Lidar with Femtosecond FiberLaser Frequency Combs

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Lidar with Femtosecond Fiber-Laser Frequency Combs

• CLRC 07
• Snowmass
• Nathan R. Newbury
• Ian Coddington
• William C. Swann
• Ljerka Nenadovic
• NIST, Boulder CO
• nnewbury_at_boulder.nist.gov
• (303)-497-4227

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Outline

• Fiber-Laser Frequency Combs
• Background
• Properties
• Initial demonstration
• LIDAR with a free-running comb
• Preliminary measurements
• LIDAR with two phase-locked combs
• Conclusion

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Frequency Comb Stable Mode-Locked Laser
repetition rate 20 ns
Power
Time Domain
pulse width 100 fs
The Comb
Time
3 THz
50 MHz
Frequency Domain
Power
Frequency

• First demonstrated with Tisapphire based
  frequency combs
• J. Hall T. Hänsch Nobel prize Jones,
  et al. Science, vol. 288 (2000)
• Udem et al. Nature, vol. 416 (2002)

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The Frequency CombTime vs Frequency Domains
Time domain (Pulses in time)
2f
E(t)
t
t
frep-1
Frequency domain (Comb of lines)
frep
fo frep f/2p
I(f)
f
0
fnnfrf0
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Fiber Frequency Combs
I(f)
Highly Nonlinear Fiber
Er-doped fiber
Fiber amp
1 mm
CW pump
2 mm
Femtosecond Fiber Laser (50-250 MHZ)
Stabilized Comb
PZT
Detect f0 fn
narrow cw laser

• Current Performance
• 50 250 MHz repetition rates
• Spans from 1 to 2 mm
• Sub-femtosec timing jitter
• Sub-radian optical phase noise
• lt 100 mHz comb linewidths
• lt mHz frequency stabilities

(all numbers with respect to cw laser reference)
I. Coddington, Nature Photonics, 1, 283 (2007) W.
Swann et al., Opt. Lett. 31, 3046 (2006). N.
Newbury and Swann, JOSA B, in press (2007)
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Using a comb for Coherent LIDARChallenges
Wide bandwidth (25 nm) ? High range resolution
(45 mm ) Narrow modes ? High velocity resolution
100 fs
20 ns
5 nJ
100 MHz Mode-locked Ring Laser
EDFA
Challenges 1) A coherent pulsed Local
Oscillator 2) That shows up at the right
time 3) Also Stretching of return pulse, speckle
noise, chirped-pulse amplification . . .
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Outline

• Fiber-Laser Frequency Combs
• Background
• Properties
• Initial demonstration LIDAR with a
  free-running comb
• Split off part of source laser to use as LO
• Fourier process of signal to relax delay line
  requirements
• Preliminary measurements LIDAR with two
  phase-locked combs
• Conclusion

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Spectrally-Resolved Coherent LIDARdivide and
conquer
Femtosec Fiber laser
signal
Local Oscillator
delay
For Range Process coherently (Fourier Transform)
_at_ fixed delay For Doppler Average shift on all
channels (speckle average)

• Why spectrally resolve channels?
• Required delay line precision set by filter
  bandwidth not pulse bandwidth (e.g. 45 mm to 2 mm
  for 0.6 nm filter)
• Spectral-averaging of Doppler shift to mitigate
  speckle effects
• Can phase-compensate channels for differential
  dispersion

Swann and Newbury, Opt. Lett., 31, 826-828 (2006)
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Laboratory Setup
Femtosec Fiber laser
Speckle Target (rough Aluminum)
200 m
0.5 W EDFA
50 MHz
signal
10 Hz rotation
LO
Delay
AOM
Conventional Channel
return
Dn30kHz
AOM
25 nm (45 mm range resolution)
Arrayed Waveguide Grating
Spectral Channels
6 Discrete Detectors
1580 nm
1540 nm
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Vibrometry Speckle-Averaging
Time Signals Uncorrelated
Femtosec Fiber laser
signal
l6
l5
delay
l4
Voltage
l3
l2
30 kHz heterodyne frequency
l1
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Example Vibration Measurements
Apply fake vibration signature of 50 Hz, 0.34
mm/s to target
Variance is six times smaller using
speckle-averaging (further reductions possible
with more detectors)
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Range ImagesFourier Processing
signal
Femtosec Fiber laser
delay and sum FFT of channel data
Delay in software
delay
N-channel filter
Similar to OFDR, Fourier-Domain OCT, Spectral
interferometry
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Spectrally-resolved coherent LIDAR

• Provides
• Relaxed control of delay line
• Speckle-averaging of Doppler signal
• Phase-compensation possible
• 10s mm range resolution
• However
• Poor range ambiguity (limited by spectral filter
  finite of detectors)
• Variable delay line still required at mm level
• Solution Use a 2nd phase-locked source as LO
• Can be viewed as spectrally resolving individual
  comb lines
• But more easily viewed in time domain .

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Coherent LIDAR with two phase-locked fiber combs

• Advantages
• No delay line whatsoever
• Widest possible range ambiguity
• Full phase-compensation possible
• Flexible tradeoff in range resolution vs
  acquisition time vs sensitivity vs Doppler
• Single photoreceiver/digitizer

pulses overlap here
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Cavity-stabilized 1550-nm cw fiber laser(for
stabilizing the femtosec lasers)
Pound-Drever-Hall Cavity Lock
Laser
stabilized output
AOM
99
PM
5 cm
1
10 cm
l/4
synthesizer
Vibration isolation
servo filter
Error out
1 Hz Linewidth
1 Hz
1.5 sec acq. time, measured vs 0.1 Hz 1126 nm
standard
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Simplified Schematic
Cavity-Stabilized CW Fiber Laser (1550 nm)
1550 nm
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Example Signal
local oscillator
backreflection from target(s)
R2
R1

• Conditions
• frep 100 MHz
• Dfrep 600 Hz 1/1.7ms
• 1.6 nm FWHM bandpass
• 5000 total comb lines detected
• Overlap for 25 shots

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Range Image(preliminary data)
1.7 ms/frame
Moveable mirror
Range (m)
4 reflector
Time (s)
Femtosec Fiber laser (LO)
Femtosec Fiber laser (source)
(Range not yet fully calibrated)
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Range Image expanded view (preliminary data)
1.7 ms/frame
0.4 mm
1 cm
Range (m)
1 cm
Time (s)
Sub-mm range resolution
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Conclusion

• Can transfer linewith/stability of a cw
  cavity-stabilized laser to a mode-locked
  femtosecond laser
• Sub-Hz linewidths on individual modes (relative)
• Sub-femtosecond pulse jitter
• Resulting source is broadband coherent
• Equivalent to multiple (10ks) or coherent
  sources
• Equivalent to coherent pulsed RADAR source but in
  optical
• Demonstrated single-source coherent lidar
• lt 45 mm resolution
• Speckle averaging for improved Doppler
  sensitivity
• Preliminary results on flexible coherent
  femtosecond lidar
• Fully resolvable comb lines
• 1.5 m range ambiguity with sub-mm range resolution