coherent beam combining of fiber amplifier arrays and ... · coherent combining of fiber amplifier...
TRANSCRIPT
![Page 1: Coherent beam combining of fiber amplifier arrays and ... · Coherent combining of fiber amplifier arrays on a remote surface after propagation through turbulent atmosphere V.Jolivet,](https://reader034.vdocuments.site/reader034/viewer/2022042110/5e8abac54fa39f7b7f1fd808/html5/thumbnails/1.jpg)
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)
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Outline
• Overview
• Experiment of coherent combining on a scattering surface after propagation through turbulence
• Experimental set up using modulation multiplexing
• Experimental results of beam combining
• Theory of the experiment
• Conclusion and future work
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Overview
• Goal: High power, high quality and high efficiency laser beam
• Fiber laser already demonstrated up to 5 kW
• However, there are power scaling limits due to:• 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
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COAT – Coherent Optical Adaptive Techniques
• COAT = technique introduced in the 1970s’ – 1980s’
-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
T.R. O’Meara, « The multidither principle in adaptive optics » J. Opt. Soc. Am. vol. 67, n°3,pp. 306-315 (1977)
target
turbulence
detector
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Lock-in amplifier 1
Feedback loop 1
LFG
cos(ν1.t)
Lock-in amplifier 2
Feedback loop 2
LFG
cos(ν2.t)
MO50:50
camera
50:50
EOM
EOM
reference beam
Polarizingbeam splitterCollimated beams
Experimental Setup
1.5 µm 2W
1.5 µm 2W
T.M.Shay, O.E. 14 (25), 12015-12021,2006
At the output of the lock-in amplifier, the error signal is:
1.5 µm 2W
( ) ( )
( ) ( )
−+
−=
∑=
N
jijisjsj
iuisusiPDISi
PPJ
PPJRS
10
1_
sin
sin2
φφβ
φφβ Proportionnal to the phase difference between the ith beam and the reference
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Coherent Combining of Three Amplifiers, Results
Phase control
locked
Theoretical far field pattern
unlocked
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Experimental results
Residual phase error:λ/28 RMS Residual phase error:λ/15 RMS
Without turbulence With turbulence
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(A)
Hot air blower
(A)
Signal processing and control feedback
Signal processing and control feedback
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Diffuse target
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence
(C)
Hot
air
blow
er
(B)Hot air blower
(B)
(C)
Turbulence in zone (B),(onward and backward path)
Scattering surface
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(A)
Hot air blower
(A)
Signal processing and control feedback
Signal processing and control feedback
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Diffuse target
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence
(C)
Hot
air
blow
er
(B)Hot air blower
(B)
(C)
Strong turbulence in A,(only on the onward path)
Scattering surface
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(A)
Hot air blower
(A)
Signal processing and control feedback
Signal processing and control feedback
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Diffuse target
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence
(C)
Hot
air
blow
er
(B)Hot air blower
(B)
(C)
Strong turbulence in C,(only on the backward path)
Scattering surface
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(A)
Hot air blower
(A)
Signal processing and control feedback
Signal processing and control feedback
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Diffuse target
Automatic Coherent Combining on a Scattering Surface After Propagation Through Turbulence
(C)
Hot
air
blow
er
(B)Hot air blower
(B)
(C)
Compensation of the onward turbulence:Coherent combining of the beams on the scattering surface, even when turbulence is strong on the backward path.
Scattering surface
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Theoretical analysis of the experiment
(A)
Signal processing and control feedback
Signal processing and control feedback
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Oscillator50:50
50:50
CCD Camera
Collimated beamsBeam 1
Reference beam
Polarizing beamsplitter cube
Beam 3
EOM
EOM
Detector
Diffuse target
(B)
(C)Scattering surfacePolarizing beam
splitter cubeλ/4
Output current of the photodiode:
Resulting phase error signal:
Onward turb
backward turb+speckle
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Theoretical analysis of the experiment
The double sum can be split betweenterms coming from the same scattering areaand the ones coming from different scattering areas
Close to zero
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Theoretical analysis of the experiment
lowering the aperture using a pinhole allow to be unsensitive to backward turbulence
Resulting phase error signal:
Onward turbulence
All the phase difference terms coming from speckle and backward turbulence are negligable, thanks to the pinhole
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Conclusion
• 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.
• Wave front shaping has been done experimentally in a turbulent path.
• 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.
• 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.
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Future Work
• Future work at ONERA involves:
• Increase servo-loop bandwidth,
• Densify the pupil using 2D arrays,
• Combine high power fiber amplifiers
• Outdoor experiment
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Thank you!