HIGH AVERAGE POWER OPERATION OF THE 1.3µm IODINE LASER

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HIGH AVERAGE POWER OPERATION OF THE 1.3µm IODINE LASER

H. Baker, T. King

To cite this version:

H. Baker, T. King. HIGH AVERAGE POWER OPERATION OF THE 1.3µm IODINE LASER.

Journal de Physique Colloques, 1980, 41 (C9), pp.C9-359-C9-362. �10.1051/jphyscol:1980949�. �jpa-

00220603�

JOURNAL DE PHYSIQUE Colloque C9, suppt6ment au nolZ, Tome 41, novembre 1980, page cg-351

A REPETITIVELY PULSED CARBON DIOXIDE LASER WITH MEAN POWER OUTPUT IN EXCESS OF

3 0

kW

J.D.L.H. Wood and P.R. Pearson

Royal SigrzaZs and Radar Establishment, Ministry o f Defence, BaZdock, Hertfordshire, England.

A b s t r a c t . - A pulsed e l e c t r o n beam s u s t a i n e d atmospheric p r e s s u r e carbon d i o x i d e l a s e r w i t h an a c t i v e volume of 17 l i t r e s has been i n c o r p o r a t e d i n a closed c y c l e g a s r e c i r c u l a t i o n system. Gas flow and e l e c t r i c a l i n p u t can be v a r i e d over a wide range. The problems a s s o c i a t e d w i t h r e p e t i t i v e o p e r a t i o n have been explored and measurements made of multimode and s i n g l e mode o u t p u t q u a l i t y , r e p r o d u c i b i l i t y and r e l i a b i l i t y . P a r t i c u l a r a t t e n t i o n has been given t o e l e c t r o n gun and f o i l window d e s i g n , and t h e mechanical s t a b i l i t y of t h e o p t i c a l c a v i t y . The e f f e c t of g a s p u r i t y , g a s flow r a t e s and damping of a c o u s t i c d i s t u r b a n c e s have been measured. Medium q u a l i t y h a s been a s s e s s e d by pulsed i n t e r f e r o m e t r y and a time l a p s e c i n e f i l m made of t h e decay of p e r t u r b a t i o n s i n t h e i n t e r - p u l s e p e r i o d . Maximum mul- timode o u t p u t a t 10.6 pm i s 600 J p e r 30 microsecond p u l s e with p u l s e r e p e t i t i o n f r e q u e n c i e s up t o 66 Hz. Mean power achieved has been up t o 36 kW f o r 0.5 second o r 22 kW f o r I second d u r a t i o n w i t h t h e c a p a b i l i t y of r e p e a t i n g t h e p u l s e t r a i n a t 30 second i n t e r v a l s .

INTRODUCTION

Operation of g a s l a s e r s a t h i g h r e p e t i t i o n r a t e s r e q u i r e s t h a t t h e heated g a s b e r e p l a c e d by c o l d g a s i n t h e working r e g i o n p r i o r t o t h e a p p l i c a t i o n of t h e n e x t d r i v e pulse. To achieve t h i s t h e l a s e r i s i n c o r p o r a t e d i n t o a c l o s e d c y c l e g a s r e c i r c u l a - t i o n system. The p r e s e n t paper i s concerned w i t h a pulsed e l e c t r o n beam s u s t a i n e d carbon d i o x i d e l a s e r having an a c t i v e volume of 17 l i t r e s . T h i s h a s been developed using d e s i g n c r i t e r i a e s t a b l i s h e d i n s m a l l e r systems (1,2,)3.

LASER AND GAS CIRCULATION SYSTEM

F i g u r e 1 shows a c r o s s - s e c t i o n of t h e e l e c t r o n gun and l a s e r . The e l e c t r o n beam i s d e r i v e d from a d i s c h a r g e i n helium a t a p r e s s u r e of about 50 mTorr and u s e s a concept devised i n c o n j u n c t i o n with GEC H i r s t Research L a b o r a t o r i e s (4). A continuous c u r r e n t of 100 mA i s maintained through t h e a u x i l i - a r y e l e c t r o d e . Imposed on t h i s i s a pulsed c u r r e n t of s e v e r a l amps during which t h e main gun p u l s e of

150 kV i s a p p l i e d t o t h e gun cathode. I o n s e x t r a c t e d from t h e d i s c h a r g e g e n e r a t e secondary e l e c t r o n s a t t h e cathode. These a r e a c c e l e r a t e d through t h e gun and t h e 25 t h i c k aluminium f o i l

window i n t o t h e l a s e r gas. C a r e f u l e l e c t r o n o p t i c s d e s i g n e n s u r e s a uniform c u r r e n t d i s t r i b u t i o n n e c e s s a r y b o t h f o r good l a s e r medium i o n i s a t i o n and f o i l l i f e exceeding 10 p u l s e s . 5

FIGURE 1 LASER CROSS SECTION

The e l e c t r o n beam l e a v e s t h e window w i t h consider- a b l e divergence due t o s c a t t e r i n t h e f o i l . The e f f e c t s of t h i s ⁱ¹¹ spreading t h e main discliarge o u t s i d e t h e o p t i c a l c a v i t y a r e l i m i t e d by i n s u l a t - ing s l a t s which a c t a s e l e c t r o n beam s t o p p e r s b u t

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980949

JOURNAL DE PHYSIQUE

The r e p e t i t i v e l y p u l s e d l a s e r h a s been used t o determine t h e consumption of i-C F I under condi-

3 7

tion.s*,of f r e e - r u n n i n g o s c i l l a t i o n . The f r e e i o d i n e qeneraced by 20,000 l a s e r p u l s e s a t a lOOmS o u t p u t en&rgy, 1Hz p u l s e r a t e was s e p a r a t e d from t h e 100gm.

charge of i - C F I and found t o weigh 0.45gm. The 3 7

i o d i n e produced corresponds t o t h e consumption o f 0.16 molecules of i - C F I f o r each l a s e r photon.

3 7

The r e v e r s i b i l i t y of p h o t o d i s s o c i a t i o n , based on c a l c u l a t i o n s of t h e number of p h o t o d i s s o c i a t i o n s from l a s e r o u t p u t energy d a t a , i s 90%. T h i s abso- l u t e d e t e r m i n a t i o n of t h e p h o t o d i s s o c i a t i o n r e v e r s i - b i l i t y a p p l i e s t o s p e c i f i c o p e r a t i n g c o n d i t i o n s a t a f i x e d pump l e v e l w i t h a 37% c a v i t y m i r r o r t r a n s - mission.

Absorption a t 633mn "L ^S

F i g 1. Absorption o f 633nm probe beam b y , m o l e c u l a r i o d i n e i n t h e a m p l i f i e r mode, w i t h A) f a s t , f l o w B) slow flow.

t h e r e s u l t s of Kuznetsova and Maslov [5] , - s t i m u l a - t e d emission g r e a t l y enhances r e v e r s i b i l i t y , be- cause o f t h e r a p i d t r a n s f e r o f t h e e x c i t e d i o d i n e atoms t o t h e ground s t a t e where t h e d e s i r a b l e r e - combination r e a c t i o n , R

+

1 + R I , can compete w i t h t h e u n d e s i r a b l e d i m e r i s a t i o n , R

+

R -t R

2 ' Excite-d i o d i n e atoms recombine with a l k y l r a d i c a l s v e r y slowly. T h e ' i n c r e a s e d y i e l d of molecular i o d i n e with n-C F I a s t h e p a r e n t molecules

3 7

r e f l e c t s t h e more r a p i d r a t e of d i m e r i s a t i o n f o r t h i s isomer.

Molecules consumed 6

(

I O " C ~ - ~ )

Amplifier mode 4

2 gscillator mode

':

4 8 12

I o d i n e p r o d u c t i o n under o t h e r c o n d i t i o n s h a s !

Molecules dissociated

(

10~~cm-3)

been monitored by measuring t h e 633nm a b s o r p t i o n F i g 2 . Comparison of C F I i n o s c i l l a t o r and a m p l i f i e r modes. 3 7

c o e f f i c i e n t of molecular i o d i n e , which peaks

I n t h e a m p l i f i e r mode, a low l e v e l o f g a i n approximately l O m s a f t e r t h e pumping f l a s h , and

s a t u r a t i o n , o r incomplete f i l l i n g o f t h e a m p l i f i e r n o r m a l i s i n g t h e r e s u l t s t o t h e s i n g l e a b s o l u t e

a p e r t u r e by t h e o p t i c a l p u l s e g i v e a l e v e l o f measurement. S t r o n g and r e p r o d u c i b l e a b s o r p t i o n

d i s s o c i a t i o n i r r e v e r s i b i l i t y which may b e unac- s i g n a l s , a s i l l u s t r a t e d i n f i g u r e 1, a r e o b t a i n a b l e .

c e p t a b l e on economic grounds i n a r e p e t i t i v e l y F i g u r e 2 compares consumption of t h e working mater-

p u l s e d , h i g h average power system. We have i a l f o r i-C F I and n-C F I , and f o r o p e r a t i o n i n

3 7 3 7 i n v e s t i g a t e d t h e s u b s t i t u t i o n of e x c i t e d s t a t e t h e f r e e r u n n i n q ' o s c i l l a t o r and a m p l i f i e r modes.

quenching f o r s t i m u l a t e d emission t o improve Changing from a m p l i f i e r mode t o a 37% coupled,

r e v e r s i b i l i t y i n t h e u n s a t u r a t e d a m p l i f i e r mode.

p l a n e p a r a l l e l , c a v i t y improves the p h o t o d i s -

.

W h i l s t pumping and energy e x t r a c t i o n t a k e p l a c e s o c i a t i o n r e v e r s i b i l i t y from 59% t o 91% f o r i-C I ,

3 7 on a 1 0 ~ s t i m e s c a l e , t h e e x g i t e d s t a t e l r f e t i m e and from 48% t o 88% f o r n-C F I . I n agreement w i t h

3 7 i s n a t u r a l l y 1 0 0 ~ s t o l O r n s , depending on g a s

composition, and can u s e f u l l y be reduced t o improve r e v e r s i b i l i t y w i t h o u t a f f e c t i n g t h e ampl- f i e r g a i n . F i g u r e 3 shows t h e improvement i n r e v e r s i b i l i t y achieved by t h e use of oxygen a s an e x c i t e d s t a t e quencher. With a 1 0 ~ s l i f e t i m e , r e v e r s i b i l i t y approaches t h a t o b t a i n e d i n t h e o s c i l l a t o r mode, b u t would i n v o l v e c o n s i d e r a b l e l o s s of g a i n with a l o p s pump p u l s e d u r a t i o n . L i f e - times of 30-4011s a r e more a c c e p t a b l e , and improve r e v e r s i b i l i t y from 50"ao 65 t o 7 0 % . An a l t e r n a t i v e t o t h i s technique i s t o induce c o n t r o l l e d para- s i t i c o s c i l l a t i o n i n t h e a m p l i f i e r t o d e p l e t e t h e e x c i t e d s t a t e p o p u l a t i o n a f t e r t h e passage of t h e o p t i c a l p u l s e being a m p l i f i e d .

Excited state lifetime

( S )

10-4

I O - ~ 2 . 1 6 ~

A

' O ° C

100 torr n-C3F71 a 7,s

X

10'~dissociations.

0 0,02

0, I

I 2

Oxygen

'

pressure

' ('

torr.

) Fig. V a r i a t i o n of p h o t o d i s s o c i a t i o n r e v e r s i b i l i t y produced by oxygen quenching of t h e i o d i n e e x c i t e d

s t a t e i n t h e a m p l i f i e r mode.

A k i n e t i c model h a s been s e t up to a l l o w c a r - c u l a t i o n of t h e degree of p h o t o d i s s o c i 6 f i o n r e v e r - s i b i l i t y from r a t e c o n s t a n t s o b t a i n e d from p u b l i s h e d m a t e r i a l . C a l c u l a t i o n s a r e i n agreement w i t h t h e expefrim6ntal o b s e r v a t i o n s f o r t h e c a s e of o s c i l - l a t o r mode o p e r a t i o n , where t h e numerical r e s u l t s depend mainly on t h e more r e l i a b l e r a t e c o n s t a n t s f o r C3F7

+

^{I (} 2 P ~ / ~ ) + C F I and 2C F I + C6F14.

3 7 3 7

C a l c u l a t i o n s f o r t h e a m p l i f i e r mode a r e l e s s r e l i a b l e s i n c e t h e e x c i t e d s t a t e recombinat'ion r e a c t i o n , C3F7

+

I ( 2 P ) + C3F71 h a s a s m a l l and

1/2

u n r e l i a b l y determined r a t e c o n s t a n t , and i m p u r i t y

quenching of t h e e x c i t e d i o d i n e i s d i f f i c u l t t o q u a n t i f y . The experimental d a t a f o r t h e a m p l i f i e r mode a r e a l s o e f f e c t e d by u n c o n t r o l l e d i m p u r i t y

quenching l e v e l s , caused t y p i c a l l y by t h e presence of up t o 1% C F H i n c o m e r c i a l l y produced C F I .

3 7 3 7

I t i s f e a s i b l e t h a t t h e degree o f r e v e r s i b i l i t y i n t h e a m p l i f i e r mode may be c o n s i d e r a b l y worse t h a n t h e measured v a l u e i f h i g h e r p u r i t y m a t e r i a l s were used.

A s t h e k i n e t i c model h a s been shown t o be r e l i a b l e f o r t h e c a s e of free-running o s c i l l a t o r s , we have extended i t t o c o n s i d e r long p u l s e and CW o p e r a t i o n of t h e i o d i n e l a s e r using low

p r e s s u r e mercury lamp pumping. We have considefed a system with a 5cm d i a m e t e r , lOOcm long a c t i v e r e g i o n i pumped by 0 , 1 w a t t cm -3 a t t h e 254nm mercury wavelength, s i m i l a r i n s i z e t o t h e e x p e r i - mental arrangement o f W i t t e e t a 1 [S]. The pump lamps c o n v e r t a t l e a s t 15% of t h e e l e c t r i c a l i n p u t to u s e f u l W energy; compared t o t y p i c a l l y 5% f o r s h o r t p u l s e , xenon flashlamps, b u t tlse average power d e n s i t y i s much lower. A s a consequence, a l a r g e volume d e v i c e i s needed t o p r o v i d e adequate g a i n and high o u t p u t p o w e r , ' s i t h a low o p e r a t i n g p r e s s u r e t o maximise t h e stimu- l a t e d emission c r o s s - s e c t i o n , t o minimise e x c i t e d s t a t e quenching and t o slow down t h e 3-body r e - combination o f atomic i o d i n e . For a 2 0 t o r r CF I

3 f i l l , t h e model p r e d i c t s a 30 w a t t peak power, 36ms d u r a t i o n o u t p u t p u l s e , w i t h l a s e r ' a c t i o n t e r m i n a t i n g because o f t h e build-up o f t h e s t r o n g l y quenching molecular i o d i n e . Due to t h e lower r a t e o f d i m e r i s a t i o n of t h e i - C F z a d i c a l , b e t t e r

3 7

performance i s e x p e c t e d w i t h i - c F I , w i t h 33 w a t t 3 7

peak power, 7 3 m ~ ~ d u r a t i o n o u t p u t p u l s e . F i g u r e 4 shows t h e v a r i a t i o n of o u t p u t p a y e r and molecular i o d i n e c o n c e n t r a t i o n f o r a f l a t - t o p p e d pump

p u l s e . I n b o t h c a s e s , the g a s temperature r i s e i s '

JOURNAL DE PHYSIQUE

l e s s t h a n 1 0 0 ~ ~ and cannot cause t e r m i n a t i o n of l a s e r a c t i o n by p y r o l y s i s .

20

0 2 0 4 0 6 0 Time ( m s l

F i g 4. C a l c u l a t e d l a s e r power and molecular i o d i n e Concentration f o r A ) CF I B) i-C3F71.

3

Operation i n t h e CW mode r e q u i r e s a g a s r e s i s - dence time i n t h e tube of 20ms f o r CF I and 40ms

3 f o r i-C F I , a c h i e v a b l e by a m u l t i - p o r t flow

3 7

system. We c o n s i d e r i n p a r t i c u l a r t h e a p p l i c a t i o n of a p u r e l y t h e r m a l l y d r i v e n flow system [2]. A condenser temperature o f - 3 5 O ~ and a r e f r i g e r a t i o n power o f 1 . 5 kW i s r e q u i r e d t o g i v e t h e n e c e s s a r y f l o w , based on t h e l a t e n t h e a t of e v a p o r a t i o n o f

t h e C F I molecule. The r e f r i g e r a t o r would pump 3 7

h e a t from t h e condenser t o t h e b o i l e r of t h e flow system, t o improve t h e thermal e f f i c i e n c y o f t h e system.

Because of t h e much lower f r e e r a d i c a l concen- t r a t i o n s , t h e CW system h a s b e t t e r r e v e r s i b i l i t y t h a n c o n v e n t i o n a l f l a s h l a m p pumped systems. Spec- i f i c C3F71 consumption i s p r e d i c t e d t o be 0.18- kJ-l, compared t o 0.3391~ kJhl f o r e x i s t i n g s h o r t p u l s e l a s e r o s c i l l a t o r s . I n a d d i t i o n , e l e c t r i c a l e f f i c i e n c y i s expected t o be 1.5%, s o t h a t t h e o v e r a l l e f f i c i e n c y , i n c l u d i n g chemical a s p e c t s , i s expected t o b e improved f i v e t i m e s o v e r f l a s h l a m p pumped p u l s e d l a s e r s .

The p r e s e n t a d d r e s s of H J Baker is:

Department of Applied P h y s i c s , U n i v e r s i t y of Hull, H u l l HU6 7RX, UK.

References

[l] W Fuss and K Hohla, 0pt.Commun.

S,

427, 1976.

[2] H J Baker and T A King, i n Laser Advances and

~ p p l i c a t i o n i , Ed: B S Wherrett, p47, Wiley 1980.

131 K J W i t t e , P Burkhard, and H R Lilthi 0pt.Commun.

8,

202, 1979.

[4] G H F i s k and F K Truby. J.Appl.Phys.

2,

94, 19 80.

[S] S V Kuznetsova and A I Maslov. Sov.J.Quant.

E l e c t r o n .

3,

468, 1974.

Support f o r t h i s work h a s been provided by t h e Science Research Council.