ADAPTIVE OPTICS WITH SEGMENTED MIRROR OR DEFORMABLE MIRROR

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ADAPTIVE OPTICS WITH SEGMENTED MIRROR

OR DEFORMABLE MIRROR

J. Gaffard, C. Davila, J. de Miscault, G. Roger

To cite this version:

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JOURNAL DE PHYSIQUE _CoZZoque_C9, _{suppZ6ment au nO1l, Tome 41, novembre}_1980,_pageC9-269

J.P. Gaffard, C. Davila, J.C. De Miscault and G. ~ o ~ e r *

Laser Department, Laboratoires de Marcoussis, Centre de Recherches de Za C.G.E., Route de Nozay, *91460 Marcoussis, France.

S c i e n t i f i c Direction.

Rdsum6.- Deux types de miroirs adaptatifs sont 6tudids : un miroir segment6 avec 19 moteurs (COAT 19) et un miroir continfiment d6formable P 7 moteurs.

Chaque miroir du COAT 19 est mG par un actuateur piezo-Qlectrique qui assure P la fois les fonctions de correction et de modulation dans la bande 12-24 kHz. L'analyse de la surface d'onde optique est faite grZce B la d6tection des signaux de modulation dans la lumiPre r6trodiffusEe par la cible. On en d6duit les ordres de correction des perturbations dues P l'atmosphzre, au laser et aux miroirs... Dans le cas du miroir dgformable, les fonctions de correction et de modulation sont assur6es par des miroirs s6parGs. Ces miroirs comportent un systsme de refroidissement pour pouvoir Btre utilis6s avec des lasers de haute gnergie.

Abstract.- Two kinds of adaptive optics (COAT) are presented : one with segmented mirrors (COAT = 19) with 19 mirrors and actuators and the other with a continuously deformable mirror and 7 actuators. Each mirror of COAT 19 is moved by a piezoelectric actuator performing correction and modulation

(dithering in the 12-24 kHz band). Detection of signal modulations in the light backscattered from the target gives means to lock the system and then to cortect disturbing effects due to atmosphere, laser, mirrors

...

In the second kind of COAT (deformable mirror), the works of correction and modulation (dithering) are performed by two separate mirrors. They both are cooled to support high-energy laser beam.

I

-

INTRODUCTION

We s t u d y i n Marcoussis two k i n d s o f a d a p t i v e o p t i c s (COAT) :

. t h e segmented p i s t o n m i r r o r s ; t h e c o n t i n u o u s deformable m i r r o r s .

The segmented p i s t o n m i r r o r i s a n assembly o f elementary p l a n and p a r a l l e l m i r r o r s which move p e r p e n d i c u l a r l y t o t h i s p l a n .

FIGURE 2

-

COAT CEFORFIABLE MIRROR

FICURE 1

-

_-

COAT SEGMENTED FIIRRORS

The c o n t i n u o u s deformable m i r r o r i s a r e f l e c t i n g s h e e t pushed b y piezoceramic motors.

The aim o f t h i s paper i s t o g i v e more d e t a i l s a b o u t t h e c o n t r o l o f t h e eeak i n t e n s i t y o f a h i g h energy l a s e r wave by t h e means o f t h e segmented m i r r o r COAT 19. I n t h e experiment, t h e t a r g e t i s l o c a t e d 600 m f a r f r o m t h e l a s e r .

+ Sponsored by D.R.E.T.

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C9-270 JOURNAL DE PHYSIQUE

The phenomena t o be compensated by t h e COAT a r e :

t h e f l u c t u a t i o n s o f the beam i n t h e l a s e r ; t h e d e f e c t s o f the o p t i c s ( p l a n m i r r o r s , r e f l e c t i n g telescopes

. .

.) ;

t h e atmospheric disturbances.

A l l those disturbances tend t o s t r e t c h t h e beam on t h e t a r g e t t o 3 t o 5 times t h e d i f f r a c t i o n l i m i t . With the use o f the COAT, we i n t e n d t o reach t h e d i f f r a c t i o n l i m i t , w h i l e i n c r e a s i n g the peak i n t e n s i t y on t h e t a r g e t .

11

-

THE MIRROR

The COAT includes 19 elementary hexagonal m i r r o r s olaced edge t o edge. Holybdenum has been chosen t o reduce t h e thermal d i s t o r t i o n o f t h e m i r r o r s when heated by the l a s e r beam.

Each m i r r o r i s moved by an a c t u a t o r which i s a stack o f piezoceramic d i s k s . An e l a s t i c r o d j o i n s each m i r r o r t o t h e socket.

Each a c t u a t o r has two f u n c t i o n s : t h e c o r r e c t i o n o f the wavefront and the phase modulation o f the l a s e r beam.

The c o r r e c t i o n o f t h e wavefront i s made w i t h a stack o f 22 d i s k s . The necessary v o l t a g e i s about

+

600 V t o o b t a i n 6.2 pm o f displacement. With an incidence anole equal t o 30°, we can produce a path d i f f e r e n c e o f 10.7 um t h a t exceeds s l i g h t l y t h e wavelength o f t h e l a s e r .

The modulation o f t h e wavefront i s made w i t h a stack o f t h r e e d i s k s which produce a sinusoTdal

around

+

1G3 V produces a .15 pm displacement. The modulation frequencies should be 1 ocated between 12 and 24 kHz.

I11

-

THE CONTROL OF THE MIRROR

CHASE DIAORAM PISTON COAT MIRROR DEFORMABLE COAT MIRROR

I

*

All mlrrors are dithered at different frequenclea f f f

1 ' 2 ' ¶ " '

i An I.R. detector records the llght backscattered from the target

JPERATION

*

Each dlther frequency IS sampled out

r Slgnals are processed

*

A microprocessor maxtmize the focused energy

FIGURE 3

-

COAT PRINCIPLE

The c o n t r o l o f t h e m i r r o r needs 4 steps :

Phah

**e. moddaticn or( *he emifted beam**

Each m i r r o r i s d i t h e r e d permanently. Each o f t h e 19 d i t h e r e d frequencies i s chosen between 12 and 24 kHz w i t h the d e s i r e d 656 Hz step.

Muuwrement o

6 *he

f i g k t 6

cattehed

b y

-the

tahget

The s i g n a l coming back from t h e t a r g e t i s assumed t o be p r o p o r t i o n a l t o the i n t e n s i t y on the t a r g e t . This s i g n a l i s modulated a t the fundamentals mi and a t t h e harmonic frequencies 2 (ai, wi

_'

"j . * .

wa~edh0& comp&ation

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INTENSITY ON TARGET

-

WAVEFRO-NT TO

CORRECT

-

HYPOTHESIS :

Segmented mirror with n identical elements Uniform illumination on each mirror

Pi : phase shift due to element i

l+J) : phase shift due to optical disturbance

cp,

=

yi+q

m i (t) : phase modulation of the optical wavefront for each mirror

A

,

: uniform illumination on mirror i

ON THE TARGET :

Amplitude At=

LAi

~p;+'i) i

FIGURE 4

-

INTENSITY ON TARGET

MODULATION PRINCIPLE

If mi<<

.

we have :

Continuous component

41t

AiAj

c o s ( y i - l & )

i &

First harmonic (I

1 Ai Aj

s i n ( y i - yj) = bi

1

Second harmonic a(

1 A;

A,

cos

(yi-yi)

= a;

1

With the harmonics of greater order, it would be possible to have all AiAj cos(W-

q )

and A,A,sin(V;-

E)

(assuming enough eneigy to make the measurements)

FIGURE

5 -

FOOULATION PRINCIPLE

ai =

pi

cOS

ei

b;

=

pi

sin

8;

aj

bi

-

a;

bi

sin

(yi

-

yj)=

Pi Pi

with fi2 = a i 2 +

b;

FIGURE 6

-

WAVEFRONT TO CORRECT

FOCUSED C.O.A.T.

with sin

(

('Pi

-

Y'j

)

+

9;

-

Yj

C O S ( Y i

-

lp,) I + , 1

A

If uniform illumination :

if not, d

we must know the illumination on each mirror and have an A.G.C. for each mirror

A.G.C. will give

-

f,

al

bi

-'a,

bj

and ( Q i - \ Q d =

Pi

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JOURNAL DE PHYSIQUE Wi modulations

*>

Modulat~on amp. generator I positions measurement

The phase d i f f e r e n c e s are f i l t e r e d through a compensator, then amp1 i f i e d . The r e s u l t i s t h e c o n t r o l v o l t a g e o f each p i e z o e l e c t r i c a c t u a t o r . I f t h e c o n t r o l voltage exceeds p o s i t i v e l y o r nega- t i v e l y the p o s s i b i l i t i e s o f the actuator, we sub- s t r a c t o r add 2 .rr r a d from the phase d i f f e r e n c e .

The passband of one channel i s a l s o t h e passband o f the wavefront c o r r e c t i o n .

v

Correct~on Repositionning amp, orders calculatton

-

FIGURE 8

-

COAT CONTROL BLOCK DIAGRAP

Detector Phase Disturbances error Phase measurements

+

detection -

I V

-

THE TEST OF THE MIRROR

detection i -r Yi - Y l

If;

- y1 +\0/

+

Telescope mlrror Wavefront calculation Beam expander ---(~sclllator 250 W

1

Defection filter F ( p )

FIGURE 11

-

EXPERIMENTAL SET-UP

output poslt~on

---

_{HE. laser beam}

amp

I b a n m a r s

54declslon Modulator

AGC.

F

(

lpi

-

YT

)

FIGURE 12

-

PROCESS LOGIC Compensation

D ( P )

actuator i G ( p )

C,

=

P,

= o

FIGURE 9

-

ACTUATOR CONTROL BLOCK DIAGRA!!

Orders

Transfer function of

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The f o l l o w i n g devices a r e used t o t e s t t h e COAT 19 i n Plarcoussis :

.

a continuous C 0 2 l a s e r wave e m i t t i n g 40 kb! a t 10.6 um wavelength ; t h e COAT 19 m i r r o r ;

.

an i n f r a r e d Hg Cd Te d e t e c t o r ;

.

a Cassegrainian telescope w i t h a 65 cm a p e r t u r e ; a s e t o f m i r r o r s t h a t connect o p t i c a l l y the telescope w i t h t h e l a s e r through t h e COAT 19; t h e hardware and software t h a t perform s i g n a l processing, actuators, c o n t r o l and data r e c o r d i n g ;

a small s i z e t a r g e t .

The r e l a t i v e e f f i c i e n c y o f the COAT 19 i s the r a t i o o f the neak i n t e n s i t y , f o c a l i z e d on the t a r g e t , t o t h e peak i n t e n s i t y obtained when t h e COAT i s replaced by a plane m i r r o r .

V

-

CONCLUSION