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A giant pulse ruby laser : construction and techniques of operation Churchland, Mark T. 1967

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A GIANT PULSE RUBY LASER CONSTRUCTION AND TECHNIQUES OF OPERATION by Mark T. C h u r c h l a n d B.Sc,  U n i v e r s i t y o f B r i t i s h Columbia, 19&7  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e Department of PHYSICS  We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e required standard  THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER, 196?  ^  In  presenting  this  an a d v a n c e d  degree  the L i b r a r y  shall  i I f u r t h e r agree for  scholarly  by h i s of  this  written  thesis at  the U n i v e r s i t y  make  tha  permission  It  of  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, Canada  SEPTEMBER l£, 1969-  Columbia  requirements I agree  r e f e r e n c e and copying of  this  that copying or  not  for  that  study, thesis  by t h e Head o f my D e p a r t m e n t  f i n a n c i a l gain shall  PHYSICS  the  B r i t i s h Columbia,  is understood  permission.  Department  of  for extensive  p u r p o s e s may be g r a n t e d  for  fulfilment of  it freely available for  representatives. thesis  in p a r t i a l  or  publication  be a l l o w e d w i t h o u t my  SIDE VIEW OF LASER CAVITY INCLUDING MIRRORS AND DYE CELL  VIEW OF OPTICAL PUMPING CAVITY (UPPER HALF REMOVED) EXPOSING FLASH TUBES AND RUBY WATER JACKET  ABSTRACT  A g i a n t p u l s e r u b y l a s e r was c o n s t r u c t e d u s i n g a 6" l o n g by •§•" d i a m e t e r  ruby c r y s t a l .  f o r c o n t r o l l i n g the temporal l a s e r output.  Techniques a r e d i s c u s s e d  and s p e c t r a l d i s t r i b u t i o n o f t h e  A "Q" s p o i l i n g dye c e l l ( c r y p t o c y a n i n e i n  methanol) was u s e d t o produce o u t p u t p u l s e s o f 150 megawatts. "Mode l o c k i n g " m o d u l a t i o n u s i n g t h e above dye c e l l . and a p e r t u r e s  (3 t o 5> mm*  o f t h e o u t p u t was a l s o  observed  Using m u l t i p l e surface f r o n t m i r r o r s i n diameter) the s p e c t r a l l i n e  of t h e o u t p u t was r e d u c e d t o .Of? A.  width  iv  ACKNOWLEDGEMENTS  I would l i k e t o s i n c e r e l y t h a n k Dr. R. A. N o d w e l l f o r h i s g u i d a n c e and encouragement t h r o u g h o u t t h i s work.  I  would a l s o l i k e t o t h a n k Dr. J . Meyer f o r h i s encouragement and h e l p f u l s u g g e s t i o n s d u r i n g the p r e p a r a t i o n o f t h i s I would a l s o acknowledge  thesis.  the assistance of  Mr. P. Haas i n t h e c o n s t r u c t i o n o f t h e a p p a r a t u s .  TABLE OF CONTENTS  11 iii iv v vi  Photographs o f L a s e r C a v i t y and Pumping C a v i t y Abstract Acknowledgements Table o f C o n t e n t s Index o f F i g u r e s  1  CHAPTER I  3  CHAPTER I I  PRINCIPLES OF LASER ACTION  5  CHAPTER I I I  DESIGN AND CONSTRUCTION OF LASER  6 6 12  i  INTRODUCTION  L'asing Medium  i i Laser  Head  i i i L a s e r D i s c h a r g e Bank  15  CHAPTER IV  DIAGNOSTIC TECHNIQUES  22  CHAPTER  RESULTS  V  2^  Laser  36  Cavity  Configurations. Configurations  \2  CONCLUSION AND FUTURE IMPROVEMENTS  U3  Bibliography  vi .  •  INDEX OF FIGURES  FIGURE  PAGE  1  5  "C" AXIS o r i e n t a t i o n i n r u b y  2  6  A b s o r p t i o n spectrum f o r r u b y  3  7  Tri-axial laser cavity flash  (with  crystal  helical  tube)  h  7  Close coupled l a s e r  5>  7  Elliptical  6  8  Cavity efficiency  laser  cavity  cavity  7  10  Double e l l i p t i c a l  8  10  Water j a c k e t f o r r u b y r o d  9  12  Bank c i r c u i t  10  13  Current pulses  11  lU  Trigger c i r c u i t f o r flashtubes  12  l£  Thermopile cone  13  16  Photo d i o d e c i r c u i t  lU  17  L a s e r l i g h t e x t r a c t i o n f o r photo d i o d e  15  18  V o l t a g e o u t p u t v s . energy i n p u t f o r d i o d e  16  19  F a b r e y - P e r o t s c h e m a t i c f o r photography  17  21  D v s . l o g E graph f o r TRI-X f i l m  18  22  Roof p r i s m s  19  23  Dye  20  25  Normal mode r e s u l t s  21  27  A b s o r p t i o n spectrum o f c r y p t o c y a n i n e i n methanol  cavity  cells  vii  INDEX OF FIGURES  FIGURE  (continued)  PAGE  22  30  Mode l o c k i n g r e s u l t s (weak s o l u t i o n )  23  32  Intermediate s o l u t i o n  2k  33  Double l a s e r p u l s e s  25  3k  S t r o n g s o l u t i o n work  26  38  Aperture r e s u l t s  27  39  Triple front reflector results  28  39  T r i p l e f r o n t r e f l e c t o r with aperture  29  Ho  Extended c a v i t y  results  ( l o w power)  results  CHAPTER I  INTRODUCTION The  technology of g i a n t pulse ruby l a s e r s i s of  i n t e r e s t t o plasma p h y s i c i s t s because o f p o t e n t i a l a p p l i c a t i o n s f o r plasma d i a g n o s t i c s .  I t has become p o s s i b l e r e c e n t l y t o make use o f  l6 scattered  l a s e r l i g h t f r o m h i g h d e n s i t y plasmas (n  as a d i a g n o s t i c  tool.  Several  =10  -3 CM  )  e x p e r i m e n t s have been done t o measure  a  the  1  i o n t e m p e r a t u r e as w e l l as t h e e l e c t r o n d e n s i t y  and t e m p e r a t u r e .  V e l o c i t y d i s t r i b u t i o n d e p a r t u r e s f r o m t h e r m a l e q u a l i b r i u m have a l s o 7 been measured u s i n g t h e l a s e r s c a t t e r i n g t e c h n i q u e . These e x p e r i m e n t s a l l u s e a g i a n t p u l s e r u b y l a s e r as a h i g h power c o h e r e n t l i g h t source.  The h i g h power i s n e c e s s a r y because t h e i n t e n s i t y o f t h e -12  s c a t t e r e d l i g h t i s about 10  the i n t e n s i t y of the i n c i d e n t  light.  P u l s e d r u b y l a s e r s have a l s o been employed i n d o u b l e exposure holography to record modes.  shock wave p o s i t i o n s and s t a n d i n g  I n t h e mode l o c k e d  multi-exposure Schlieren The laser scattering.  c o n f i g u r a t i o n , l a s e r s have been u s e d . f o r  photographs.  most u s e f u l o f these d i a g n o s t i c 17  I n dense plasmas (n  = 10  : e l e c t r o m a g n e t i c f i e l d s between t h e e l e c t r o n s istic oscillations.  wave v i b r a t i o n a l  T  g  techniques i s  = l . ^ e V ) the  and i o n s s e t up  character-  I f t h e s c a t t e r i n g v e c t o r , A k, i s much l o n g e r  t h a n t h e Debye l e n g t h f o r t h e p l a s m a , t h e i n t e r a c t i o n o f t h e l a s e r ' s e l e c t r o m a g n e t i c f i e l d and t h e s e plasma o s c i l l a t i o n s produces s a t e l l i t e s .  - 2 -  These o c c u r a p p r o x i m a t e l y a t ' t h e b e a t f r e q u e n c y o f t h e two f i e l d s . For  the o s c i l l a t i o n s associated with the e l e c t r o n  acoustic  wave  d i s t i n c t and w e l l r e s o l v e d s a t e l l i t e s o c c u r t y p i c a l l y a t ± 30 A f r o m the  l a s e r wavelength.  acoustic  The i n f o r m a t i o n a s s o c i a t e d w i t h t h e i o n  wave i s c o n t a i n e d i n t h e c e n t r a l peak.  above plasma has a h a l f w i d t h o f .2 A. the  T h i s peak f o r t h e  I n order t o a c c u r a t e l y  shape of t h e c e n t r a l peak i t i s n e c e s s a r y t o use a l a s e r  a s p e c t r a l l i n e w i t h a h a l f w i d t h of much l e s s t h a n .2 A.  determine emitting  Most  c o m m e r c i a l l a s e r s cannot p r o d u c e t h i s n a r r o w a s p e c t r a l l i n e .  Those  that can are p r o h i b i t i v e l y expensive. T h i s work was i n i t i a t e d a f t e r an attempt was made w i t h a T.R.G. -10L|-A l a s e r t o o b s e r v e t h e s c a t t e r e d plasma due t o t h e i o n d e n s i t y  fluctuations.  signal i n a j e t  The l a c k o f r e p r o d u c i b i l i t y  of t h e s p e c t r a l d i s t r i b u t i o n o f t h e l a s e r made i t i m p o s s i b l e t o o b t a i n any  i n f o r m a t i o n about t h e i o n s . I t i s t h e purpose o f t h i s work t o d e v e l o p a  t e c h n o l o g y i n r u b y l a s e r d e s i g n and c o n s t r u c t i o n the  which w i l l  i n v e s t i g a t i o n of t h e i o n t e m p e r a t u r e and d e n s i t y .  would a l s o be u s e f u l i n h o l o g r a p h y , S c h l i e r e n , m e t r i c t e c h n i q u e s o f plasma  investigation.  allow  Such t e c h n o l o g y  and o t h e r i n t e r f e r o -  CHAPTER I I  PRINCIPLES OF LASER ACTION 'The quite simple.  p h y s i c a l p r i n c i p l e s behind l a s i n g a c t i o n are  I n g e n e r a l , some medium ( s o l i d , l i q u i d , o r g a s ) i s  o p t i c a l l y pumped t o produce a p o p u l a t i o n t h a t are o p t i c a l l y connected.  i n v e r s i o n between two s t a t e s  A resonant o p t i c a l c a v i t y i s then s e t  up "so t h a t a photon s p o n t a n e o u s l y e m i t t e d by t h e medium i s a m p l i f i e d t o produce a s t a n d i n g  wave p a t t e r n w i t h i n t h e o p t i c a l c a v i t y .  Some  p o r t i o n o f t h e energy i n t h e s t a n d i n g wave e s c a p e s t h r o u g h a p a r t i a l m i r r o r as t h e e m i t t e d l a s e r beam. population  F o r a m p l i f i c a t i o n t o occur, the  i n v e r s i o n must be g r e a t enough so t h a t t h e g a i n p e r passage  t h r o u g h t h e c a v i t y exceeds t h e l o s s e s a t t h e ends and s i d e s o f t h e cavity.  I t i s n o t d i f f i c u l t t o see t h a t t h e g r e a t e r t h e p o p u l a t i o n  i n v e r s i o n the higher  the gain.  To o b t a i n h i g h e r p o p u l a t i o n of "Q" s p o i l i n g i s used.  i n v e r s i o n s , the technique  T h i s means t h a t t h e e f f i c i e n c y o r "Q" of-  the c a v i t y i s , by some s u i t a b l e method, h e l d a t a s m a l l enough v a l u e so t h a t d e p l e t i o n o f t h e upper s t a t e b y s t i m u l a t e d e m i s s i o n and  the population  o f t h e upper l e v e l i s l i m i t e d o n l y b y t h e pumping  r a t e and the l o s s e s .  When t h e p o p u l a t i o n  i n v e r s i o n has r e a c h e d t h e  d e s i r e d l e v e l , the Q of the c a v i t y i s q u i c k l y i n c r e a s e d . produces a l a s e r p u l s e o f h i g h e r without Q s p o i l i n g .  i s small,  This  i n t e n s i t y t h a n c o u l d be a c h i e v e d  The two most commonly u s e d methods o f Q s p o i l i n g  - k -  are r o t a t i n g m i r r o r s and s a t u r a b l e dyes We c a n f o r m an a n a l o g y between l a s e r c a v i t i e s and microwave c a v i t i e s .  I n microwave c a v i t i e s c e r t a i n modes o r s t a n d i n g  wave p a t t e r n s w i l l be s e t up i f t h e p r o p e r d r i v e r f r e q u e n c y i s applied.  I n a l a s e r c a v i t y v e r y many modes o f a p p r o x i m a t e l y t h e  same f r e q u e n c y a r e p o s s i b l e  due t o t h e l a r g e  compared t o t h e w a v e l e n g t h o f l i g h t .  s i z e of the c a v i t y  Which mode t h e l a s i n g  action  i s i n i t i a t e d i n i s d e t e r m i n e d by. the Q o f t h e c a v i t y f o r t h a t mode and  on t h e o c c u r e n c e o f a p h o t o n s p o n t a n e o u s l y e m i t t e d i n t h a t mode.  The  l a t t e r e v e n t i s e n t i r e l y random.  We c a n see t h e n t h a t i f t h e Q  of t h e c a v i t i e s f o r two modes i s about t h e same, i t i s i m p o s s i b l e t o t e l l which w i l l l a s e The Q o f a l l b u t a few,  first. p r o c e s s o f mode s e l e c t i o n e n t a i l s r e d u c i n g t h e o r i d e a l l y , a s i n g l e mode t o a l e v e l which w i l l  not produce l a s e r a c t i o n . but  I f we can s u f f i c i e n t l y reduce t h e Q o f a l l  one mode, t h e e m i t t e d s p e c t r a l l i n e w i l l be v e r y narrow. I t s  h a l f w i d t h w i l l depend on t h e l e n g t h and t h e Q o f t h e c a v i t y . h i g h e r t h e Q, t h e n a r r o w e r t h e l i n e .  More complete d i s c u s s i o n s  modes a r e g i v e n i n books b y L e v i n e and b y L e n g y e l . consideration  Experimental  t h e o r y o f modes and mode s e l e c t i o n i s n o t  c o m p l e t e l y i n agreement w i t h e x p e r i m e n t .  The p r o b l e m , i n p a r t , i s  t o t h e d i f f i c u l t i e s i n a p p l y i n g boundary c o n d i t i o n s  electromagnetic theory. the  on  on mode s e l e c t i o n w i l l be g i v e n i n Chapter V. The  due  The  t o the  E x p e r i m e n t a l l y , problems a r i s e because o f  i n h o m o g e n i e t i e s i n t h e l a s i n g medium.  -  $ -  CHAPTER I I I DESIGN AND CONSTRUCTION OF LASER i  The l a s i n g medium u s e d f o r t h i s work i s " p i n k " r u b y c r y s t a l . I n r u b y c r y s t a l each u n i t c e l l c o n t a i n s two A ^ O ^  molecules.  3+ I n " p i n k " r u b y c r y s t a l about .0$% of t h e A l  i s replaced  w i t h C r ^ . The c r y s t a l a x i s o r "C" a x i s f o r r u b y can be +  oriented  i n any d i r e c t i o n w i t h r e s p e c t t o t h e l a s i n g  axis.  I f t h e " C " a x i s and l a s i n g a x i s a r e p a r a l l e l , t h e l a s e r !  l i g h t i s unpolarized.  I f t h e r o d i s c u t so t h a t t h e c r y s t a l  a x i s i s 60° o r 90° t o t h e l a s e r a x i s t h e l i g h t i s l i n e a r l y polarized.  The p l a n e of p o l a r i z a t i o n i n t h e l a t t e r case i s  p e r p e n d i c u l a r t o t h e p l a n e o f t h e "C" a x i s and t h e l a s e r I n t h i s work a 60° c r y s t a l was u s e d .  L A S E R AXIS  F(G  I  axis.  - 6 -  I n d e s i g n i n g t h e o p t i c a l pumping system i t i s i m p o r t a n t t o know the a b s o r p t i o n spectrum o f the ruby.  T h i s has been .  6 i n v e s t i g a t e d by Maiman e t a l . shown i n F i g .  3.6 3.2 7 2.8 E 1| 2.0 8 }.6 2A  I 1.2 Ci.  The r e s u l t s  o f t h e i r work are  2.  1  1  1  1  Incident light ||  . .  parallel to c - a x i s Incident  J[  /  1 1  I  / W — 1 / A \  lielit  perpendicular to c - a x i s  / /  /  I /  1 \  /  ~WV''  l  _  \  it  / W\ — 1 1  \\  // \ 1/ //  l  \  _ 1.5 x 10"  . —  \  \  8 0.8 r \ x> - i-A / \f 0.4 1 V — < l l^7! I 1V 3000 4000 5000 0 100 6000 Wavelength, A  2.0 x 10"  —  -•  19  0.5 x 10"  1 1  FIG.  1.0 x 10~  1  kVi.)0  7000  2  We c a n see t h e r e a r e two main a b s o r p t i o n bands, one a t lj.000 A and  one a t 5500 A, each about 1000 A wide.  To o p t i c a l l y pump  t h e s e bands we use a xenon f i l l e d f l a s h t u b e .  Xenon i s u s e d  because i t p r o d u c e s an a l m o s t b l a c k body spectrum, w i t h a maximum near t h e pumping bands. ii  The l a s e r head The l a s e r head c o n s i s t s o f a r u b y r o d , f l a s h t u b e s and  t h e o p t i c a l pumping r e f l e c t o r or c a v i t y  t h a t c o u p l e s the  - 7 -  e n e r g y from t h e f l a s h tubes t o the configurations f o r laser  ruby rod.  There a r e many  The most common a r e :  heads  \  t h e t r i a x i a l system o f \ /  ruby r o d , h e l i c a l f l a s h tube'and c y l i n d r i c a l reflector;  the  c l o s e c o u p l e d system  of l i n e a r f l a s h tubes c l o s e , to the ruby i n a s m a l l cavity;  the  elliptical  cylinder  w i t h t h e f l a s h tube a t one f o c u s and t h e r u b y at the other.  FIG. $  The e f f i c i e n c y of t h e s e c o n f i g u r a t i o n s has been s t u d i e d by C o n g l e t o n e t a l and the r e s u l t s a r e g i v e n i n FIG.  6.  I n s p i t e o f the above, the c h o i c e of c a v i t y  configuration  i s more l i k e l y t o be made f r o m a p o i n t o f v i e w of c o n v e n i e n c e i n c o n s t r u c t i o n and maintenance.  F o r example, the h e l i c a l  f l a s h t u b e s a r e t h e most e f f i c i e n t and c a p a b l e of h i g h energy d i s s i p a t i o n , b u t . t h e y are a l s o the most d i f f i c u l t t o mount and m a i n t a i n . M o u n t i n g problems o c c u r because the t h e r m a l shocks produce s p r i n g l i k e movements w h i c h c r a c k the f l a s h tubes C o n s e q u e n t l y , the c o n f i g u r a t i o n used i n t h i s work i s a m o d i f i c a t i o n of the s i n g l e e l l i p t i c a l c y l i n d e r .  Two  elliptical  - 9 -  c y l i n d e r s were machined i n a s o l i d aluminium b l o c k .  The two  c y l i n d e r s have a common f o c a l l i n e and a l l t h e f o c a l  lines  lie  i n t h e same p l a n e .  The aluminium b l o c k was c u t a l o n g  the p l a n e o f t h e t h r e e f o c i t o f o r m two h a l f s h e l l s .  The  r u b y was p l a c e d a t t h e common f o c u s and t h e f l a s h tubes were p l a c e d one a t each r e m a i n i n g  focus.  I t was f o u n d i n work w i t h a TRG IOI4 l a s e r t h a t a i r c o o l i n g h a s two d i s a d v a n t a g e s . ruby r o d i s i n e f f i c i e n t .  F i r s t , the cooling" of the  A r e c y c l e time o f more t h a n one  s h o t p e r minute r e s u l t s i n power l o s s .  The second, and  p r o b a b l y more s e r i o u s i s t h a t a i r t a k e n f r o m t h e room c o n t a i n s sundry v a p o u r s and d u s t p a r t i c l e s .  As t h e a i r i s blown o v e r  the s u r f a c e o f t h e r o d and m i r r o r s , t h e vapours and dust p a r t i c l e s form d e p o s i t s .  This not only reduces t r a n s m i s s i o n  of l i g h t b u t a l s o forms c e n t e r s f o r a b s o r p t i o n .  This u s u a l l y  r e s u l t s i n permanent damage t o "the d i e l e c t r i c c o a t i n g s , o r i n extreme c a s e s , t o t h e r u b y o r m i r r o r s u b s t r a t e .  What appears  t o be a v e r y s m a l l p i t .on t h e s u r f a c e i n t h e l a s e r p a t h u s u a l l y produces v e r y h i g h d i f f r a c t i o n l o s s e s .  This can r e s u l t i n a  g a i n o f l e s s than one and thus no l a s i n g a c t i o n . To improve t h i s s i t u a t i o n t h e r o d and f l a s h t u b e s were water c o o l e d .  The a i r around t h e r u b y 'ends and m i r r o r s t h e n i s  r e l a t i v e l y stagnant.  A l s o the duty c y c l e i s l i m i t e d mainly by  - 10 -  - 11 -  the  30 s e c . r e c h a r g e t i m e ) .  power s u p p l y  The f l a s h t u b e and water j a c k e t assembly used was E.G.  & G. - FX - 65 - B6 xenon f i l l e d .  the  Each tube i s c a p a b l e  2000 J o u l e s energy d i s s i p a t i o n , g i v i n g 1;000 J o u l e s maximum  of  The r u b y rod,was a " p i n k " r u b y , 60°  pumping energy.  6 i n c h e s l o n g by \ i n c h d i a m e t e r . cut  cut,  The ends of the r o d were  a t t h e B r e w s t e r a n g l e t o r e d u c e l o s s e s a t the s u r f a c e .  The r o d was e n c l o s e d i n a p y r e x tube whose s u r f a c e was b l a s t e d t o d e f o c u s the i n c i d e n t l i g h t .  sand-  Rubber " 0 " r i n g s i n  b r a s s f i t t i n g s were u s e d t o s e a l the r u b y t o the p y r e x water jacket.  (See FIG.  8)  The c a v i t y was c u t by t i l t i n g a 2|" 6l  0  and c u t t i n g one h a l f - s h e l l a t a t i m e .  e x c e n t r i c i t y of 0.56  m i l l ing  head a t  T h i s g i v e s an  .  The r e f l e c t i n g s u r f a c e o f the pumping c a v i t y  was  formed by p o l i s h i n g t h e s u r f a c e o f the a l u m i n i u m b l o c k . Aluminium,under  t h e s e c o n d i t i o n s , r e f l e c t s about 80% of the  i n c i d e n t l i g h t i n the a b s o r p t i o n band r e g i o n of the r u b y . The whole assembly i s mounted i n a l u c i t e base box t o p r o t e c t a g a i n s t h i g h v o l t a g e breakdowns. i s mounted a t 30° B r e w s t e r ends. to  and  The l a s e r  head  t o t h e o p t i c a l bench t o compensate f o r the  The c o o l i n g water i s r u n i n s e r i e s , g o i n g f i r s t  t h e r u b y and t h e n t o each f l a s h t u b e .  To a l l o w the a i r  b u b b l e s i n t h e c o o l i n g water t o e s c a p e , t h e water o u t p u t was always p l a c e d a t t h e h i g h e s t p o i n t i n the water  jacket.  - 12 -  The c a p a c i t o r bank f o r t h e l a s e r power s u p p l y must be d e s i g n e d in  conjunction with a suitable flashtube.  The R.C. time o f t h e  bank and f l a s h t u b e s h o u l d be l e s s t h a n one m i l l i s e c . f o r e f f i c i e n t pumping.  Care must a l s o be t a k e n t h a t t h e i n i t i a l  c u r r e n t s u r g e i s l o w enough so t h a t t h e maximum power r a t i n g o f the tube i s n o t exceeded. The i n i t i a l  surge o f c u r r e n t i n t h e f l a s h t u b e c a n  be r e d u c e d w i t h t h e a i d o f s u i t a b l e i n d u c t a n c e s .  In a delay  l i n e c o n f i g u r a t i o n and w i t h t h e c o r r e c t t e r m i n a t i o n t h i s c a n produce a s q u a r e p u l s e o u t p u t . end  T h i s square wave c a n be made t o  j u s t about t h e same t i m e l a s e r a c t i o n s t a r t s .  The main  advantage o f t h i s t e c h n i q u e - i s t h a t i t i n c r e a s e s u s e f u l f l a s h tube l i f e . The bank u s e d i n t h i s work was s p l i t s i d e s (one f o r each t u b e ) o f about 800 uF e a c h . flashtube l i f e  three inductances  h e l p shape t h e p u l s e .  'TRIGGER  2oo/^  ~~~ I 75>/F  To i n c r e a s e  were p l a c e d i n t h e c i r c u i t t o  (See F I G . 9)  Pulse;  prrrwrnn  i n t o two e q u a l  UJUL \m.H  —MO^  FIG.  9  FLASH  TUBE J  - 13 -  CURRENT PULSES  WITHOUT INDUCTANCES  WITH INDUCTANCES  The c u r r e n t p u l s e i s shown i n the o s c i l l o s c o p e t r a c e b e f o r e and a f t e r the i n d u c t a n c e s were added.  This technique  increased  f l a s h tube l i f e by a f a c t o r - o f two or t h r e e . There a r e two commonly u s e d t r i g g e r c i r c u i t s i n l a s e r f l a s h tube d i s c h a r g e s . pin.  The most common i s t h e e x t e r n a l t r i g g e r  Here a h i g h v o l t a g e s p i k e i s a p p l i e d t o a w i r e i n c l o s e  p r o x i m i t y t o the f l a s h tube.  T h i s produces enough  i o n i z a t i o n t o s t a r t the d i s c h a r g e . series injection triggering.  initial  The second method i s  Here a low i n d u c t a n c e  transformer  i s p l a c e d w i t h i t s secondary i n s e r i e s w i t h the bank and f l a s h tube.  T h i s produces a h i g h v o l t a g e s p i k e a c r o s s the f l a s h  tube e l e c t r o d e s i n excess o f the tube's breakdown v o l t a g e . The s e c o n d a r y o f t h e t r a n s f o r m e r must c a r r y the f u l l bank current.  We can see t h e s e l f i n d u c t a n c e  o f t h e secondary  must be l o w enough n o t t o slow the main d i s c h a r g e  significantly.  The s e r i e s i n j e c t i o n removes the d i f f i c u l t y o f h i g h v o l t a g e breakdowns between t h e t r i g g e r p i n and the c a v i t y o r . o t h e r metallic parts.  The s e r i e s i n j e c t i o n method i s e s p e c i a l l y  u s e f u l i f the c a v i t y and f l a s h tube are i n the same water b a t h .  - m  ^  -  I n t h e l a t t e r case t a p water can be c i r c u l a t e d t o c o o l the r u b y w i t h o u t f e a r of h i g h v o l t a g e breakdowns. s e r i e s i n j e c t i o n t r i g g e r i n g was u s e d .  Two E.G.  I n t h i s work & G.  179  t r i g g e r t r a n s f o r m e r s were s e t up i n s e r i e s w i t h the bank and the f l a s h tubes.  A c i r c u i t d i a g r a m ( i n b l o c k form) i s shown  i n F I G . 11.  8oov  ~0  100 V  O——~  rH  U J J J  SPAR K GAP  A  3)  3  h-O  ISKV  O  ImF  ZOOOV.  nnm.  FIG. 11  xz  - 15 -  CHAPTER IV  DIAGNOSTIC TECHNIQUES I n l a s e r d i a g n o s t i c s we a r e u s u a l l y concerned w i t h the  o u t p u t beam energy, i t s t e m p o r a l d i s t r i b u t i o n , and i t s  spectral distribution.  T h i s c h a p t e r w i l l d i s c u s s methods u s e d  to monitor these. The energy o u t p u t o f a p u l s e l a s e r i s u s u a l l y measured w i t h c a l o r i m e t r i c methods.  The beam i s absorbed by  some medium and t h e t e m p e r a t u r e change o f the medium i s monitored.  I n t h i s work a commercial (T.R.G. 109) t h e r m o p i l e  p a i r was u s e d .  The u n i t employs  two i d e n t i c a l c a r b o n cones and  two t h e r m o p i l e s t o m o n i t o r t h e t e m p e r a t u r e o f t h e cones.  One  cone i s used as a r e f e r e n c e and one cone i s used as t h e e n e r g y absorber.  These a r e c o n n e c t e d i n a b r i d g e network and t h e  t e m p e r a t u r e d i f f e r e n c e between them i s c a l i b r a t e d d i r e c t l y i n Joules.  To ensure an a c c u r a t e energy r e a d i n g i t i s n e c e s s a r y t o  c o n s t r u c t t h e cones i n such a way as t o maximize t h e p e r c e n t a g e of l i g h t a b s o r b e d .  The a b s o r b i n g s u r f a c e i s t h e i n s i d e o f a  h i g h l y p o l i s h e d narrow cone.  (See F I G . 12)  FIG. 12.  - 16 -  Thus a l l r e f l e c t i o n s a r e f o c u s e d t o t h e apex o f t h e cone and the m a j o r i t y e v e n t u a l l y a b s o r b e d . M o n i t o r i n g t h e t e m p o r a l development o f t h e l a s e r p u l s e requires very f a s t e l e c t r o n i c s .  The r i s e time o f t h e Q - s p o i l e d  l a s e r p u l s e can be o f t h e o r d e r o f one nanosecond. Model 519 o s c i l l o s c o p e  A Tektronix.;;  was used t o r e c o r d t h e l a s e r l i g h t o u t p u t .  I t has a r i s e time o f 0.3 nanoseconds.  The $19 has o n l y one  v e r t i c a l s e n s i t i v i t y w h i c h i s o f t h e o r d e r o f 10 v o l t s p e r c«ntimeter. To  detect a f a s t s i g n a l of t h i s s i z e a Hewlett-Packard  Lp I4.203 p h o t o d i o d e was used.  The r i s e time o f t h i s d i o d e i s  quoted as l e s s t h a n one nanosecond.  To o b t a i n a s i g n a l o f t h e  o r d e r o f 10 v o l t s a c r o s s a 125 ohm c a b l e t e r m i n a t o r , a 100 v o l t power s u p p l y was u s e d .  I t c o n s i s t e d o f a 100 v o l t b a t t e r y and  two 0.1 uF ceramic c a p a c i t o r s .  FIG.  (FIG.  13  13)  - 17 -  The  c a p a c i t o r s a r e t h e e f f e c t i v e power s u p p l y f o r s h o r t p u l s e s  ( f a s t r i s e ) due t o t h e i n d u c t a n c e o f t h e b a t t e r y .  To o b t a i n  a l i g h t s i g n a l f r o m t h e l a s e r beam a c l e a r g l a s s p l a t e i s u s u a l l y placed i n the l a s e r path approximately a t the Brewster angle.  Because t h e l a s e r l i g h t i s p o l a r i z e d and because t h e  glass i s placed a t the Brewster angle, very l i t t l e l i g h t i s r e f l e c t e d by t h e g l a s s p l a t e .  A v e r y s m a l l p a r t o f t h e beam  can be e x t r a c t e d t h i s way.  BREWSTER ANGLE  L l 5 E f  A  GLASS  FLAT  B E A M .  I n t h i s work t h e l a s e r back m i r r o r was a 99-9% r e f l e c t i n g dielectric mirror. transmitted.  Some p o r t i o n o f t h e r e m a i n i n g 0.1$ was  'The photo d i o d e was p l a c e d b e h i n d t h e back m i r r o r  i n t h e 0.1$ beam w i t h s u i t a b l e n e u t r a l d e n s i t y f i l t e r s . a l l o w e d a constant m o n i t o r i n g of the l i g h t output.  This  It is  p o s s i b l e , a f t e r c a l i b r a t i o n w i t h t h e t h e r m o p i l e energy meter, t o a s s o c i a t e an energy w i t h a g i v e n a r e a under t h e c u r v e and  - 18 -  thus t o c a l c u l a t e power o u t p u t .  To do t h i s we must be s u r e  t h a t the d i o d e has a r e a s o n a b l y l i n e a r r e s p o n s e t o l i g h t i n p u t o v e r t h e range used  ( ^ 10 v o l t s ) .  The f o l l o w i n g graph g i v e s  the v o l t a g e o u t p u t compared w i t h l i g h t  input.  A .Volts  \ Energy  Z  3  (Arbitrary Units)  FIG.  1$  4  5  - 19 -  To i n v e s t i g a t e t h e s p e c t r a l d i s t r i b u t i o n a F a b r y P e r o t i n t e r f e r o m e t e r was u s e d . f r i n g e p a t t e r n s on T r i - X - P a n s t u d i e d on a d e n s i t o m e t e r Ash)  Photographs were t a k e n o f t h e  (KODAK) f i l m .  The n e g a t i v e s  and c h a r t r e c o r d e r assembly.  were (Jarell  The d i a g r a m b e l o w g i v e s t h e s c h e m a t i c o f t h e F a b r y - P e r o t  technique.  9 0  0  V  \  FIG.  16  A narrow s p i k e f i l t e r was u s e d t o e l i m i n a t e a l l l i g h t 69I43 A ± k A.  except  Lens "A" was u s e d t o f l o o d t h e F a b r y - P e r o t  w i t h t h e l a s e r l i g h t d i s p e r s e d b y t h e ground g l a s s  screen.  A PENTAX R e f l e x camera w i t h a 200 mm. l e n s was used t o photograph t h e f r i n g e p a t t e r n .  The camera was u s e d a t t h e b u l b  s e t t i n g o f t h e s h u t t e r which was m a n u a l l y h e l d open d u r i n g laser  firing.  - 20 -  To o b t a i n t h e a c t u a l i n t e n s i t y d i s t r i b u t i o n o f the f r i n g e p a t t e r n , a c a l i b r a t i o n curve was made o f f i l m d e n s i t y vs. integrated l i g h t input. D = log where i  1 0  (  The f i l m d e n s i t y i s d e f i n e d by:  V.)  i s the i n t e n s i t y of l i g h t transmitted  t h r o u g h unexposed and d e v e l o p e d f i l m . and i i s t h e i n t e n s i t y o f l i g h t t r a n s m i t t e d t h r o u g h exposed and d e v e l o p e d f i l m . The i n t e g r a t e d l i g h t i n p u t , o r t h e e x p o s u r e , E, i s d e f i n e d b y : E = I t where I i s t h e i n t e n s i t y and  t i s the time. The e x p r e s s i o n f o r E shows t h a t i f we h a l v e t h e  i n t e n s i t y I , and double t h e exposure t i m e , t , we o b t a i n t h e same E. This i s not t r u e i n g e n e r a l .  The f a i l u r e o f t h e f i l m t o f o l l o w  t h i s expression f o r E i s c a l l e d r e c i p r o c i t y f a i l u r e .  To a v o i d  this  p r o b l e m a l l f i l m , i n c l u d i n g t h e c a l i b r a t i o n s , were exposed w i t h the l a s e r w i t h about 20 n s e c . d u r a t i o n .  The c a l i b r a t i o n c u r v e s  were c a l c u l a t e d f r o m e x p o s u r e s w i t h a n e u t r a l d e n s i t y s t e p (Hilger & Watts).  filter;  This c o n t r o l s the exposure i n convenient s t e p s . th  The exposure t h r o u g h t h e i  s t e p o f t h e wedge i s g i v e n by:  E. = I t T. x 1 where T. i s t h e t r a n s m i s s i o n o f t h e i  ^ step.  - 21 -  - 22 CHAPTER V RE.SULT5 Chapter I I I was d e v o t e d t o t h e methods u s e d i n t h e o p t i c a l pumping o f r u b y r o d s .  I n t h i s c h a p t e r we w i l l  discuss  the v a r i o u s l a s e r c a v i t i e s u s e d t o produce l a s e r a c t i o n . Experimental  r e s u l t s w i l l be g i v e n f o r each c o n f i g u r a t i o n u s e d .  B e f o r e d o i n g t h i s t h e l a s e r c a v i t y components o t h e r t h a n t h e r u b y w i l l be d i s c u s s e d . In working w i t h g i a n t pulse ruby l a s e r s i t i s d i f f i c u l t to  a v o i d damage t o o p t i c a l s u r f a c e s .  The damage r e s u l t s f r o m  t h e a b s o r p t i o n o f energy f r o m t h e l a s e r beam i t s e l f .  To a v o i d  the problem o f a b s o r p t i o n w i t h h i g h l y r e f l e c t i n g back m i r r o r s two t e c h n i q u e s  a r e commonly u s e d .  d i e l e c t r i c back m i r r o r s .  The s i m p l e s t i s t h e u s e o f  These can be made t o w i t h s t a n d powers  o f t h e o r d e r o f s e v e r a l hundred megawatts p e r square Such m i r r o r s a r e c o m m e r c i a l l y o p t i c a l companies.  The o t h e r -technique  r e f l e c t i n g 90° r o o f prisms.-  centimeter.  a v a i l a b l e from s e v e r a l i s t o use t o t a l  internal  Here a r i g h t t r i a n g u l a r p r i s m i s  u s e d t o r e f l e c t t h e l i g h t back p a r a l l e l t o i t s i n c i d e n t a x i s .  FIG.  18  - 23 -  The major p r o b l e m w i t h t h e s e p r i s m s i s t h e d i f f i c u l t y i n p o l i s h i n g a 90° c o r n e r .  Imperfections  i n the corner n o t only  produce d i f f r a c t i o n l o s s e s b u t a l s o f o r m c e n t e r s f o r a b s o r p t i o n and e v e n t u a l i r r e p e r a b l e damage.  The o t h e r p r o b l e m w i t h r o o f  p r i s m s i s t h e r e f l e c t i o n from the f r o n t s u r f a c e . problem d i e l e c t r i c a n t i r e f l e c t i o n  To a v o i d  this  c o a t i n g s c a n be a p p l i e d o r a  B r e w s t e r a n g l e can be c u t on t h e f r o n t s u r f a c e  (See F I G . 18).  F r o n t m i r r o r s f o r h i g h power l a s e r s a r e u s u a l l y d i e l e c t r i c coated t o produce t h e d e s i r e d r e f l e c t i o n f o r f e e d back.  I n some c a s e s where a l o w f e e d b a c k i s r e q u i r e d , a p a r a l l e l  faced sapphire  o r dense g l a s s f l a t can be u s e d as a f r o n t m i r r o r .  These r e f l e c t about 8$ t o 10% p e r s u r f a c e , depending on t h e refractive pitting.  index.  These f l a t s a r e much l e s s s u s c e p t i b l e t o  The "Q" c e l l which c o n t a i n s m e t h a n o l and c r y p t o c y a n i n e  a l s o has a n t i - r e f l e c t i o n c o a t i n g s on t h e o u t s i d e s u r f a c e . (See F I G .  19)  FIG.  19  - 2U -  The  a l t e r n a t i v e t o u s i n g a n t i - r e f l e c t i o n c o a t i n g s i s mounting  the o p t i c a l windows a t t h e B r e w s t e r a n g l e .  The  disadvantages  of t h e B r e w s t e r angle mount i s t h e l o s s o f t h e l i g h t a t t h e methanol-glass' The is c r i t i c a l .  interface.  T h i s , however, i s s m a l l .  a l i g n m e n t of t h e v a r i o u s s u r f a c e s o f t h e l a s e r The m i r r o r s u r f a c e s must be p a r a l l e l t o each  and p e r p e n d i c u l a r t o t h e e f f e c t i v e l a s e r a x i s .  other  The method most  commonly u s e d t o a l i g n t h e s u r f a c e s employs an He-Ne gas l a s e r as an a u t o - c o l i m a t o r .  The l a s e r beam i s d i r e c t e d t h r o u g h a  p i n h o l e and a l l s u r f a c e s a r e a l i g n e d such t h a t t h e r e f l e c t i o n s f r o m t h e m i r r o r s f a l l c o n c e n t r i c a l l y on t h e p i n h o l e .  The v a r i o u s  components can a l s o be c e n t e r e d on t h e beam o f t h e He-Ne l a s e r . T h i s i s e s p e c i a l l y u s e f u l i f t h e r u b y o r o t h e r components a r e mounted t o compensate f o r B r e w s t e r a n g l e s u r f a c e s .  The r e f r a c t i v e  i n d e x o f r u b y and g l a s s does n o t v a r y s i g n i f i c a n t l y f r o m 69U3 A (ruby l a s e r w a v e l e n g t h ) t o 6328 A (He-Ne l a s e r w a v e l e n g t h ) . t h e s e s u r f a c e s can be c e n t e r e d on t h e Ne-He beam.  Thus  I t i s not  d i f f i c u l t t o a d j u s t t h e r e f l e c t e d beam t o w i t h i n a few m i l l i m e t e r s over 3 o r U m e t e r s .  T h i s means we can a l i g n t h e s u r f a c e s  p a r a l l e l t o each o t h e r w i t h i n 10 ^ r a d i a n s . An a u t o - c o l i m a t i n g t e l e s c o p e i s a n o t h e r method u s e d to  align laser mirrors.  The image o f a s e t o f c r o s s h a i r s a t  the f o c a l p o i n t o f t h e t e l e s c o p e main l e n s i s c e n t e r e d on t h e cross h a i r s .  This i s repeated  f o r each s u r f a c e .  T h i s method  i s n o t u s e f u l f o r c e n t e r i n g t h e v a r i o u s components on a common a x i s .  - 25 -  NORMAL MODE A  100 nSEC/CM  B  0.1 mSEC/CM  FABREY-PEROT  1.0 A INTERORDER SEPARATION FIG. 20  Laser  Configurations The most b a s i c c o n f i g u r a t i o n f o r l a s e r o p e r a t i o n i s  r e f e r r e d t o as "normal mode."  I n t h i s c o n f i g u r a t i o n the c a v i t y  i n which l a s e r a c t i o n o c c u r s i s formed b y two m i r r o r s .  The Q  of t h e c a v i t y i s c o n s t a n t and l a s e r a c t i o n s t a r t s as soon as t h e population mirror  i n v e r s i o n i s h i g h enough t o s u s t a i n i t .  The back  i s u s u a l l y as c l o s e t o t o t a l l y r e f l e c t i n g as p o s s i b l e  and t h e f r o n t m i r r o r  i s p a r t i a l l y transmitting.  The p e r c e n t  - 26 -  t r a n s m i s s i o n of the f r o n t m i r r o r f o r optimim l a s e r a c t i o n .is a f u n c t i o n of r u b y l e n g t h and q u a l i t y .  I n t h i s work a  r e f l e c t i n g u n c o a t e d . s a p p h i r e f l a t was  used.  a 99-9%  input.  The back m i r r o r  r e f l e c t i n g d i e l e c t r i c coated g l a s s f l a t .  t h r e s h o l d f o r l a s e r a c t i o n v a r i e d between The  20% was  In t h i s c a v i t y  1800 and 2200 J o u l e s  v a r i a t i o n i n t h r e s h o l d energy i s a f u n c t i o n of f l a s h -  tube c o n d i t i o n , pumping c a v i t y c o n d i t i o n , and the c l e a n l i n e s s of m i r r o r s and r u b y s u r f a c e s .  The  s h o r t s p i k e s t y p i c a l l y 50 t o 100 randomly a t a r a t e of about one  a c t u a l l a s e r o u t p u t c o n s i s t e d of nsec. i n d u r a t i o n . e v e r y 3 uSEC.  The  These o c c u r d u r a t i o n of  t h i s p r o c e s s i s of the same o r d e r of t i m e as the f l a s h t u b e i n t h i s case, 5 m i l l i s e c . The  (See FIG.  The  of a l l the n o r m a l mode s p i k e s .  was  q u i t e broad, t y p i c a l l y  2.25  2 K.V. K.V.  .5  The  the  spectral distribution  A f u l l width h a l f i n t e n s i t y  (FWHl).  20) The  at  recorded  exposure of the r i n g p a t t e r n was  sum  (See F I G .  20)  s p e c t r a l d i s t r i b u t i o n of the l i g h t was  w i t h the F a b r y - P e r o t .  pulse;  (=  e n e r g y o u t p u t of the l a s e r was  t y p i c a l l y 3 Joules  3000 J o u l e s i n p u t ) t o a maximum of 10 J o u l e s a t  (U000 J o u l e s ) .  T h i s g i v e s an e f f i c i e n c y of  .2%.  "Q"-Spoiling As was  discussed e a r l i e r  (see Page 3 ) ,  "Q"-Spoiling  i s the p r o c e s s i n w h i c h the Q of the l a s e r c a v i t y v a r i e s The  "Q"  of the c a v i t y i s h e l d a t a low v a l u e u n t i l h i g h  temporally. population  - 27 -  i n v e r s i o n i s achieved.  I t i s t h e n r a p i d l y changed t o a h i g h  v a l u e and t h e h i g h g a i n system produces a s h o r t d u r a t i o n h i g h energy p u l s e . In t h i s work c r y p t o c y a n i n e was d i s s o l v e d i n m e t h a n o l . C r y p t o c y a n i n e i n methanol has an a b s o r p t i o n band c e n t e r e d a t about 70U0 A which i s 370 A w i d e .  (FWHl) (See F I G . 21)  WAVELENGTH (ANGSTROMS) A b s o r p t i o n s p e c t r u m of c r y p l o c y a n i i i e i n m e t h a n o l .  . FIG." 21 The p r o c e s s o f " Q - s w i t c h i n g " w i t h c r y p t o c y a n i n e i n v o l v e s the o p t i c a l pumping o f t h e dye's a b s o r p t i o n band.  The  i n i t i a l a b s o r p t i o n i s c o n t r o l l e d b y t h e c o n c e n t r a t i o n o f the dye  and t h e l e n g t h o f t h e dye c e l l .  When t h e p o p u l a t i o n i n v e r s i o n  i n t h e r u b y i s h i g h enough t o s t a r t l a s e r a c t i o n i n s p i t e o f t h e dye  c e l l l o s s e s , t h e dye c e l l i s q u i c k l y pumped t o t r a n s p a r e n c y .  - 28 -  This increases the Q of the c a v i t y . p o p u l a t i o n i n v e r s i o n and h i g h  -Q  The c o m b i n a t i o n  of high  t h e n produces a " g i a n t p u l s e . "  Throughout t h i s work an a n t i r e f l e c t i o n coated dye c e l l 10 mm. l o n g ( i n t e r n a l l e n g t h ) was u s e d .  I t was p o s i t i o n e d  between t h e r u b y and t h e 99.9% r e f l e c t i n g d i e l e c t r i c back m i r r o r . The  c o n c e n t r a t i o n s u s e d were t y p i c a l l y f r o m 0.2 x 10 ^ M o l a r t o  6.0 x 10  Molar.  "Q" s p o i l i n g c h a r a c t e r i s t i c s w i t h dye c e l l s Weak S o l u t i o n s Weak s o l u t i o n s a r e s o l u t i o n s w i t h a c o n c e n t r a t i o n o f between .2 x 10 ^ and 2 x 10 ^ m o l a r .  The most i n t e r e s t i n g  c h a r a c t e r i s t i c about l a s i n g a c t i o n i n t h i s r e g i o n i s "mode locking."  Mode l o c k i n g i s t h e m o d u l a t i o n o f t h e o u t p u t l i g h t  at a frequency to  which c o r r e s p o n d s t o t h e t i m e r e q u i r e d f o r l i g h t  t r a v e l twice the length of the c a v i t y .  I n t h i s work t h e  c a v i t y was o f t h e o r d e r o f Q.5> metres l o n g and t h e p e r i o d between mode l o c k e d maxima was o f t h e o r d e r o f LM x 3 n s e c . / ^ = 3 n s e c . The  t e m p o r a l w i d t h o f t h e mode l o c k e d p u l s e v a r i e d between about  l.Q and 3 n s e c .  The d u r a t i o n o f t h e m o d e ^ l o c k i n g sequence o f  p u l s e s was about 50 t o 100 n s e c .  F i g . 22 A shows a t y p i c a l  -6 p u l s e w i t h a dye c e l l c o n c e n t r a t i o n o f .67 X 10 base i s 50 nsec./cm.  M.  The t i m e  F I G . 22 E shows a s i m i l a r t r a c e w i t h a  t i m e base o f 10 nsec./cm.  The above r e s u l t s were o b t a i n e d  with  a s a p p h i r e f r o n t r e f l e c t o r , and a 99->-9% r e f l e c t i n g d i e l e c t r i c back mirror.  - 29 -  MODE LOCKING  A  $0 nSEC/CM  B  10 nSEC/CM  C  10 nSEC/CM  D  10 nSEC/CM  E  10 nSEC/CM  F  10 nSEC/CM FABREY - PEROT  DYE CELL CONCENTRATION 1 A  = .6 x 1 0 ~ M. 6  INTERORDER SEPARATION FINESSE = 30  FIG. 22  I f t h e pumping energy i s i n c r e a s e d i n t h i s c o n f i g u r a t i o n double p u l s i n g o c c u r s . pulse i s l i m i t e d .  Because o f t h i s , t h e energy i n any one  The s p e c t r a l d i s t r i b u t i o n o f t h e mode l o c k e d  p u l s e i s f a i r l y broad.  Fabrey-Perot  w i d t h s o f t h e o r d e r o f .3 t o .It A.  measurements g i v e l i n e  - 30 -  I t i s i n t e r e s t i n g t o n o t i c e the s e c o n d a r y m o d u l a t i o n of some of the mode l o c k e d p u l s e s .  In FIG.  the m o d u l a t i o n s u g g e s t s a b e a t i n g of two  shorter existed.  22D We  frequencies  approximately  T h i s c o u l d be e x p l a i n e d i f a s e c o n d a r y c a v i t y k%  8% a p a r t .  i n FIG.  22 E, f o r example,  22B,  No such c a v i t y e x i s t s .  C, D, F.  S i m i l a r t r a c e s are shown  It. i s c l e a r from t r a c e s l i k e FIG.  22B  t h a t the b e a t i n g e f f e c t i s ' i n c o n s i s t e n t from- shot t o can see f r o m t r a c e s i n FIG.  i s not a m p l i t u d e  modulation.  22C  and  22D  and  shot.  t h a t the b e a t i n g  effect  Here the p u l s e s are much more s p i k e  shaped and the o u t - o f - p h a s e components can be r e s o l v e d .  The  ' o r i g i n of t h e s e d i f f e r e n t f r e q u e n c i e s i s not known. I n a mode l o c k e d s p i k e the e n e r g y output, i s t y p i c a l l y .05  Joules.  The  o r d e r of 2 n s e c . .05  J/2  t e m p o r a l h a l f w i d t h of each s p i k e i s of  the  T h i ^ g i v e s an average power per s p i k e of  n s e c . = 25 M  watts.  I n somo cases mode l o c k i n g does n o t o c c u r .  Then a  50 n s e c . d u r a t i o n p u l s e w i t h about 1 J o u l e o f energy i s t y p i c a l . The  s p e c t r a l d i s t r i b u t i o n i s e f f e c t i v e l y the same as f o r mode  locking.  I f a pulse of t h i s nature  i s required consistently  i t can be o b t a i n e d by p l a c i n g a second f r o n t m i r r o r i n t h e l a s e r cavity.  I n t h i s work a s a p p h i r e f l a t was  a d i s t a n c e of about 10 I t was  added t o the system a t  i n c h e s f r o m the f r o n t m i r r o r ( a l s o s a p p h i r e ) .  p o s i t i o n e d by t r i a l and e r r o r u n t i l mode l o c k i n g  suppressed.  Then an o u t p u t p u l s e of .5  was,  J o u l e s i n \\0 n s e c .  was  - 31 -  typical.  T h i s i s f o r 2.2 KV i n p u t .  was 1 X 10~  concentration  Molar.  6  Intermediate  The dye c e l l  Solutions Intermediate  s o l u t i o n s a r e those w i t h a m o l a r i t y  between 2.0 X 10 ^ and 2.5 X 10 ^ m o l a r . l o c k i n g occurs  infrequently.  I n t h i s r e g i o n mode  About one i n twenty s h o t s  will  have r e g u l a r m o d u l a t i o n w i t h t h e c h a r a c t e r i s t i c p e r i o d i c i t y . Even i n t h e s e  cases t h e p e r c e n t m o d u l a t i o n i s l o w compared w i t h  weak s o l u t i o n work. non r e p r o d u c i b l e .  I n g e n e r a l t h e p u l s e s a r e n o n - u n i f o r m and T h i s i s a t t r i b u t e d t o t h e random s t a r t i n g  t i m e o f t h e v a r i o u s modes t h a t f o r m t h e l a s e r o u t p u t .  The p u l s e  l e n g t h f o r weak s o l u t i o n s was o f t h e o r d e r o f 50 t o 100 n s e c . The  p u l s e l e n g t h f o r i n t e r m e d i a t e s o l u t i o n s was between 25 and  50 n s e c .  The t o t a l e n e r g y o u t p u t f o r weak and s t r o n g s o l u t i o n s  was about e q u a l f o r t h e same pumping energy.  We c a n see t h e n  t h a t t h e power o u t p u t , n o t t h e e n e r g y o u t p u t ,  i s a f u n c t i o n of  the dye c e l l  concentration. The  The  s p e c t r a l d i s t r i b u t i o n i s s t i l l q u i t e broad.  l i n e t y p i c a l l y has a h a l f w i d t h o f .25 A.  F I G . 23 shows the.  s p e c t r a l and t e m p o r a l d i s t r i b u t i o n o f an i n t e r m e d i a t e  concentration  laser pulse. As w i t h weak s o l u t i o n s , i f t h e pumping energy i s i n c r e a s e d the l a s e r gives double or t r i p l e pulse output.  I t was  - 32 -  INTERMEDIATE SOLUTION 2.5 X 1 0 ~ M. 6  FIG.  10 nSEC/CM.  23  noticed.^, i n t h e F a b r e y - P e r o t p i c t u r e s of double p u l s e s d o u b l e t s t r u c t u r e appeared.  that a  This i n d i c a t e s that i f the l a s e r  p u l s e s t w i c e i t does so a t d i f f e r e n t w a v e l e n g t h s . l e n g t h d i f f e r e n c e i s o f t h e o r d e r of .2 A ( F I G . 2k).  The waveThe  wavelength s h i f t i s p o s s i b l y due t o t h e h e a t i n g o f t h e r u b y r e d by t h e f l a s h t u b e s .  T h i s would change t h e e f f e c t i v e c a v i t y  l e n g t h and c o n s e q u e n t l y t h e modes s e l e c t e d by t h e c a v i t y .  I f we  assume t h a t t h e r a t e o f change o f t h e temperature o f t h e r o d i s constant  over t h e t i m e between p u l s e s , we c a n c a l c u l a t e t h e  - 33 -  DOUBLE PULSES 0.2 mSEC. APART  20 nSEC./CM  INTERORDER SEPARATION  1.0 A  F I G . 2U  expected  s p e c t r a l broadening  o f each l i n e due to' h e a t i n g .  The  time between p u l s e s i n double p u l s i n g i s o f t h e o r d e r o f 300 usee. The d u r a t i o n o f each p u l s e i s t y p i c a l l y wavelength s h i f t d u r i n g a p u l s e i s onn  5>0 nsec.  U 5 e C  *  T h e r e f o r e , the  X .2 A = 3 X 10 ^ A.  T h i s s h i f t i s c l e a r l y n e g l i g i b l e compared t o t h e .2 A h a l f of t h e l i n e .  width  - 3U -  STRONG SOLUTION 3 X 10 1.0  v  M  A INTERORDER SEPARATION 20 nSEC/CM FIG.  Strong  2$  Solutions S t r o n g s o l u t i o n s a r e those w i t h a m o l a r i t y between  2.5 X 10~ M 6  and 6 X 10~  6  M.  Above c o n c e n t r a t i o n o f 6 X 10~  6  M,  l a s e r a c t i o n d i d not o c c u r . The s t r o n g s o l u t i o n r e g i o n i s the most commonly u s e d i n dye Q s p o i l e d ruby l a s e r s .  S t r o n g s o l u t i o n s produce the  s h o r t e s t p u l s e s and t h e h i g h e s t power o u t p u t . as s h o r t as 10 nsec. were r e c o r d e d . a c e l l c o n c e n t r a t i o n o f 3-0 X 10 ^ M.  Pulse  lengths  F I G . 25 was o b t a i n e d  with  The energy c o n t a i n e d i n  - 35 -  t h i s p u l s e was 1.5  Joules.  T h i s g i v e s a power o u t p u t o f 150  MW.  I n 13 s h o t s t a k e n a t t h i s c o n c e n t r a t i o n t h e r e was no double p u l s i n g recorded.  Also i n the Fabrey-Perot  p u l s e s , no d o u b l e t s t r u c t u r e was e v i d e n t . t y p i c a l Fabrey-Perot  p i c t u r e s o f these  13  F I G . 25 shows a  p i c t u r e o f s t r o n g s o l u t i o n l a s e r spectrum.  The h a l f w i d t h o f t h e l i n e i s o f t h e o r d e r o f .2.A.  I t was  n o t i c e d i n t h e work w i t h s t r o n g s o l u t i o n s t h a t t h e t e m p o r a l development o f two d i f f e r e n t p u l s e s was q u i t e s i m i l a r . FIG.  (See  25A, B, C ) . T h i s r e p r o d u c a b i l i t y i n d i c a t e s t h a t f e w e r modes  are producing" l a s e r a c t i o n and t h a t p o s s i b l y t h e modes a r e l a s i n g i n t h e same o r d e r each t i m e . I n a l l t h e dye c e l l work t o " t h i s p o i n t a 99-9% r e f l e c t i n g d i e l e c t r i c back m i r r o r and a s a p p h i r e f r o n t m i r r o r were u s e d . r u b y and back m i r r o r . f r o n t m i r r o r was added.  (uncoated)  The dye c e l l was p l a c e d between t h e I n mode l o c k s u p p r e s s i o n  an e x t r a  sapphire  - 36 -  Cavity Configurations We d i s c u s s e d e a r l i e r i n t h i s c h a p t e r the dye c e l l c o n c e n t r a t i o n on l a s e r a c t i o n .  the e f f e c t of  I n t h i s p a r t we w i s h  t o d i s c u s s some o f t h e d i f f e r e n t c a v i t y c o n f i g u r a t i o n s and t h e i r e f f e c t on l a s e r a c t i o n . spectral line  We a r e s p e c i f i c a l l y i n t e r e s t e d i n  width. The  s p e c t r a l l i n e w i d t h i s d e t e r m i n e d by t h e modes  t h a t propagate w i t h i n the c a v i t y .  We u s u a l l y speak o f two c l a s s i -  f i c a t i o n s o f modes, t r a n s v e r s e and l o n g d i t u d i n a l .  Longditudinal  modes occur because s t a n d i n g wave p a t t e r n s w i t h a d i f f e r e n t number of w a v e l e n g t h s can occur a l o n g t h e same a x i s i n t h e same l e n g t h o f cavity.  Transverse  modes occur because s t a n d i n g waves c a n produce  d i f f e r e n t phase p a t t e r n s a c r o s s the l a s e r c a v i t y .  These a r e e x a c t l y  a n a l a g o u s t o microwave t r a n s v e r s e modes. The room t e m p e r a t u r e .  r u b y e m i s s i o n band i s s e v e r a l Angstroms wide a t F o r a one meter l o n g c a v i t y t h i s would r e s u l t  i n s e v e r a l hundred l o n g d i t u d i n a l modes, a l l o f h i g h Q.  To s e l e c t  l o n g d i t u d i n a l modes we must reduce t h e Q o f t h e c a v i t y f o r those modes we w i s h t o e l i m i n a t e .  T h i s i s done by a d d i n g r e f l e c t i n g  surfaces t o the f r o n t m i r r o r . combination  The t r a n s m i s s i o n t h r o u g h t h i s  i s t h e same as f o r a F a b r y - P e r o t .  As a r e s u l t , t h e  o n l y modes t h a t p r o p a g a t e a r e t h o s e modes whose w a v e l e n g t h s a r e common t o a l l t h e r e s o n a n t  c a v i t i e s i n t h e system.  - 3.7 -  These modes must s a t i s f y where  =  2 d^N^Cos 8  ru = number of w a v e l e n g t h s i n c a v i t y of l e n g t h A. = w a v e l e n g t h of  d^  light  d^ = l e n g t h of c a v i t y = r e f r a c t i v e index i n c a v i t y 9  a n g l e of d e v i a t i o n f r o m the normal of the m i r r o r .  The modes must a l s o l i e i n the e m i s s i o n band of the r u b y . An u n c o a t e d s a p p h i r e f l a t was  u s e d as a f r o n t  mirror.  T h i s gave two s u r f a c e s t o s e l e c t l o n g d i t u d i n a l modes.  I n a few  cases a t h i r d s u r f a c e , a d i e l e c t r i c 30$ m i r r o r  was  added t o the f r o n t m i r r o r . The  s i n g l e s a p p h i r e f l a t was  not s u c c e s s f u l i n  r e d u c i n g the s p e c t r a l l i n e w i d t h , even a t low powers. expected that f i r i n g  the l a s e r c l o s e t o t h r e s h o l d  It  was  should  reduce t h e l i n e w i d t h by r e d u c i n g the number of modes w i t h a h i g h enough p o p u l a t i o n i n v e r s i o n t o l a s e . technique  was  The f a i l u r e o f  a t t r i b u t e d to inhomogeneities  this  i n the r u b y r o d .  To overcome t h i s , s m a l l d i a m e t e r a p e r t u r e s were added t o the c a v i t y n e a r the s u r f a c e s of the r u b y .  Using  only a small  p a r t of the r u b y f o r l a s i n g and w o r k i n g near t h r e s h o l d we a r e l a t i v e l y narrow l i n e .  L i n e w i d t h s between 0.15  were r e c o r d e d w i t h a p e r t u r e s between 0.5  cm.  I t i s a l s o p o s s i b l e t h a t these a p e r t u r e s had the number of t r a n s v e r s e modes.  and 0.2  obtained  and 0.03 cm.  in  A diameter.  some e f f e c t i n r e d u c i n g  The main drawback t o t h i s method  - 38 -  k.5  X 10~  b  3 n.m.  M  APERTURE  LOW PUMPING POWER 10 nSEC/CM FIG.  i s t h e low power output. were t y p i c a l . FIG.  Energies  26  o f .05 J o u l e s i n 20 n s e c .  This g i v e s a power o f o n l y 2.5 megawatts. 26 shows a t y p i c a l F a b r e y - P e r o t  power a p e r t u r e work.  p i c t u r e f o r low  I n an attempt t o improve t h i s r e s u l t a 30%  d i e l e c t r i c m i r r o r was added t o t h e s a p p h i r e f r o n t r e f l e c t o r system. I t was p o s i t i o n e d about 5 mm. f r o m t h e s a p p h i r e f l a t . shows t h e F a b r e y - P e r o t dye  c e l l solution).  F I G . 27  trace f o r t h i s c o n f i g u r a t i o n (with strong Three d i s t i n c t wavelengths a r e e v i d e n t .  The l a s e r d i d n ' t double p u l s e d u r i n g these s h o t s so we a t t r i b u t e these s p e c t r a l l i n e s t o t h e same l a s e r p u l s e .  Densitometer t r a c e s  - 39 -  TRIPLE FRONT REFLECTOR 20 nSEC/OM  NO APERTURE  3.2 X 1 0 ~ M. 6  F I G . 2?  A  B 3 m.m.  APERTURE  3.2 X 1 0 ~ M.  20 nSEC/CM F I G . 28  6  - 40 -  EXTENDED CAVITY ( 1 . 0 Meter) TRIPLE FRONT REFLECTOR 3.2 X 1 0 " M 6  FIG.  10 nSEC/CM 29  gave the h a l f width of the most intense l i n e to be .11 A - .01 A. The u n c e r t a i n t y i n measurement i s due t o the g r a i n of the f i l m producing i n c o n s i s t e n t h a l f width measurements. were both measured t o be .10 A t .01 A. aperture (D  a  U mm.)  The other l i n e s  At t h i s point a small  wag added between the ruby and f r o n t m i r r o r .  Working near threshold with t h i s aperture the f o l l o w i n g two Fabrey-Perot p i c t u r e s were t y p i c a l . (FIG 28 A and B) These two p i c t u r e s were taken with the aperture i n d i f f e r e n t transverse p o s i t i o n s .  Densitometer t r a c e s of F I G . 28A  - m. -  gave a h a l f w i d t h o f .11  -  w i d t h of .06 A - .01  The power o u t p u t was t y p i c a l l y  A.  .01  A  and o f F I G . 28B gave a h a l f .1  J o u l e s i n 20 n s e c . or 5 megawatts. I t i s p o s s i b l e the a p e r t u r e t o some degree  controlled  the t r a n s v e r s e modes. I n an attempt t o reduce t h e Q of some o f the t r a n s v e r s e modes, t h e main c a v i t y was extended t o about 1 metre l o n g f r o m .6 m e t r e s .  The same t e c h n i q u e s as above produced a s p e c t r a l h a l f  w i d t h of .0I4 A ± .01 (See F I G .  29)  A.  The power was of the o r d e r of $ megawatts.  - U2 -  CONCLUSIONS AND FUTURE IMPROVEMENTS T h i s work has p r e s e n t e d t e c h n i q u e s u s e f u l i n p r o d u c i n g h i g h power l a s e r p u l s e s lines.  as w e l l as n a r r o w s p e c t r a l  However, t h e b a s i c i n c o m p a t i b i l i t y o f t h e s e two  o b j e c t i v e s has a l s o been d e m o n s t r a t e d . powers can be o b t a i n e d tolerable. of  One hundred megawatt  i f l i n e w i d t h s o f t h e o r d e r o f .3 A a r e  With s l i g h t m o d i f i c a t i o n s  t o t h e system, l i n e widths  .05 A a r e p o s s i b l e i f powers o f 5 megawatts a r e u s a b l e .  With  t h i s l a t t e r s e t u p i t would be p o s s i b l e t o d e t e r m i n e t h e i o n t e m p e r a t u r e and d e n s i t y i n a plasma j e t w i t h t h e t e c h n i q u e o f laser scattering. Further  improvement on t h e s p e c t r a l l i n e w i d t h w i t h  t h e l a s e r r o d p r e s e n t l y employed would be v e r y d i f f i c u l t t o t h e inhomogeneities o f the ruby.  owing  I f a b e t t e r r o d were a v a i l a b l e ,  more s o p h i s t i c a t e d t e c h n i q u e s o f mode c o n t r o l c o u l d be u s e d . p a r t i c u l a r , spherical mirrors  In  i n a c o n f o c a l c o n f i g u r a t i o n have  been u s e d t o produce s i n g l e mode l a s e r a c t i o n .  The b i g advantage  to confocal mirror configurations  i s t h e degeneracy i n t h e wave-  lengths  T h i s would make narrow s p e c t r a l  of the transverse  modes.  l i n e s p o s s i b l e a t r e l a t i v e l y h i g h powers. u s e f u l a ruby r o d of higher necessary.  F o r t h i s system t o be  o p t i c a l q u a l i t y t h a n was used h e r e i s  - U3 -  BIBLIOGRAPHY  1.  Chan P. ¥. , and N o d w e l l R.A., Phys. Rev. L e t t e r s ,  16 (1966)  122  2.  C o n g l e t o n R.S., Sooy W.R., D e w h i r s t D.R., and R i l e y L.D., i n Quantum E l e c t r o n i c s . (P. G r i v e t and N. Bloembergen, e d s . ) Columbia U n i v e r s i t y P r e s s , New Y o r k , 1961)-, Volume I I I , p. I i i l 5  3-  K a f a l o s P., M a s t e r s J.T., and Murray E.M.E. A p p l i e d P h y s i c s , Volume '35, No. 8, August,- 196U. P« 23U9.  li.  L e n g y e l B.A., I n t r o d u c t i o n t o L a s e r P h y s i c s John W i l e y and Sons, I n c .  $.  L e v i n e A.K., L a s e r s , Volume 1, 1966 M a r c e l D e k k e r , I n c . , New Y o r k .  6.  Maiman, T.H., H o s k i n s R.H., D'Haenens J.D., Asawa C.K., and E v t u h o v V., Phys. Rev. 123,  1151  (1961).  7.  R i n g l e r H. and N o d w e l l R.A. P r o c e e d i n g s o f t h e Conference on Plasma P h y s i c s and C o n t r o l l e d F u s i o n . U t r e c t N e t h e r l a n d s , June 1969-  8.  Rohr H.,  P h y s i c s L e t t e r s 2^A, No. 2 (1967) p.167.  \  

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