Optimum Laser PRF Study for Pulsed Wind Lidars
M. J. KavayaNASA Langley Research Center
toWorking Group on Space-Based Lidar Winds
8-9 Feb 2011
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• This is a notional presentation with many assumptions
• Please don’t place emphasis on exact numbers
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4 Different Cases Considered
5 Figures of Merit
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
1. Wind measurement performance
2. Laser design difficulty (optical power)
3. Laser wallplug power
4. Optical damage
5. Computer speed and data rate
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Importance1 Wind
Performance
2 Laser Design
Difficulty
3 Wall Plug Power
4 Optical Damage
5 Computer Speed & Data
Rate
Ground XX XX X
Airborne XX XX X X
Space ISS JEM EF
XX XX X XX X
Space FF XX XX XX XX XX
Guess atRelative Importance of Figures of Merit
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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
CPERF L LW E f
POW L L
WPE
W E f
DATA LW f
DAM LW E
DPERF L LW E f
[ ] [1/ ] [ / ] [ / ] [ / ] [ ] [ / ] [ ] [1/ ]L L CPERF DPERF LASER WPE POW DAM DATA
Units
E J f s W s J W s J W s J W J s W J W s
LASER L LW E f
W’s are weighting constantsKavaya 04/22/23 5
1
2
[ ]
Two cases: E = or E =
POW L LCPERF L L LASER L L DAM L DATA L
CWPE
COPT CWP CWPEL LL L COPT L CWP L
L CWPE L
COPT POW COPT COPTCPERF LASER COPT DAM DATA L
CWPE LL
W E fFOM W E f W E f W E W f
P PE fE f P P
f f
P W P PFOM W W P W W f
ff
FOM
CWP CWPE CWP CWPECPERF LASER CWP CWPE POW CWP DAM DATA L
LL
P PW W P W P W W f
ff
Cases 1 & 2. Coherent Detection Wind LidarConstant Laser Optical Power & Constant Laser Wallplug Power
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0/
0
Per Jirong Yu and Mulugeta Petros, assume: 1
where f = 939 Hz, and 0.5
Using our space mission studies: (10Hz) 0.012
K 2.266
(formula is actually for a differ
L Cf fCWPE CWPE COPO
C COPO
CWPE
CWPE
K K e
K
01 /
2
ent laser and does not apply for very small or large PRF values anyway, but carry on)
1
1
L C
COPT POW COPT COPTCPERF LASER COPT DAM DATA Lf f
LL CWPE COPO
CWP CWPE COPO
CPERF
P W P PFOM W W P W W f
ff K K e
or
P K K eFOM W
0
0
0
/
/
/
1
1
L C
L C
L C
f f
f fLASER CWP CWPE COPO
L
f fCWP CWPE COPO
POW CWP DAM DATA LL
W P K K ef
P K K eW P W W f
f
Cases 1 & 2. Coherent Detection Wind LidarConstant Laser Optical Power & Constant Laser Wallplug Power
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3
4
[ ]
Two cases: E = or E =
POW L LDPERF L L LASER L L DAM L DATA L
DWPE
DOPT DWP DWPEL LL L DOPT L DWP L
L DWPE L
POW DOPT DOPTDPERF DOPT LASER DOPT DAM DATA L
DWPE L
W E fFOM W E f W E f W E W f
P PE fE f P P
f f
W P PFOM W P W P W W f
f
or
FOM
0/
3
Due to ignorance of 0.355 micron laser model, assume: 1 Forcing (100Hz) 0.03, K 0.59L C
DWP DWPEDPERF DWP DWPE LASER DWP DWPE POW DWP DAM DATA L
L
f fDWPE DWPE COPO DWPE DWPE
DPERF DOPT
PW P W P W P W W f
f
K K e
FOM W P
0
0 0
0
/
/ /4
/
1
1 1
1
L C
L C L C
L C
POW DOPT DOPTLASER DOPT DAM DATA Lf f
LDWPE COPO
f f f fDPERF DWP DWPE COPO LASER DWP DWPE COPO
f fDWP DWPE COPO
POW DWP DAM DATA LL
W P PW P W W f
fK K e
FOM W P K K e W P K K e
P K K eW P W W f
f
Cases 3 & 4. Direct Detection Wind LidarConstant Laser Optical Power & Constant Laser Wallplug Power
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All 4 Formulae
0
0
0
0
1 /
/
/2
/
1
11
1
L C
L C
L C
L C
COPT POW COPT COPTCPERF LASER COPT DAM DATA Lf f
LL CWPE COPO
f fCWP CWPE COPO f f
CPERF LASER CWP CWPE COPO
L
f fCWP CWPE COPO
POW CWP DAM
P W P PFOM W W P W W f
ff K K e
P K K eFOM W W P K K e
f
P K K eW P W
0
0 0
3 /
/ /4
1
1 1
1
L C
L C L C
DATA LL
POW DOPT DOPTDPERF DOPT LASER DOPT DAM DATA Lf f
LDWPE COPO
f f f fDPERF DWP DWPE COPO LASER DWP DWPE COPO
DWP DWPE COPO
POW DWP DAM
W ff
W P PFOM W P W P W W f
fK K e
FOM W P K K e W P K K e
P K K eW P W
0/L Cf f
DATA LL
W ff
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9 Different Dependences on fL
0 0 0
0 0
1 1/2 1
1 1/2 1/ / /
/ /1 1/2
constant
1 1 1
1 1
L C L C L C
L C L C
L L L
f f f f f f
f f f fL L
f f f
e e e
e f e f
Parameter Values for Calculations
Coherent Direct
f0 939 Hz 939 Hz*
KOPO 0.5 0.5*
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
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Equal Weightings, Performance x 100
-1000
-800
-600
-400
-200
0
4 44 84 124 164 204 244 284 324 364 404 444 484 524 564 604 644 684 724
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 1 1 1 1 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 1 1 1 1 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
• 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
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-1000
-800
-600
-400
-200
0
4 44 84 124 164 204 244 284 324 364 404 444 484 524 564 604 644 684 724
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 1 1 1 5 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 1 1 1 5 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
Equal Weightings, Performance x 100, Data x 5
• Higher data rate weight moved direct PRF down more than coherent
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Equal Weightings, Performance x 100
-1000
-800
-600
-400
-200
0
4 44 84 124 164 204 244 284 324 364 404 444 484 524 564 604 644 684 724
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 1 1 1 1 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 1 1 1 1 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
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-1000
-800
-600
-400
-200
0
4 44 84 124 164 204 244 284 324 364 404 444 484 524 564 604 644 684 724
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 1 1 100 1 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 1 1 100 1 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
Equal Weightings, Performance x 100, Damage x 100
• Large damage weight only slightly increases optimum PRF (hence slightly lower energy)
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Equal Weightings, Performance x 100
-1000
-800
-600
-400
-200
0
4 44 84 124 164 204 244 284 324 364 404 444 484 524 564 604 644 684 724
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 1 1 1 1 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 1 1 1 1 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
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-1000
-800
-600
-400
-200
0
4 44 84 124 164 204 244 284 324 364 404 444 484 524 564 604 644 684 724
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
Equal Weightings, Performance x 100, Laser Difficulty x 10
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 10 1 1 1 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 10 1 1 1 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
• Moderately weighting laser difficulty lowers optimum PRF for fixed wallplug power
• Does not change optimum PRF for fixed optical power, as expected
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Equal Weightings, Performance x 100
-1000
-800
-600
-400
-200
0
4 44 84 124 164 204 244 284 324 364 404 444 484 524 564 604 644 684 724
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 1 1 1 1 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 1 1 1 1 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
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Equal Weightings, Performance x 100, Wallplug Power x 10
-11000-9000-7000-5000-3000-10001000
20 220 420 620 820 1020 1220 1420 1620 1820 2020 2220 2420 2620 2820 3020 3220 3420 3620
Laser PRF (Hz)
COH OPT COH WP DIR OPT DIR WP
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 100 1 10 1 1 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 100 1 10 1 1 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
• 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
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1
10
100
1000
10000
100000
0 5 10 15 20 25 30 35 40 45 50 55 60
FOM
Laser PRF (Hz)
Series1 Series2 Series3 Series4
1
10
100
1000
10000
100000
3 203 403 603 803 1,003 1,203 1,403 1,603 1,803 2,003 2,203 2,403
FOM
Laser PRF (Hz)
Series1 Series2 Series3 Series4
WPERF WLASER WPOW WDAM WDATA POPT (W) KWPE KOPO f0 (Hz) PWP(W)
Coherent 10000 0 0 10000 100 2.5 2.266 0.5 939 208Multiplier 1 1 1 1 1
Direct 10000 0 0 10000 100 32 0.59 0.5 939 1067Multiplier 1 1 1 1 1
Performance = Damage = 10,000. Data = 100. Others 0.
• Broadest range of PRF = direct, constant WP. Narrowest = Coherent constant OP.
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
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