Nd:YAG Laser Power Cycling

1
NdYAG Laser Power Cycling

• The Direct Detection Doppler Lidar uses a
  frequency tripled NdYAG 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
  _at_ 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?
  10lt duty cycle lt 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
  (NTotal power) (1-N)0.1Total Power
• Orbit averaged thermal load is also reduced.
  AverageThermal0.91AveragePower. 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
Duty cycle () Orbit Avg Power Thermal load Shots/year
10 156W 142W 315 million
30 303.4W 276W 945 million
50 451W 410W 1.58 billion
70 600W 546W 2.20
100 820W 750W 3.15 billion
4

• Laser Power Cycling Backups

5
HOMER Diode Arrays Long Term Performance

• Two sets of HOMER LDAs installed on in-house
  lifetest station, operated at HOMERs 17 mJ pulse
  energy specs _at_ 242 Hz.
• Set A (top) Power Cycled Operation
• - 4 G4s, 25C, 50A, 80us
• - gt 5.74 B shots
• - gt 21900 cycles
• - no measureable decay (within 2 cumulative
  instrument noise)
• Set B (top) Continuous Operation
• - 4 G4s, 25C, 50A, 80us
• - gt 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 gtgt 10B
  shots
• 3. Power cycling high power QCW arrays does NOT
  reduce liftime, under these conditions.

6
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
LPWR 5
ARMED 4
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 Heate
rs 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 _at_ 50 Hz) 1064 nm
• output was 687 W (27.7 V, 24.8 A)
• 6.6 system level wall plug efficiency _at_
  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 _at_ 50 Hz
• 2.2 system level wall plug
  efficiency _at_ 355 nm
• Expected 355 nm results - gt410 mJ _at_ 50 Hz (gt45
  THG)
• gt3 system level wall plug
  efficiency _at_ 355 nm