Optimum Laser PRF Study for Pulsed Wind Lidars

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 Kavaya 12/14/2021 1

• This is a notional presentation with many assumptions • Please don’t place emphasis on exact numbers Kavaya 12/14/2021 2

4 Different Cases Considered 1. Coherent detection wind lidar, constant laser optical power 2. Coherent detection wind lidar, constant laser wallplug power 3. Direct detection wind lidar, constant laser optical power 4. Direct detection wind lidar, constant laser wallplug power 5 Figures of Merit 1. Wind measurement performance 2. Laser design difficulty (optical power) 3. Laser wallplug power 4. Optical damage 5. Computer speed and data rate Kavaya 12/14/2021 3

Guess at Relative Importance of Figures of Merit Importance 1 Wind Performance 2 Laser Design Difficulty Ground XX XX Airborne XX XX X X Space ISS JEM EF XX XX X Space FF XX XX XX 3 Wall Plug Power 4 Optical Damage 5 Computer Speed & Data Rate X Kavaya 12/14/2021 4

Optimum Laser PRF f. L (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 Kavaya 12/14/2021 5

Cases 1 & 2. Coherent Detection Wind Lidar Constant Laser Optical Power & Constant Laser Wallplug Power Kavaya 12/14/2021 6

Cases 1 & 2. Coherent Detection Wind Lidar Constant Laser Optical Power & Constant Laser Wallplug Power Kavaya 12/14/2021 7

Cases 3 & 4. Direct Detection Wind Lidar Constant Laser Optical Power & Constant Laser Wallplug Power Kavaya 12/14/2021 8

All 4 Formulae Kavaya 12/14/2021 9

9 Different Dependences on f. L Kavaya 12/14/2021 10

Parameter Values for Calculations Coherent Direct f 0 939 Hz* KOPO 0. 5* h. WPE 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 Kavaya 12/14/2021 11

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 Kavaya 12/14/2021 12

Equal Weightings, Performance x 100, Data x 5 • Higher data rate weight moved direct PRF down more than coherent Kavaya 12/14/2021 13

Equal Weightings, Performance x 100 Kavaya 12/14/2021 14

Equal Weightings, Performance x 100, Damage x 100 • Large damage weight only slightly increases optimum PRF (hence slightly lower energy) Kavaya 12/14/2021 15

Equal Weightings, Performance x 100 Kavaya 12/14/2021 16

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 Kavaya 12/14/2021 17

Equal Weightings, Performance x 100 Kavaya 12/14/2021 18

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 f. L • Does not change optimum PRF for fixed wallplug power as expected Kavaya 12/14/2021 19

Performance = Damage = 10, 000. Data = 100. Others 0. • Broadest range of PRF = direct, constant WP. Narrowest = Coherent constant OP. Kavaya 12/14/2021 20

Other Results • 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 Kavaya 12/14/2021 21