Optimum Laser PRF Study for Pulsed Wind Lidars M. J. Kavaya NASA Langley Research Center to

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


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


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)





Equal Weightings, Performance x 100, Data x 5

• Higher data rate weight moved direct PRF down more than coherent

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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)





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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)





Equal Weightings, Performance x 100, Damage x 100

• Large damage weight only slightly increases optimum PRF (hence slightly lower energy)

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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)





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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)


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

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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)





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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)





• 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




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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Optimum Laser PRF Study for Pulsed Wind Lidars M. J. Kavaya NASA Langley Research Center to

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