass amplifier) Output

18. Comparison between two candidates Laser oscillator with two laser heads Master oscillator and power amplifier (MOPA) 3.1 J (1.5 J+1.6 J) 2.9 J 125 mJ 125 mJ 166 ns 81 ns 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.

19. Summary • An average output power higher than 7 W was demonstrated in a 100-mJ-class Tm,Ho:YLF laser operating at -80ºC. • In this laser, a Q-switched pulse energy of 45 mJ was obtained even at -40ºC. • 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 We10

20. 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 HR HR Output mirror Laser head f=3m Q-switch HR HR A. Sato et al., CLEO-PR 2015, paper 25F3-4.

21. Beam quality