Velocity Sensing FSPT Modulated Coherent Lidar

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• Velocity Sensing FSPT Modulated Coherent Lidar

• FSPT modulation
• FSPT-generator _at_ 1.5 µm
• Proof-of-principal

Petter Lindelöw DTU Ørsted, Technical University
of Denmark
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Frequency Stepped Pulse Train (FSPT)
frequency
Emitted Frequency Stepped Pulse Train
?f
?f
Tpulse
Tinter
Time
frequency
f0 fdoppler(x3..x3) 2?f
Returned Scatter at time t0
f0 fdoppler(x2..x2)?f
f0 fdoppler(0..x1)
Scatter distance
cTpulse/2
c(TpulseTinter)/2
c(2TpulseTinter)/2
fd(Range 3) 2?f
fd(Range 1)
fd(Range 2) ?f
Detected Power Spectrum
?5f/2
Frequency
?f/2
3?f/2
3
Unique range cell to frequency window mapping
If ?f gt 2fDoppler max
Scatter distance
Range cell
fx 2?f fDoppler
fx ?f fDoppler
fx - ?f fDoppler
fx fDoppler
Frequency slot
3rd Unmatched with 2?f (Td2TiTp)..
(Td3Tp2Ti) c/2
3rd 3?f/2..5?f/2
fx - 2?f fDoppler
2nd Unmatched with ?f (TdTi ).. (Td2TpTi)
c/2
2nd ?f/2..3?f/2
fx - 3?f fDoppler
1st Matched with LO (Td -Tpulse) ..
(TdTpulse) c/2
1st 0..?f/2
t
Tdelay
fLO
t
fx
fx - 3?f
fx - ?f
fx - 2?f
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Light-wave Synthesized Frequency Sweeper (LSFS)
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LSFS results

• Pulse duration 225 ns - 850 ns
• Period 225 ns 1.3 µs
• Train duration 780 µs, 870 pulses
• Frequency step -27.12 MHz ? 21 m/s
• Frequency pseudo tuning 23 GHz, 190 pm
• Output peak power 0.25 mW

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LSFS results
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FSPT LIDAR PROTOTYPE
LSFS
EDFA
Circulator
Telescope
OPTIC LEVEL
105 m
Amplifier
Detector
ELECTRIC LEVEL
Digitizer
TRIGGER
DIGITAL LEVEL
FFT
Save
SAMPLE
SAMPLE
475 ns pulses Train length 17
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FSPT LIDAR RESULTS
Fan at 105 m (2nd range cell) Doppler
shift N.A. Leakage noise at 54.24 MHz
Fan at 105 m (2nd range cell) Fan
Doppler 38-27.12 10.9 MHz Leakage noise at
54.24 MHz
Fan at 105 m (2nd range cell) Fan
Doppler N.A. Noise at 27.12 MHz
Fan at 105 m (2nd range cell) Fan
Doppler 38.4-27.12 11.2 MHz Leakage noise at
54.24 MHz
Fan at 105 m (2nd range cell) Fan
Doppler 37.6-27.12 10.5 MHz Leakage noise at
54.24 MHz
Fan at 105 m (2nd range cell) Fan
Doppler 37.7 -27.12 10.6 MHz Leakage noise at
54.24 MHz
Fan at 105 m (2nd range cell) Fan
Doppler 38.5-27.12 11.4 MHz Leakage noise at
54.24 MHz
Fan at 105 m cw sensing Doppler shift
10.9 MHz
Fan at 105 m cw sensing Doppler shift
10.5 MHz
Fan at 105 m cw sensing Doppler shift
10.7 MHz
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Advantages of FSPT

• FSPT has the following advantages
• High duty cycle sensing without introducing range
  ambiguities and giving the resolution of a pulsed
  system
• Possibility to measure the sign of the Doppler
  shift and to avoid low frequency RIN noise
  without offseting the LO

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Conclusion

• Introduced and described the advantages of
  Frequency Stepped Pulse Train modulated coherent
  lidars for velocity sensing of distributed
  targets.
• Identified and evaluated a suitable FSPT
  generator.
• Proof-of-principle on a hard target with a
  modified lidar.

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Acknowledgements

• I would like to thank the wind metrology group at
  Risø for letting me borrow and modify one of
  their wind lidars. I would especially like to
  thank Jakob Mann and Ferhat Bingöl for helping me
  with the set up and with the initial
  measurements.
• I would also like to thank Mike Harris at Qinetiq
  for giving valuable input on the details of the
  lidar prototype.