1. Optimum Laser PRF Study for Pulsed Wind Lidars
M. J. Kavaya
NASA Langley Research Center to
Working Group on Space-Based Lidar Winds
8-9 Feb 2011

2. This is a notional presentation with many assumptions
Please don’t place emphasis on exact numbers

3. 4 Different Cases Considered
Coherent detection wind lidar, constant laser optical power
Coherent detection wind lidar, constant laser wallplug power
Direct detection wind lidar, constant laser optical power
Direct detection wind lidar, constant laser wallplug power
5 Figures of Merit
Wind measurement performance
Laser design difficulty (optical power)
Laser wallplug power
Optical damage
Computer speed and data rate

4. Relative Importance of Figures of Merit
Guess at
Relative Importance of Figures of Merit
Importance
1 Wind Performance
2 Laser Design Difficulty
3 Wall Plug Power
4 Optical Damage
5 Computer Speed & Data Rate
Ground XX X
Airborne
Space ISS JEM EF
Space FF

5. Optimum Laser PRF fL (Energy = EL) Benefits and Costs
Figure of Merit
Coherent Detection
Direct Detection
1 Wind measurement performance
2 Laser design difficulty (optical power)
3 Laser wallplug power
4 Optical damage
5 Computer speed and data rate
W’s are weighting constants

6. Cases 1 & 2. Coherent Detection Wind Lidar
Constant Laser Optical Power & Constant Laser Wallplug Power

7. Cases 1 & 2. Coherent Detection Wind Lidar
Constant Laser Optical Power & Constant Laser Wallplug Power

8. Cases 3 & 4. Direct Detection Wind Lidar
Constant Laser Optical Power & Constant Laser Wallplug Power

9. All 4 Formulae

10. 9 Different Dependences on fL

11. Parameter Values for Calculations
Coherent
Direct f0
939 Hz
939 Hz*
KOPO
0.5
0.5*
hWPE
0.012 at 10 Hz
0.030 at 100 Hz
KWPE
2.266
0.590
POPT
2.5 W
32 W
PWP
208 W
1067 W
*same as coherent due to ignorance of model value

12. Equal Weightings, Performance x 100
Optimum PRF: COH OPT < COH WP < DIR OPT < DIR WP
Coherent favors higher EL more than direct.
Wallplug power introduces efficiency, which favors higher PRF

13. Equal Weightings, Performance x 100, Data x 5
Higher data rate weight moved direct PRF down more than coherent

14. Equal Weightings, Performance x 100

15. Equal Weightings, Performance x 100, Damage x 100
Large damage weight only slightly increases optimum PRF (hence slightly lower energy)

16. Equal Weightings, Performance x 100

17. Equal Weightings, Performance x 100, Laser Difficulty x 10
Moderately weighting laser difficulty lowers optimum PRF for fixed wallplug power
Does not change optimum PRF for fixed optical power, as expected

18. Equal Weightings, Performance x 100

19. Equal Weightings, Performance x 100, Wallplug Power x 10
Moderately weighting wallplug power greatly flattens PRF dependence of all cases
Terms with WPOW either independent of or gently depend on fL
Does not change optimum PRF for fixed wallplug power as expected

20. Performance = Damage = 10,000. Data = 100. Others 0.
Broadest range of PRF = direct, constant WP. Narrowest = Coherent constant OP.

21. Other Results Conclusions
Increasing optical power increases optimum frequency for fixed optical power cases
Conclusions
The optimum laser PRF may be different from the laser designer’s point of view, the lidar technique and measured geophysical parameter point of view, or the total space mission point of view?
The numbers herein should not be used, only the concepts