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DIAGNOSTICS OF THE LASER-LIQUID
INTERACTION
D. Emmony
To cite this version:
DIAGNOSTICS O F THE LASER-LIQUID INTERACTION
D.C. Emmony
Department o f Physics, Loughborough University o f Technotogy, Leicestershire, U.K.
Abstract.- The interaction of TEA C02 laser pulse with a free water surface has been studied using shadowgraphy and optical probe techniques.
INTRODUCTION
A number o f workers have s t u d i e d t h e i n t e - r a c t i o n o f l a s e r l i g h t w i t h l i q u i d s anc? (1) i d e n t i f i e d d i f f e r e n t a b s o r p t i o n reqimes
.
I n s t r o n g l y a b s o r b i n g l i q u i d s and w i t h h i g h l a s e r r a d i a t i o n f l u x e s t h e ?recess i s d o ~ i n a t e z by v a p o r i z a t i o n of t h e l i q u i d a t t h e s u r f a c e whereas a t low l i g h t i n t e n s i - t i e s o r i n weakly a b s o r b i n g l i q u i c t s t h e p r o c e s s l e a d s t oheat in^
of t h e l i q u i d and expansion g i v i n q r i s e t o l e s s v i o l e n t t h e r m o e l a s t i c e f f e c t s i n t h e body o f t h e l i q u i d . The a b s o r p t i o n c o e f f i c i e n t of w a t e r a t 310.6pm
i s
s 1 0 cm-I and even modest l a s e r e n e r g i e s (-mJ) l e a d t o s t r o n g i n t e r a c t i o n s when a l a s e r p u l s e i s focused t o a s m a l l a r e a on a f r e e w a t e r s u r f a c e . Shock waves a r e q e n e r a t e d b o t h i n t h e l i q u i d and i n t h e a i r above. I n p r i n c i p l e i t s h o u l d b e p o s s i b l e t o e q u a t e t h e i n c i d e n t energy i n t h e l a s e r p u l s e t o t h e l i s s i p a t i v e p r o c e s s e s i n t h e l i q u i d and gaseous r e g i o n s . The measurement of shock wave p a r a n - e t e r s i n s m a l l 3 d i m e n s i o n a l e v e n t s o f t h e o r d e r 0-
l O m m i s n o t s t r a i g h t f o r -ward. High s p e e d photography o f t h e s u r - f a c e r e g i o n w i t h arqon s p a r k , Q-switched ruby l a s e r anc? n i t r o g e n l a s e r p u l s e s shows t h e d e v e l o p n e n t o f t h e c a r b o n d i o x i d e l a s e r q e n e r a t e d shock s t r u c t u r e . Two o p t i c a l probe t e c h n i q u e s have been developed t o s t u d y t h e shock f r o n t s t r u c - t u r e s . The S c h l i e r e n probe which depends upon t h e d e f l e c t i o n of a narrow l a s e r beam, h a s been used t o d e t e r m i n e t h e shock wave p r o f i l e i n t h e w a t e r ( 2 ) and i n t h e a i r above t h e s u r f a c e . A l a s e r probe i n t e r f e r o m e t e r measures t h e t i m e develop- ment o f t h e a i r shock. EXPERIMERTS A TEA C02 l a s e r ( 3 ) w i t h a t o t a l p u l s e energy o f 50mJ i n 8011s FWHM was f o c u s e d by a gernanium l e n s a t normal i n c i d e n c e on t o t h e f r e e s u r f a c e of w a t e r c o n t a i n e d i n a c u b i c g l a s s c e l l . The f o c a l s p o t d i a m e t e r was 0.3n-rn w i t h c o r r e s p o n d i n q e n e r g y and power d e n s i t i e s o f 50scrn-~ and 6 x 1 0 * ~ c m - ~ . These f i q u r e s a r e p o r e t h a n an o r d e r of magnitude l a r g e r t h a n 3 3 t h e v a p o r i z a t i o n t h r e s h o l d ( 2 . 6
x
1 0 Jcm ) when t h e a b s o r p t i c n c o e f f i c i e n t o f w a t e r 16C9-232 JOURNAL DE PHYSIQUE
a t 10.6pm (870cm-I) i s t a k e n i n t o a c c o u n t .
A Blumlein e x c i t e d n i t r o q e n l a s e r ( 4 ) w i t h a p u l s e d u r a t i o n o f < 2 . 5 n s was used a s a l i g h t s o u r c e f o r hiGh speed shadow photo- graphy. T h i s l a s e r i s s u p e r a d i a n t w i t h o n l y one m i r r o r which l e a d s t o a r a t h e r d i v e r g e n t beam,
=
2O, and c o r r e s p o n d i n g unsharp image. The image q u a l i t y i s c o n s i d e r a b l y improved by u s i n p a f o c u s e d shadow system which i n t h e o r y h a s SchlTeren p r o p e r t i e s due t o t h e l i m i t e d a p e r t u r e o f t h e l e n s , f i g u r e 1. The p l a n e of i n t e r e s t L a s e rF
,
Lens cameraf
N 2 l a s e rT
I
Water c e l l DELAY F i g u r e I Shadowgraph arrangement i s f o c u s e d on t h e f i l m p l a n e o f a 35mm s i n g l e l e n s r e f l e x camera. The carbon d i o x i d e l a s e r was t r i g g e r e d by t h e camera s h u t t e r and a v a r i a b l e d e l a y u n i t was used t o g i v e e x p o s u r e s a t d i f f e r e n t t i m e s a f t e r t h e l a s e r i n p a c t . M u l t i p l e e x p o s u r e shadowgraphs were o b t a i n e d by u s i n g up t o t h r e e n i t r o g e n l a s e r s i n s e r i e s ( 4 ) . How- e v e r , a l t h o u g h t h e n i t r o q e n l a s e r does n o t work a s an i d e a l Blumlein, t h e d e l a y between i n d i v i d u a l l a s e r s f o r p r a c t i c a l t r a n s m i s s i o n l i n e dimensions was o n l y 20ns which i s t o o s m a l l t o r e c o r d d i s p l a c e m e n t s o f t h e w a t e r shock which o n l y t r a v e l s 0.03mm i n t h i s t i m e . Delays up t o 300ns between t h e i n d i v i d u a l i l l u m i n a t i n g n i t r o g e n l a s e r s were obtainec? by u s i n g c o a x i a l c a b l e d e l a y l i n e s . The S c h l i e r e n probe t e c h n i q u e h a s been d e s c r i b e d e l s e - w h e r e ( 2 ) . The p r i n c i p l e o f t h e method i s t h a t t h e d e f l e c t i o n of a r a y o f l i g h t w h i c h i s t r a v e r s e d by an unchanging r e f r a c t i v e i n d e x p r o f i l e may b e u s e d t o c a l c u l a t e t h a t p r o f i l e . I n t h e p r e s e n t e x p e r i m e n t a f o c u s e d He-Ne l a s e r beam i s a r r a n g e d t o l i e i n t h e p l a n e o f i n c i d e n c e o f t h e i n t e r - a c t i n g C02 l a s e r beam. The beam d e f l e c - t i o n was measured by a k n i f e edge and h i g h speed p h o t o d e t e c t o r combination. Bys u i t a b l e c h o i c e o f r e l a y l e n s e s d e f l e c - t i o n s o f 1 x d e q r e e s have been meas- u r e d i n t i m e s o f z 5 n s . The probe beam d e f l e c t i o n a s a f u n c t i o n of t i m e was r e c o r d e d on a f a s t s t o r a g e o s c i l l o s c o p e . The r e f r a c t i v e i n d e x p r o f i l e o f t h e shock v7as o b t a i n e d by modeling and computer matching. The probe arrangement a l s o o f f e r s a s i m p l e t e c h n i q u e f o r measurement of t h e shock f r o n t d i s p l a c e m e n t w i t h t i m e . The l a s e r probe h a s been f u r t h e r m o d i f i e d
t o form an i n t e r f e r o m e t e r a l l o w i n g m e a s u r e ment o f t h e o p t i c a l p a t h l e n g t h a s a
f u n c t i o n o f t i m e d i r e c t l y r a t h e r t h a n t h e complex computer i t e r a t i o n p r o c e d u r e . The i n t e r f e r o m e t e r i s e s s e n t i a l l y a Ifichelson arranqement. I n t h e c o n v e n t i o n a l
Michelson i n t e r f e r o m e t e r p a r a l l e l beams I
1
1
S p a t i a l f i l t e r C 0 2 l a s e r LULLLW0
He Ne L a s e r Si / The q u a s i c o h e r e n t p r o p e r t i e s of t h e n i t r o q e n l a s e r l e a & t o d i f f r a c t i o n and i n t e r f e r e n c e e f f e c t s on t h e shadowqraphs. I n p a r t i c u l a r t h a t p a r t of t h e h e m i s p h e r i - c a l w a t e r shock o p o s i t e t h e s u r f a c e i s n o t c l e a r l y d e f i n e d and t h e p h o t o c r a p h s a r e n o t e a s i l y i n t e r p r e t e d . A S c h l i e r e n probe r e c o r d i n g o f t h e d e f l e c - t i o n due t o t h e a i r shocki s
shown i n f i g u r e 111.I ,
,
F i g u r e I1 Probe Michelson i n t e r f e r o m e t e r Two camera l e n s e s were used t o produce t h e probe beam. The l e n s e s a r e p o s i t i o n e d s o t h a t t h e two f o c i i a r e c o i n c i d e n t which i m p l i e s t h a t t h e m o d i f i e d i n t e r f e r o m e t e r i s n o t s e n s i t i v e t o a n g u l a r movements o f
/
L1 \ l a t e r c e l l
t h e probe beam about t h e f o c u s . The i n t e r f e r o m e t e r was a d j u s t e d s o t h a t t h e recon?bined w a v e f r o n t s were p a r a l l e l , t h a t i s a s i n g l e f r i n g e f i l l e d t h e f i e l d of t h e f i n a l l e n s b e f o r e b e i n g f o c u s e d on t h e f a s t s i l i c o n p h o t o d i o d e . The s i g n a l was r e c o r d e d u s i n g t h e same a m p l i f i e r and s t o r a g e o s c i l l o s c o p e a s i n t h e S c h l i e r e n probe e x p e r i m e n t .
A Q-switched ruby l a s e r was used a s t h e l i g h t s o u r c e i n a Kach Zehnder i n t e r f e r o - m e t e r t o o b t a i n i n t e r f e r o g r a m s of t h e w a t e r and a i r shocks. The p u l s e d u r a t i o n v a s 40ns which i s a d e q u a t e f o r r e c o r d i n q t h e a i r shock wave b u t t o o l o n q t o a l l o w a c c u r a t e measurements on t h e w a t e r shock wave. D i r e c t p r e s s u r e measurenents on t h e
shock waves above and below t h e s u r f a c e were c a r r i e d o u t u s i n g l i t h i u m n i o b a t e p i e z o e l e c t r i c p r e s s u r e t r a n s 6 u c e r s .
F i g u r e I11
The a i r shock h a s a f i n i t e r i s e t i m e o f 200ns. The computer d e r i v e d d e f l e c t i o n curve assuming a s t e ~ shock f r o n t and r e f r a c t i v e i n d e x , n r , a t a d i s t a n c e
r
f o l l o w i n q t h e e x p r e s s i o n 3 nr = n+
(nm-
no) ( r / R ) 0 g i v e s t h e b e s t f i t w i t h t h e e x p e r i m e n t a l d e f l e c t i o n , F i q u r e I V . Here n and no a r em
t h e maximum and ambient r e f r a c t i v e i n d i c e s r e s p e c t i v e l y and Ri s
t h e r a d i u s of t h e shock f r o n t . The decay of r e f r a c t i v e i n d e x i s g i v e n i n F i g u r e V.The shock f r o n t d i s p l a c e m e n t a s a f u n c t i o n of t i m e i s shown i n F i g u r e V I . When t h e s e r e s u l t s a r e d i s p l a y e d on l o g a r i t h m i c s c a l e s t h e g r a d i e n t i s 0.65.
C9-234 JOURNAL DE PHYSIQUE
i n t e g r a t e d o p t i c a l p a t h l e n g t h t o b e
o r d s t h e f r i n g e p a t t e r n from t h e i n t e r - f e r o m e t e r a s t h e o p t i c a l p a t h l e n g t h chanqes w i t h time. T h a t i s a s t h e shock wave expands t h r o u g h t h e probe beam. I n p r i n c i p l e t h e r e c o r d i n g s a r e r e l a t i v e l y
s i w l e t o i n t e r p r e t b u t t h e shock f r o n t rise t i n e and r a p i d l y changinq o p t i c a l p a t h n e a r s h a l l o w c h o r d s make t h e a n a l y s i s d i f f i c u l t . The y - a x i s o f t h e o s c i l l o s c o p e r e c o r d s t h e i n t e r f e r o m e t e r f r i n g e i n t e n - s i t y which f o l l o w s t h e o p t i c a l p a t h l e n q t h chanqes a s t h e shock wave expands throuph t h e beam. The
start in^
p o i n t o f t h e t r a c e depends upon t h e low frequency q u i e s c e n t s t a t e o f t h e i n t e r f e r o n e t e r . The two extremes b e i n c , A, c o n s t r u c t i v e o r B des- t r u c t i v e . I n t h e example shown h e r e t h e f r i n y e p a t t e r n i s between t h e s e two ex- tremes which h e l p s t o renove a m b i q u i t i e s and i s n e a r t h e n o s t s e n s i t i v e p a r t of t h e i n t e r f e r o m e t e r r e s p o n s e . The t o t a l o p t i - c a l p a t h l e n g t h i n c r e a s e s t o p o i n t C and t h e n f a l l s . P o i n t C c o r r e s p o n d s t o t h e naxinum o p t i c a l p a t h l e n q t h which i s t h e s u n o f t h e c o n t r i b u t i o n s of t h e r e f r a c t i v e i n d e x a l o n g t h e p a t h through t h e shock. The r i s e and f a l l i n o p t i c a l p a t h l e n q t h i s e a s i l y u n d e r s t o o d when t h e shock f r o n t i s c o n s i d e r e d a s a s p h e r i c a l s h e l l . IS.ea- l l y t h e number o f f r i n g e s t o t h e p o i n t C s h o u l d e q u a l t h o s e a f t e r w a r d s b u t t h e o p t i c a l p a t h i s f u r t h e r c o m p l i c a t e d by t h e a r r i v a l o f t h e vapour plume. I n o r d e r t o o b t a i n t h e r a d i a l d i s t r i b u t i o n o f t h e r e f r a c t i v e i n d e x f r o n t h e i n t e r f e r o - n e t e r measurements some form o f i n v e r s i o n t e c h n i q u e must be used. I n t h e c a s e o f t h eF i q u r e X F r i n q e s h i f t w i t h t i m e and shock r a d i u s .
two diniensional Kach Zehnder i n t e r f e r o a r a m t h i s i s r e l a t i v e l y s i r . p l e whereas i n t h e probe i n t e r f e r o m e t e r t h e chanqinq p r o f i l e must be t a k e n i n t o a c c o u n t . A f u l l des- c r i p t i o m o f t h i s t i m e dependent i n v e r s i o n w i l l be g i v e n e l s e w h e r e . The maximm r e f r a c t i v e i n d e x change i s 3.8 x a t 2mm above t h e w a t e r s u r f a c e and 2.3 x a t 4mm. The maxinum y e s s u r e i n t h e shock wave a t 0 . 5 m above t h e w a t e r s u r f a c e i s
6 5
4 x 1 0 Pa anc? t h i s f a l l s t o 2 x 1 0 Pa a t
4rn..
The w a t e r shock wave i s much t h i n n e r t h a n t h e a i r shock wave, t h e i n i t i a l p r e s s u r e p u l s e i s 30pm and t h e t o t a l t h i c k n e s s i s 100rm. P r e l i m i n a r y measurenents u s i n g l i t h i u m n i o b a t e p i e z o e l e c t r i c p r e s s u r e gauges i n d i c a t e t h a t t h e r e s o l u t i o n of t h e o p t i c a l probe t e c h n i q u e s may b e s u p e r i o r where s ~ h e r i c a l geometry i s i n v o l v e d . However t h e r e i s qood aqreement between t h e p r e s s u r e t r a n s d u c e r and t h e o p t i c a l d i a g n o s t i c t e c h n i q u e s d e s c r i b e d above. CONCLUSIOIGS AND DISCUSSION
C9-236 JOURNAL DE PHYSIQUE
which i s d i f f i c u l t t o measure because o f t h e dimensions and time s c a l e of t h e e v e n t . S i n g l e exposure s h o r t d u r a t i o n shaeowgraphy shows t h e main f e a t u r e s of t h e e v e n t b u t i n t e r p r e t a t i o n
i s
n o t s i m p l e . The r a j o r i t y o f t h e work d e s c r i b e d h e r e i s concerned w i t h t h e a i r shock wave. The d i s p l a c e r e n t-
t i m e
r e s u l t s i n d i c a t e t h a t t h e shock wave cannot b e d e s c r i b e d by b l a s t wave t h e o r y where t h e power of t h e t i r e dependence i s 0.4 compared w i t h t h e measures v a l u e o f 0 . 6 5 .There i s good agreen?ent between t h e probe d e f l e c t i o n , Each Zehnder and probe i n t e r - f e r o m e t e r r e s u l t s i n s p i t e o f t h e u s e o f a c o n s t a n t v a l u e f o r t h e maxinurr. v a l u e f o r t h e r e f r a c t i v e i n d e x , nm i n t h e d e f l e c t i o n c a l c u l a t i o n s .
ACKETOWLE DGEMENT S
The ~ u t h o r would l i k e t o acknowledqe t h e a s s i s t a n c e o f G . P . Davidson, E . G e r r i t s o n , T. Geerken and L.A. Mahmood. The work v:as s u p p o r t e d by g r a n t s from t h e S c i e n c e Research Council an2 t h e Procurement
E x e c u t i v e ( M O D ) . The l o a n o f t h e C02 l a s e r by RSPE i s g r a t e f u l l y acknowledned.
REFERENCES
(1) P . W . S i g r i s t and F.K. Kneubuhl, J . Acoust. 80c. &n.
64,
1652, 1978.( 2 ) G.P. Oavidson and D . C . Emmony, J . Phys. E .
2 ,
92, 1980.( 3 ) D.S. S t a r k , P . H . Cross and 11. P o s t e r , IEEE J . Cuantun E l e c t r o n . Q.E.11, 774, 1975.
