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

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

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

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

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

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

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

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

All 4 Formulae

9 Different Dependences on fL

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

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

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

Equal Weightings, Performance x 100

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

Equal Weightings, Performance x 100

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

Equal Weightings, Performance x 100

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

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

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