coherent beam combining of fiber amplifier arrays and ... · coherent combining of fiber amplifier...

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)

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

2

01/2

3/20

08

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

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

3

01/2

3/20

08

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

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

4

01/2

3/20

08

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

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

5

01/2

3/20

08

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

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

6

01/2

3/20

08

Coherent Combining of Three Amplifiers, Results

Phase control

locked

Theoretical far field pattern

unlocked

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

7

01/2

3/20

08

Experimental results

Residual phase error:λ/28 RMS Residual phase error:λ/15 RMS

Without turbulence With turbulence

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

8

01/2

3/20

08

(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

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

9

01/2

3/20

08

(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

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

10

01/2

3/20

08

(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

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

11

01/2

3/20

08

(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

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

12

01/2

3/20

08

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

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

14

01/2

3/20

08

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

V.J

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

15

01/2

3/20

08

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

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

17

01/2

3/20

08

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

Wes

t

18

01/2

3/20

08

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

oliv

et, O

NE

RA

, pap

er 6

873-

41, P

hoto

nics

Wes

t

19

01/2

3/20

08

Thank you!