Transcript
Coherent combining of fiber amplifier arrays on a remote surface after propagation through turbulent atmosphere V.Jolivet, P.Bourdon, B.Bennai, L.Lombard, D.Goular, G.Canat, O.Vasseur (Presented by C.Besson)
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Outline
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Overview
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Experiment of coherent combining on a scattering surface after propagation through turbulence •
Experimental set up using modulation multiplexing
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Experimental results of beam combining
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Theory of the experiment
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Conclusion and future work
Overview
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Goal: High power, high quality and high efficiency laser beam
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Fiber laser already demonstrated up to 5 kW
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However, there are power scaling limits due to: •
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
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• •
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Thermal effects, Non linear effects, Pump diode power limited coupling, Damage to surface
Beam combining is a promising way to overcome these limits • •
Spectral combining Coherent combining with active phase control
COAT – Coherent Optical Adaptive Techniques •
COAT = technique introduced in the 1970s’ – 1980s’ T.R. O’Meara, « The multidither principle in adaptive optics » J. Opt. Soc. Am. vol. 67, n°3, pp. 306-315 (1977)
turbulence
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
target
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detector -Adaptative optic techniques applied to coherent light sources - goal = to maximize the power density on the surface -Turbulence compensation demonstrated for a glint target -For fiber amplifier coherent combining, first demonstrated (without turbulence) by T.M.Shay, O.E. 14 (25), 12015-12021,2006
Experimental Setup LFGthe lock-in amplifier, the error signal is: At the output of cos(ν .t) Proportionnal to the phase S Si _ I = 2 RPD J1 (β i ) Pus Pis sin(φu − φi ) 1
( )
NFeedback J 0 1β j + ∑loop j =1
EOM
(
Lock-in Pjs Pisamplifier sin φ j 1− φi
1.5 µm 2W
Collimated beams 50:50 01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
MO
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50:50
1.5 µm 2W
EOM
)
difference between the ith beam and the reference
Polarizing beam splitter
reference beam
1.5 µm 2W
Feedback loop 2
Lock-in amplifier 2 camera
LFG cos(ν2.t)
T.M.Shay, O.E. 14 (25), 12015-12021,2006
Coherent Combining of Three Amplifiers, Results
Phase control unlocked
locked
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Theoretical far field pattern
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Experimental results
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Without turbulence
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Residual phase error:λ/28 RMS
With turbulence
Residual phase error:λ/15 RMS
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence (C) Hot air blower
Signal processing and control feedback
(C)
EOM
Polarizing beam splitter cube
Scattering Diffuse surface target
Beam 3 50:50 Oscillator
50:50
Reference beam Beam 1
(A) Collimated beams
Detector
(B)
EOM
(A) 01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Hot air blower
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Signal processing and control feedback
Turbulence in zone (B), (onward and backward path)
Hot air blower
(B)
CCD Camera
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence (C) Hot air blower
Signal processing and control feedback
(C)
EOM
Polarizing beam splitter cube
Scattering Diffuse surface target
Beam 3 50:50 Oscillator
50:50
Reference beam Beam 1
(A) Collimated beams
Detector
(B)
EOM
(A) 01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Hot air blower
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Signal processing and control feedback
Strong turbulence in A, (only on the onward path)
Hot air blower
(B)
CCD Camera
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence (C) Hot air blower
Signal processing and control feedback
(C)
EOM
Polarizing beam splitter cube
Scattering Diffuse surface target
Beam 3 50:50 Oscillator
50:50
Reference beam Beam 1
(A) Collimated beams
Detector
(B)
EOM
(A) 01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Hot air blower
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Signal processing and control feedback
Strong turbulence in C, (only on the backward path)
Hot air blower
(B)
CCD Camera
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence (C) Hot air blower
Signal processing and control feedback
(C)
EOM
Scattering Diffuse surface target
Polarizing beam splitter cube
Beam 3 50:50 Oscillator
50:50
Reference beam Beam 1
(A) Collimated beams
Detector
(B)
EOM
(A) 01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Hot air blower
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Signal processing and control feedback
Hot air blower
CCD Camera
(B)
Compensation of the onward turbulence: Coherent combining of the beams on the scattering surface, even when turbulence is strong on the backward path.
Theoretical analysis of the experiment Signal processing and control feedback
EOM Beam 3
Polarizing beam splitter cube
50:50 Oscillator
50:50
λ/4
(C)
Polarizing beam splitter cube
Reference beam Beam 1
Detector
Collimated beams
(A)
EOM
(B) CCD Camera
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Signal processing and control feedback
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Output current of the photodiode: Resulting phase error signal:
Onward turb backward turb +speckle
Scattering Diffuse surface target
Theoretical analysis of the experiment
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
The double sum can be split between terms coming from the same scattering area and the ones coming from different scattering areas
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Close to zero
Theoretical analysis of the experiment
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Resulting phase error signal:
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All the phase difference terms coming from speckle and backward turbulence are negligable, thanks to the pinhole
lowering
Onward turbulence
the aperture using a pinhole allow to be unsensitive to backward turbulence
Conclusion
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We have performed coherent combining of three 2W-fiber amplifiers on a remote scattering surface using the modulation multiplexing technique. λ/28 residual phase error was obtained.
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Wave front shaping has been done experimentally in a turbulent path.
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
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Coherent combining of fiber amplifiers on a scattering surface has been demonstrated, even after propagation through turbulent atmosphere, and without any external turbulence measurement system.
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Theoretical analysis of the pinhole effect has been given: limiting the aperture allow to phase lock on a surface in the far field, inspite of turbulence on the backward path.
Future Work
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
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Future work at ONERA involves: •
Increase servo-loop bandwidth,
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Densify the pupil using 2D arrays,
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Combine high power fiber amplifiers
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Outdoor experiment
01/23/2008 V.Jolivet, ONERA, paper 6873-41, Photonics West
Thank you!
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