Nd:YAG Laser Power Cycling

1. Nd:YAG Laser Power Cycling • The Direct Detection Doppler Lidar uses a frequency tripled Nd:YAG laser, the same laser technology used in MOLA, GLAS, MLA, CALIPSO. Significantly higher pulse energy/ average power is required for the Doppler measurement. • GWOS DD laser has nominal pulse energy of 360 mJ @ 355 nm at a repetition frequency of 100 pps  36W optical power. • Assuming 4.4% wall plug efficiency to 355 nm the electrical power draw is about 820W. • Since only about 9% of the electrical power is converted to photons the remaining power (750W) will need to be dissipated by the thermal system. • 3.15 billion shots/year. • Question: What are the benefits of operating the laser at a lower duty cycle on a per orbit basis? 10%< duty cycle < 100%.

2. Potential Savings from Laser Power Cycling • Laser lifetime improves • Operation at a duty cycle of N% reduces the shot count proportionally • Recent testing of 808 nm pump diodes at 30% duty cycle shows little or no degradation of the output. Tests are up to 6B shots • Supposition: If power is cycled on every orbit, the orbit averaged electrical power can be reduced by some fraction. • When not acquiring science data we will assume 10% of total average power is required to maintain the laser in standby mode. • For duty cycle N, the orbit averaged power = (N*Total power)+ (1-N)*0.1*Total Power • Orbit averaged thermal load is also reduced. AverageThermal=0.91*AveragePower. Note – to take advantage of this the thermal system must be able to operate with variable heat load to avoid significant over-cooling of the laser when in standby mode.

3. GWOS DD Laser Power Cycling

4. Laser Power Cycling Backups

5. HOMER Diode Arrays’ Long Term Performance • Two sets of HOMER LDA’s installed on in-house lifetest station, operated at HOMER’s 17 mJ pulse energy specs @ 242 Hz. • Set A: (top) Power Cycled Operation • - 4 G4’s, 25C, 50A, 80us • - > 5.74 B shots • - > 21900 cycles • - no measureable decay (within 2% cumulative instrument noise) • Set B: (top) Continuous Operation • - 4 G4’s, 25C, 50A, 80us • - > 6.43 B shots • - no measureable decay (within 2% cumulative instrument noise) • Early Conclusions: • 1. Extensive in-house screening procedures are proving accurate and repeatable. • 2. Proper derating insures long life: >> 10B shots • 3. Power cycling high power QCW arrays does NOT reduce liftime, under these conditions. • Research jointly funded by Biomass Monitoring Mission (BioMM) and Laser risk reduction program (LRRP)

6. Ring Resonator Expansion telescope Optical isolator Amplifier #1 Amplifier #2 Fiber port Power amplifier LBO doubler LBO tripler Fiber-coupled 1 mm seed laser 355 nm output Raytheon 1 J Risk Reduction Laser Optical Layout Final System Optical Configuration Both the original NASA Ozone amplifiers and the power amplifier have been shown to be capable of 100 Hz operation

7. Raytheon Laser Transmitter Alternate Duty Cycle Operation Measured 1064 nm output during typical Off/On cycle “Off” operation is in Armed mode (87 W) “On” operation in HPWR mode (687 W) 88% of full power is reached in 1.5 minutes 93% of full power is reached in 2 minutes 10% duty cycle - 147 W average power - 687 W peak power 50% duty cycle - 387 W average power - 687 W peak power 100% duty cycle - 687 W average power - 687 W peak power

8. Raytheon Laser TransmitterModes and Power Consumption Power-up WARMUP FAULT ARMED LPWR HPWR DIAG Blue text indicates alternative command characters when operating laser system from Hyperterminal serial interface CNTRL INITIALIZE “1” COLD 1 HPWR 6 687 W 28 W CNTRL HPWRMODE “C” CNTRL HPWRMODE ARMED LPWR HPWR DIAG CNTRL HTRSON “C” CNTRL LASERDISARM “4” CNTRL LPWRMODE “D” 87 W “A” WARMUP 2 ARMED 4 LPWR 5 CNTRL LPWRMODE CNTRL LASERARM 687 W 32 W “A” “7” CNTRL STOP CNTRL CLRINT “2” CNTRL DIAGMODE “-” (hyphen) “8” LPWR HPWR DIAG FAULT 3 DIAG 7 687 W WARMUP ARMED LPWR HPWR DIAG Any active fault

9. Raytheon Laser TransmitterState Definitions COLD: Control electronics on Heaters off Faults suppressed Diode power supplies off All diode & QS pulses off WARMUP: THG and SHG heaters on Faults acknowledged Diode power supplies off All diode & QS pulses off FAULT: Active fault detected/latched Heaters on (unless heater fault is active) Diode power supplies off All diode & QS pulses off ARMED: THG and SHG heaters on THG and SHG at nominal temperatures Faults acknowledged Seed laser on Diode power supplies on All diode & QS pulses off HPWR: Heaters on Faults acknowledged Diode power supplies on All diode pulses on, nominal PW QS on Full optical output power (after ramp-up) LPWR: Heaters on Faults acknowledged Diode power supplies on All diode pulses on, nominal PW QS on Low optical output power DIAG: Heaters on Faults acknowledged Diode power supplies on All diode pulses on QS off No significant optical output

10. Raytheon Laser Transmitter Measured System Performance Current system, 100% duty cycle, 50 Hz operation Total DC power consumption (nominal 28 V) at 45.6 W (912 mJ/pulse @ 50 Hz) 1064 nm output was 687 W (27.7 V, 24.8 A) 6.6% system level wall plug efficiency @ 1064 nm Laser mass - 43 kg Laser volume - 10 cm x 42 cm x 69 cm = 29,000 cm3 • Preliminary 355 nm results - 300 mJ @ 50 Hz 2.2% system level wall plug efficiency @ 355 nm • Expected 355 nm results - >410 mJ @ 50 Hz (>45% THG) >3% system level wall plug efficiency @ 355 nm