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

V.J

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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

V.J

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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

V.J

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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

V.J

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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.

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

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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

V.J

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Thank you!