THERMAL BLOOMING OF HIGH POWER LASER BEAMS

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Submitted on 1 Jan 1980

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THERMAL BLOOMING OF HIGH POWER LASER

BEAMS

M. Philbert, M. Billard, G. Fertin, J. Lefèvre

To cite this version:

JOURNAL DE PHYSIQUE CoLLoque C9, suppZ6ment au nO1l, Tome 41, nouenbre 1980, page C9-149

T H E R M A L B L O O M I N G O F H I G H POWER L A S E R B E A M S

M. P h i l b e r t , M. B i l l a r d , G. F e r t i n and J . LefPvre.

Office National d ' ~ t u d e s e t de Recherches Ae'rospatiaZes (ONERA), 92320 Cha^tiZZon, France.

Abstract.- With a view t o b e t t e r p r e d i c t i n g t h e e f f e c t s of thermal defocusing w i t h i n t h e atmosphere, an experimental s i m u l a t i o n set-up has been designed a t ONERA. This c o n s i s t s e s s e n t i a l l y of a v e r t i - c a l a i r t i g h t c e l l c o n t a i n i n g a gas o r gas mixture s u f f i c i e n t l y absorbing t o induce "blooming" of a C02 l a s e r beam over a d i s t a n c e of about 3 m.

A r e t u r n wind t u n n e l , i n t e g r a t e d w i t h i n t h e c e l l , c r e a t e s a uniform wind on t h e beam propagation path; t h e wind v e l o c i t y may be p r e c i s e l y a d j u s t e d between 0.1 and 2 m/sec.

A r o t a t i n g drum photometric a n a l y s e r , a s s o c i a t e d with a d a t a a c q u i s i t i o n and p r o c e s s i n g u n i t , provi- des an automatic s e q u e n t i a l e x p l o r a t i o n of t h e i r r a d i a n c e model observed a t t h e beam f o c u s .

Some c h a r a c t e r i s t i c r e s u l t s a r e p r e s e n t e d t o i l l u s t r a t e t h e p o s s i b i l i t i e s o f f e r e d by t h e set-up.

1. I n t r o d u c t i o n

-

Propagation of l i g h t beams through t h e atmosphere is accompanied by a n energy l o s s by molecular a b s o r p t i o n and by s c a t t e r i n g on a e r o s o l s and molecules. For h i g h t power l a s e r s , t h e a i r h e a t i n g due t o a b s o r p t i o n induces a l o n g t h e p r o p a g a t i o n p a t h a r e d u c t i o n of r e f r a c t i o n index i n t h e beam a x i s , whose e f f e c t i s t o spread t h e beam and t o n o t i c e a b l y reduce i x r a d i a n c e r e l a t i v e t o i t s expected v a l u e : it i s i t h e "thermal blooming" phenomenon, which h a s been, f o r over t h e n y e a r s , t h e s u b j e c t of much t h e o r e t i c a l and experimental work [1,21.

To a n a l y s e t h e e f f e c t s of thermal blooming, computing models and codes have been e s t a b l i s h e d by v a r i o u s a u t h o r s . The experimental v e r i f i c a t i o n of t h e r e s u l t s of t h e s e c a l c u l a t i o n s i n f r e e f i e l d over long p a t h s p r e s e n t s g r e a t d i f f i c u l t i e s i n view of t h e l a c k of p r e c i s i o n i n our knowledge of t h e v a r i o u s p h y s i c a l parameters involved i n t h i s phenomenon.

That i s why, i n o r d e r t o v e r i f y some computing models developed a t CGE (Compagnie G n B r a l e d 1 E l e c t r i c i t 6 ) , a g a s s i m u l a t i o n c e l l

h a s been r e a l i z e d by ONERA, w i t h t h e f i n a n c i a l Support of DRET ( t h e Research D i r e c t o r a t e of t h e French M i n i s t r y of Defence).

This experimental set-up should a l s o be used t o e v a l u a t e t h e p o s s i b i l i t i e s o f f e r e d by phase c o r r e c t i n g systems w i t h a d a p t i v e o p t i c s , intended f o r compensating thermal blooming e f f e c t s .

2. D e s c r i p t i o n of t h e s i m u l a t i o n c e l l

-

The s i m u l a t i o n c e l l , whose b a s i c d e s i g n i s represen- t e d on f i g u r e 1, i s made of a v e r t i c a l c y l i n d r i c a l body, 2 m i n d i a m e t e r , i n s e r t e d between two q u a s i hemispheric caps ends. The o v e r a l l h e i g h t of t h e t a n k i s 5.20 m. The lower bottom comprises an 0.60 -m-diam. c e n t r a l p a r t , h o l d i n g t h e f o c u s i n g system of t h e l a s e r beam. This system i n c l u d e s a ZnSe l e n s of 3.16 m f o c a l l e n g t h and a window, a l s o i n ZnSe, e n s u r i n g a i r t i g h t n e s s . The u s e f u l diameter i s 50 nun. The l a s e r i s focused on a r o t a t i n g drum beam a n a l y s e r i n s t a l l e d i n t h e upper p a r t , and a d j u s t a b l e between 3 m and 2.40 m from t h e window. This a n a l y s e r p r o v i d e s i n q u a s i r e a l t i m e t h e

JOURNAL DE PHYSIQUE

Upstream

Laser

Fan Downstream

deflector

beam

I

deflector

Honeycomb

Airtight

~ e s t

straightener

wall

section

Fig. 1 : Simulation c e l l .

d i s t r i b u t i o n of i r r a d i a t i o n energy i n t h e analysed s e c t i o n of t h e beam. The absorbing g a s , u s u a l l y C02,is s e t i n motion by an i n t e r n a l mini wind t u n n e l , t h e s t r u c t u r e of which can be seen on t h e c r o s s s e c t i o n of t h e c e l l . This wind tunnel i s made of two contoured v e r t i c a l w a l l s occupying t h e whole h e i g h t of t h e c y l i n d r i c a l body ( 3 m)

.

The t e s t s e c t i o n , where t h e beam propagates, i s 0 . 2 0 m wide; it i s preceded by an upstream c o l l e c t o r with a c o n t r a c t i o n r a t i o of 3 , and followed by a s h o r t d i f f u s e r of 14" apex angle. Gas c i r c u l a t i o n i s ensured by two f a n s i n s t a l l e d through an a i r t i g h t wall i n t h e median s e c t i o n of t h e body. To ensure flow uniformity i n t h e t e s t s e c t i o n , a f i l t e r i n g medium i s d i s t r i b u t e d over t h e honeycomb s t r a i g h t e n e r mounted a t t h e c o l l e c t o r i n l e t . This arrangement makes it p o s s i b l e t o o b t a i n a q u a s i h o r i z o n t a l flow within t h e t e s t i n g volume, with a v e l o c i t y c o n s t a n t t o within

2

5% over t h e t e s t s e c t i o n height. A guard p l a t e , s o l i d with t h e a n a l y s e r , ensures t h e c o n t i n u i t y of t h e flow a t t h e drum l e v e l .

The f a n s a r e driven by two hydraulic motors of a power of 4 kW each, a t t h e maximum speed of 800 r.p.m. The motors a r e supplied by a pumping u n i t c o n t r o l , l o c a t e d under t h e c e l l . Adjustment of t h e r o t a t i n g speed i s continu- ously ensured, between 30 and 800 r.p.m., t o within

'r

1 r.p.m., allowing a p r e c i s e

v a r i a t i o n of t h e wind v e l o c i t y between 0.1 and 2 m/sec. The v e l o c i t y c a l i b r a t i o n has been r e a l i z e d by means of an antmometric paddle-wheel, and i t s uniformity checked with a hot wire probe.

P r e s s u r i z a t i o n c i r c u i t s ensure t h e evacu- a t i o n of t h e c e l l , i t s f i l l i n g with a i r o r CO

between 0 and 1 5 b a r s and i t s d r a i n i n g t o A molyMenum mirror s e t

a t

45O t u r n s t h e beam atmosphere; f i l l i n g with an air-C02 mixture i s v e r t i c a l towards t h e c e l l .

a l s o possible. The c e l l checking devices f o r p r e s s u r i z a t i o n ,

A c o n t r o l console, located i n t h e measurement of physical parameters of t h e g a s , a s

-

measuring room, ensures t h e following f u n c t i o n s : well a s t h e d a t a a c q u i s i t i o n and processing u n i t , p r e s s u r i z a t i o n c o n t r o l , pumping u n i t c o n t r o l , a r e p u t together i n an independent measuring room. measurement of physical parameters of t h e g a s i n 4. Beam analyser

-

This a n a l y s e r , f a b r i c a t e d by t h e c e l l : p r e s s u r e , temperature and wind ETCA (Central Technical Establishment f o r

v e l o c i t y . Armament

,

French Ministry of Defence), i s made

3. General d e s c r i p t i o n of t h e experimental s e t - u p - of a 35-cm-dia. drum, r o t a t i n g a t 600 r.p.m. One The g e n e r a l layout of t h e set-up

i s

presented on hundred 0.3-mm-dia; holes a r e d i s t r i b u t e d over f i g u r e 2. A t t h e lower l e v e l of t h e c e l l a r e i t s periphery, and spaced over a h e i g h t of 20 mm. located : t h e CO l a s e r , whose power may be

2 A window i n t h e instrument casing g i v e s t o t h e

adjusted between 0 and 400 W , an HeNe l a s e r f o r l a s e r beam access t o t h e drum, and f i x e s t h e alignment, a removable checking c a l o r i m e t e r , an boundaries of t h e r e c t a n g u l a r a n a l y s i s frame : a f o c a l o p t i c a l system whose magnification may be 20 x 11

mm.

changed a t

w i l l

f o r modifying t h e t r a n s v e r s e A t each t u r n , t h e beam s e c t i o n i s dimension of t h e beam e n t e r i n g t h e c e l l , hence s e q u e n t i a l l y explored over t h e height of t h e t h e c h a r a c t e r i s t i c value of t h e Fresnel parameter. window. A p y r o e l e c t r i c d e t e c t o r , i n t e g r a t e d

3

1

Pressurization

I

3

1

control

I

_{,Operations room}

C9- 152 JOURNAL DE PHYSIQUE

within the instrument, receives successively the flux whjch has crossed each hole of the drum, and thus gives rise to a succession of analysis signals similar to television video signals. Complementary holes, associated with optical couplers, deliver an electric pulse of line synchronization at the passage of each analysis hole, and a pulse of frame synchronization at each turn of the drum.

5. Data acquisition and processing

-

In the initial concept of the measuring-unit, acquisition of the electric signals issued from the analyser is ensured in two steps : analog magnetic recording at the rate of

152

cm/sec., then digitization on the magnetic tape of an H.P. 21 MX computer by reading of the analog recording at the reduced rate of 38 cm/sec.

This operating procedure was chosen with a view to ensure compatibility between the sampling frequency related to the time resolving power of the analyser (55 kHz) and the digiti- zation frequency imposed by the present analog- to-digital converter. A faster converter will allow, at a later stage, the suppression of the analog recorder.'

Before being recorded, the signals are received on the analyser control unit, which ensures the following functions : generation of 55 pulses'per line, f a digitization of the video signal, "zero recovery" for compensating the drift of the video signal related to the capacitive effect of the pyroelectric detector.

Before digitization, the recorded analog signals a,re ditgcted towards an adaptation unit

initial frame and final frame of the sequence. A "control gate", triggered by the counters, selects the digitization pulses that are sent, together with the video signal, to the analog- to-digital converter of the computer.

The digital data stored on magnetic tape are then processed by means of codes recorded on disc. The main code provides a digital representation, by stitches, of the irradiation distribution observed in the analysis window; the information,,displayed on a screen or printed, is restituted frame by frame or calculated as mean values over a predetermined set of frames.

Other codes are also available : graphic representation of lines of equal intensity, three-dimensional representation by perspective effects, plotting of curves representing, as a function if the experimental parameters (laser power, wind velocity), the evolution of some characteristic results : maximum irradiation, mean irradiation within a circlecenteredon the bary centre and containing 63% of the energy, radius of this circle, etc...

6. Results obtained

-

Figure 3 is an example of three-dimensional representation obtained in the case of relatively important blooming.

On figure 4 are compared, for identical operating conditions, the results provided by the CGE computing code and the experimental distributions observed in maintaining constant the ratio of laser power by wind velocity; these latter results seem to prove that blooming does not only depend on the P/V ratio, but tends to increase when P and

V

increase together. enslirGg the selection of the sequence to

be

. Figure 5 represents the evolution of the

-

Fig, 3 : Three-dimensional representation of irradiance

.

Fig. 4 : Comparison with the result of the CGE code of mcdels observed at P/V

=

4

(N = 9) for various values of laser power

and

wind velocity

P

: power, in

W

V

: velocity, in cm/sec.

Fig. 5 : Defocusing c w e s obtained for various values of the wind velocity (Fresnel number NF = 10)

For the moment it is not possible to draw a significant conclusion on the comparaison of the absolute energy levels provided by

experimental means and computing codes, because of delays in the running of the planned

experimentation, due to technological difficulties, and of one insufficiently precise knowledge of

some characteristic elements (beam size at the cell inlet, quality factor).

7. Project of experimentation of corrective systems with adaptive optics

-

A complementary experimentation on this set-up is planned, it aims at studying the behaviour of corrective systems with adapted optics in the presence of thermal blooming.

The first system is a multidither C.O.A.T. mirror, constructed by the Ecole Polytechnique Laboratory. This mirror comprises 21 elements 40

mm

wide, controlled in translation

by

JOURNAL DE PHYSIQUE

adjustment and the periodic correction of long- been realized at ONERA. This cell, originally term shifts inherent to piezoelectric supports.

The second device is a single plane mirror, built by ONERA (POLA), intended for an experiment of automatic pointing on aerial targets; this fine pointing system, mobile along two orthogonal directions, provides a conical spatial exploration within a 1.3 m a d sighting field.

An appropriate optical bench has been realized, allowing the spreading of the beam to

conceived as an automatic and operational test set-up, has been associated with elaborate measuring and computing means.

The results presented in this paper show, on the qualitative view point,the real interest offered by this set-up, in spite of some diffi- culties that retarded the running of the planned experimentation.

New experiments will now be performed, the diameter required for adaptive optics, then _{after addition of a second analyser which will} its reduction for access to the cell. _allow

_a

_{more precise characterization of the}

A second bench, placed above the set-up, _{laser beam entering into the simulation cell.} will allow simultaneously picking up the

References information for the mirror servocontrol and

[ I ] Smith, David C., "High-Power Laser Propaga- using the beam analyser. _{tion : Thermal Blooming" Proceedings of}

the

IEEE

Vol. 65 N012, December

1977.