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A two-mode interference measurement for
high accuracy absolute laser ranging in space
Duy Hà PHUNG, Alain BRILLET, Michel LINTZ,
ARTEMIS, Observatoire de la côte d'Azur, Nice
Christophe ALEXANDRE
CEDRIC-LAETITIA, CNAM, Paris
M.Lintz, ICSO 2012
11/10/2012
michel.lintz @ oca.eu
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Recent evolution in the field of laser ranging of long distances:
the femtosecond laser frequency comb
Coddington et al. Nature Phot. 3, 2008, 351
Optics Express 19 (2011), 18501
"Optical sampling" method
(pulse interference)
M.Lintz, ICSO 2012
11/10/2012
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Recent evolution in the field of laser ranging of long distances:
the femtosecond laser frequency comb
S. van den Berg et al.
PRL 108, 183901 (2012)
"spectral (dispersive) interferometry"
M.Lintz, ICSO 2012
11/10/2012
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Yet we believe that femtosecond laser frequency combs
are not mandatory for high accuracy laser ranging !
M.Lintz, ICSO 2012
11/10/2012
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Naive idea
Optical
interference
1 cycle = opt=
"non-ambiguity range"
Larger path lengths are
wrapped into [0, opt]
Ideally: Combination of 3
Ideally: Combination of 2 measurements
- optical interference (=> accuracy/resolution << opt )
- time-of-flight measurement
- and "synthetic wavelength" measurement (E. Samain's proposal)
using microwave-modulated optical source
=> Requirement:
accuracy of the "synthetic phase" measurement better than opt/ =10-4
But 20 GHz phase measurements are NOT stable enough on the long term!
=> optical and microwave phase measurements are merged into 1 single measurement
=> two-mode interference measurement solves the problem
time of flight
1km
M.Lintz, ICSO 2012
11/10/2012
1m
"synthetic
wavelength"
1 cycle = = 15mm
for frequency 20GHz.
Larger path lengths are
wrapped into [0, ]
Optical interference
?
1mm
long-term phase drifts
in µw chains
1µm
1nm
1pm
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Ranging with a simple set-up: Two-mode optical interference
Meas
2-mode
laser beam
20GHz beat
Ref
L, 1-
l, 
l0
reference
photodiode
signal
photodiode
micro-wave modulated terms at F=c/ (= synthetic wavelength) :
Meas
Ref
(1   ) cos 2 (ct  L) / 

  cos 2 (ct  l ) / 

Ll


 2  cos 2 ( L  l ) / opt cos 2 (ct 
) / 
2


Int
L scan
opt scan

segment

"hedgehog"
Int
L =
two-mode interference
signal
straight!
(ideally!)
Ref
Meas
phase
M.Lintz, ICSO 2012
11/10/2012

2 ( L  l0 ) / 
2
ei 2 l0 / 
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The recorded two-mode interference signal
synth =15mm
opt =1.5µm
10.000 "spikes" !
target
moved by
/2 =7,5mm
dead zones:
acquisition is blocked
(data transfer; slow link,
replaced by TCP/IP link)
Difficulties:
- stray interference (<10-8 power)
- 20 GHz signal measurement:
accuracy, noise <10-4
- complex procedure
M.Lintz, ICSO 2012
11/10/2012
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Complex signal :
How to deal with it ?
If time-of-flight data are taken beforehand
=> (L-l) already known to better than 10-5
=> 3 successive measurements
of the 2-mode interference signal, at

1= opt,
a
2= opt+ FSR/4
3= opt+ FSR/2
1
a2

a3
=> extraction of all the parameters !
2
3 = opt+FSR/2
synthetic phase is measured
in 100 µs
as a geometrical property
(an angle) of the 2-mode signal
=> not affected any more
by the long term phase drifts
M.Lintz, ICSO 2012
11/10/2012
½(synth. phase)
<=
2 ( L  l ) / 2

a3


a2
2 = opt+FSR/4
opt

a1
phase 2 ( L  l0 ) / 
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ToF:
Non-linear shaping:
S. Pitois (Univ. Dijon)
Control: Etienne Samain (OCA)
Set-up
two-mode beat
pulse train!
8km
two-mode
1, 2
MI
two-mode beat
fast
frequency
update
target
L
pol. contr.
Glan 45°
BS
l
PBS
20 GHz
PhD0
PhD1
20 MHz
complex
procedure:
Acquisition
(phase & amplitude)
M.Lintz, ICSO 2012
11/10/2012
FPGA-based electronics
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Typical length measurements ( raw data )
( slow target movement: 20µm in 10 minutes )
Raw data: considerable systematic effects!
M.Lintz, ICSO 2012
11/10/2012
Origin: electronic distorsion of the signals
- electronic cross-talk
- saturation
- amplitude-to-phase coupling
=> have to be corrected for
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Typical
length recording
measurements
data)
" Iliade
": typical
(target is(corrected
moved 20µm in
10 minutes)
( slow target movement: 20µm in 10 minutes)
7,49278
measurement
length (m)
7,49277
10µm
with correction for
electronic distorsion :
much less systematics!
7,49276
7,49275
7,49274
10 mn
7,49273
M.Lintz, ICSO 2012
11/10/2012
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Typical length measurements (target is moved 20µm in 10 minutes)
7,49278
measurement
length (m)
10µm
1µm
7,49277
correct values
7,49276
7,49275
7,49274
2mn
7,49273
M.Lintz, ICSO 2012
11/10/2012
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Typical length measurements (target is moved 20µm in 10 minutes)
1 measurement = 320 elementary measurements (1/130µs)
7,49278
measurement
length (m)
7,49277
7.4926988829 m
10µm
10
nm
1µm
7.5 m path in air:
noise @ 0.1s 10 nm
200nm
7,49276
10 ms
7,49275
Zoom x25
each measurement cycle (43ms)
is a series of 320
10
elementary
measurements over 130 µs
nm
7,49274
7,49273
M.Lintz, ICSO 2012
11/10/2012
400pm
interference blocked
2mn 5 s
(=>noise only; no signal)
nombre de mesures élémentaires
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" Iliade ": typical recording (target is moved 20µm in 10 minutes)
1µm
What's wrong with the data ?
25% of wrong values
( shifted by  opt )
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11/10/2012
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â3
â3
Electronic distorsion: instantaneous and retarded AM-to-PM
â1
amplitude
â1
60µs
60µs
(data processing)
(data processing)
EM4 20 GHz photodiode
â4 â5
â5
Electronic distorsion of theâ4signals
â2 cross-talk (  2x10-3 )
- electronic
- easy to measure
- easy to correct for
=> no real problem
phase
- amplitude-to-phase coupling
optical power varies by up to X3
65 µs
during the â1=>â2=>â3 sequence!
Acq
0
opt scan
Acq
15µs
Acq Acq Acq
â2
5µs
depends on PhD
bias voltage
Acq
75µs
135µs
"straight"
segment... is curved!
Acq
Acq Acq
Acq
hysteresys !
retarded AM-to-PM !
instantaneous AM-to-PM
amplitude
0.5
M.Lintz, ICSO 2012
11/10/2012
1
ratio
time in µs
210µs
amplitude
1.5
0.5
1
ratio
1.5
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Electronic distorsion: retarded AM-to-PM, thermal origin
Retarded AM-to-PM coupling:
very likely of thermal origin
power (VbiasX photocurrent) dissipated in the photodiode junction => several kW/cm² !
Retarded AM-to-PM is difficult to model and to correct for
(it involves the whole history of the signal amplitude)
opt scan
"segment"
5µs
65µs
amplitude
0.5
M.Lintz, ICSO 2012
11/10/2012
1
ratio
amplitude
1.5
0.5
1
ratio
1.5
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Length measurement results (target movement 2µm/min)
with "slow acquisition"
=> solve retarded AM-to-PM coupling! <=
µm
L-7.492717m
µm
better convergence (94%)
slow acquisition
reduces
retarded AM/PM
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11/10/2012
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How to reduce retarded AM-to-PM transfer?
Characterization of AM/PM in the photodiodes ( Phung et al., submitted )
=> depends on the applied voltage
=> depends on the photodiode
(smaller in New Focus than in EM4 photodiodes)
AM/PM also differs with differents amplifiers
AM/PM larger when amplifier gain is larger?
=> still to be optimized
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11/10/2012
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Conclusion : Laser vs incoherent
C. Courde et al.,ICSO 2010
two-mode interference
Resolution
measured (expected)
@ 10s
incoherent source
obtained (expected)
gain 103 10nm (1nm)
20nm (3nm)
|
@ 1s
@ 0,1s
3nm (10pm)
40nm (10nm)
@10ms
3nm (15pm)
not yet measured
@ 1ms
5nm (30pm)
"
@100µs
1nm (100pm)
Pol. Controller?
System
 simple
Source
2 phase-locked CW lasers wideband+modulator
Signature
AOM
polarization switch
Procedure
complex
simple
Measurt scheme
M.Lintz, ICSO 2012
11/10/2012
OK ...
with less retarded AM/PM!
simple
OK
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Thanks to ...
ANR " ILIADE " (grant 07-BLAN-0309-01),
collab. with Etienne Samain (OCA, Nice),
S. Pitois, C. Finot, J. Fatome (Univ. Dijon)
CNES (R&T R-S06/SU-0001-013)
PhD Grants:
Thalès, région PACA and Univ. Nice-Sophia
michel.lintz @ oca.eu
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11/10/2012
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