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DEVELOPMENT OF A LASER OSCILLATOR-AMPLIFIER COMBINATION AND A MULTI -CHANNEL SPECTRAL DETECTION SYSTEM FOR LIGHT SCATTERING EXPERIMENTS by GARY GEORGE ALBACH B . S c , U n i v e r s i t y o f W a t e r l o o , 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e D e p a r t m e n t o f 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 r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA J u l y , 1972 In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University o,f B r i t i s h Columbia Vancouver 8, Canada ABSTRACT I n p r e p a r a t i o n f o r l a s e r l i g h t s c a t t e r i n g e x p e r i -m e n t s on p l a s m a s i n m a g n e t i c f i e l d s a p u l s e d r u b y l a s e r has b e e n d e v e l o p e d i n c o n j u n c t i o n w i t h a m u l t i c h a n n e l s p e c t r a l a n a l y s e r f o r d e t e c t i o n o f t h e s c a t t e r e d l i g h t . T h e l a s e r , c o n s i s t i n g o f s e p a r a t e o s c i l l a t o r and o a m p l i f i e r r o d s has a s p e c t r a l l i n e w i d t h o f .08 A a t p o w e r s up t o 100 M e g a w a t t s . The u s e o f a P o c k e l s C e l l as t h e Q - s w i t c h p e r m i t s a c c u r a t e s y n c h r o n i z a t i o n w i t h t h e s p e c t r a l a n a l y s e r and a l l e x t e r n a l e l e c t r o n i c s . F o r t h e m u l t i c h a n n e l d e t e c t i o n s y s t e m f i v e f i b e r o p t i c s s l i t b u n d l e s t r a n s m i t l i g h t f r o m t h e o u t p u t o f a m o n o c h r o m a t e r t o f i v e p h o t o m u l t i p i i e r t u b e s , w h i c h a r e g a t e d on f o r 100 n s e c d u r i n g t h e l a s e r p u l s e . The p u l s e s a r e d i s p l a y e d s e q u e n t i a l l y t o g i v e an i n t e n s i t y v s . w a v e l e n g t h p r o f i l e on an o s c i l l o s c o p e s c r e e n . TABLE OF CONTENTS Page ABSTRACT. i i LIST OF FIGURES v ACKNOWLEDGEMENTS v i i C h a p t e r 1 INTRODUCTION . 1 PART I 2 THE LASER SYSTEM • . 3 2.1 L a s e r A m p l i f i e r T h e o r y 3 2.2 O s c i l l a t o r D e s i g n 8 2.2.1 S p e c t r a l Width and Power O u t p u t . . . 8 2.2.2 R e p r o d u c i b i l i t y and S y n c r o n i z a t i o n . 10 2.2.3 Mode S e l e c t i o n 12 2.3 O s c i l l a t o r P e r f o r m a n c e . 17 2.3.1 S p e c t r a l Width . 17 2.3.2 P u l s e C h a r a c t e r i s t i c s 25 2.3.3 Summary . 26 i i i Chapter Page 2.4 A m p l i f i e r Performance 27 2.4.1 Energy Gain . 27 2.4.2 P u l s e D i s t o r t i o n 32 2.5 D i s c u s s i o n . 34 PART I I 3 THE DETECTION SYSTEM 37 3.1 Ge n e r a l O u t l i n e of P o l y c h r o m e t e r . . . . . . 37 3.2 F i b e r O p t i c s . 39 3.3 P h o t o m u l t i p l i e r s . 41 3.3.1 Design C o n s i d e r a t i o n s 41 3.3.2 Performance 46 3.4 D i s c u s s i o n 50 4 DISCUSSION 52 REFERENCES • • 54 APPENDICES I COMPARISON OF PHOTOMULTIPLIER PULSING TECHNIQUES . 58 I I PHOTOMULTIPLIER ELECTRONICS 64 i v LIST OF FIGURES F i g u r e Page 1. P o c k e l s C e l l C o n f i g u r a t i o n f o r \ Wave O p e r a t i o n . . 11 2. O p t i c s f o r A n a l y s i s o f S p e c t r a l C h a r a c -t e r i s t i c s o f L a s e r O s c i l l a t o r O u t p u t 17 3. O s c i l l a t o r O u t p u t : C o n f o c a l R e s o n a t o r 21 4. O s c i l l a t o r O u t p u t : P l a n e M i r r o r s ( 7 5 % f r o n t ) 21 5. O s c i l l a t o r O u t p u t : P l a n e M i r r o r s a)s,t>) ( 3 0 % f r o n t ) S h o w i n g Two C o n s e c u t i v e S h o t s One M i n u t e A p a r t 23 6. O s c i l l a t o r O u t p u t : P l a n e M i r r o r s and Dye C e l l 23 7. F r a c t i o n a l P o p u l a t i o n I n v e r s i o n v s . Pumping E n e r g y , 31 8. A m p l i f i e r G a i n v s . Pumping E n e r g y 33 9. L a s e r A m p l i f i e r P u l s e s , a) I n p u t , b) O u t p u t , N o r m a l i z e d t o Same E n e r g y 34 10. L a s e r O p e r a t i o n . 35 11. F i b e r O p t i c s S l i t P a c k a g e 40 v Fi gure Page 12. Photomultipiier End-Window Showing Method of Total Internal Reflection 41 13. Circui t of LED Driver . 47 14. Noise in PM Gating Electronics . 49 15. Photomultipiier Outputs: a) DC Light Input, b) 30 nsec Light Pulse. 49 16. Detection System Operation . . . 51 A- l Transmission Gate Circui t . 65 A-2 Avalanche Transistor Gate Pulse Generator. . . . 68 A-3 Power Supply for Gate Pulse Generator 70 vi ACKNOWLEDGEMENTS I w i s h t o t h a n k D r . J . M e y e r f o r s u g g e s t i n g and s u p e r v i s i n g t h e c o u r s e o f t h i s w o r k . Many t h a n k s a l s o go t o a l l t h e members o f t h e P l a s m a P h y s i c s G r o u p f o r many h o u r s o f u s e f u l d i s c u s s i o n . I w o u l d l i k e t o e x p r e s s my a p p r e c i a t i o n f o r t h e t e c h n i c a l a s s i s t a n c e o f Mr. D. S i e b e r g and Mr. J . Z a n g a n e h d u r i n g d e v e l o p m e n t o f t h e d e t e c t i o n e l e c t r o n i c s . I w o u l d a l s o l i k e t o a c k n o w l e d g e S h a r i H a l l e r f o r a f i n e j o b i n t h e t y p i n g o f t h i s t h e s i s . F i n a n c i a l a s s i s t a n c e f r o m t h e N a t i o n a l R e s e a r c h C o u n c i l i s g r a t e f u l l y a c k n o w l e d g e d . T h i s w o r k i s s u p p o r t e d by a g r a n t f r o m t h e A t o m i c E n e r g y C o n t r o l B o a r d o f C a n a d a . v i i Chapter 1 INTRODUCTION In any laser l i g h t scattering experiment the two essential pieces of diagnostic equipment are the laser i t s e l f and the detection system for the scattered l i g h t . The work presented here i s the development and operation of a complete system, consisting of a pulsed ruby laser in conjunction with a multichannel spectral analyser, for plasma diagnostics. As o r i g i n a l l y proposed, the apparatus was to be used to study pulsed plasmas in magnetic f i e l d s . This goal dictated that the laser have the following c h a r a c t e r i s t i c s : 1) h i g h s p e c t r a l b r i g h t n e s s 2) r e p r o d u c e i b l e p u l s e s 3) a c c u r a t e s y n c h r o n i z a t i o n t o e x t e r n a l e l e c t r o n i c s Chapter 2 describes the design considerations and performance of the laser system. The f i n a l design consists of a three-inch ruby rod in a low power o s c i l l a t o r with a Pockels Cell 1 2 Q - s w i t c h . T h i s i s f o l l o w e d b y a s i x i n c h r o d a s a p o w e r a m p l i f i e r . T h e v a r i o u s s e c t i o n s o f C h a p t e r 2 d e a l w i t h : ( 1 ) t h e r e l e v a n t t h e o r y n e c e s s a r y t o e v a l u a t e t h e l a s e r a m p l i f i e r p e r f o r m a n c e ; ( 2 ) t h e r e a s o n s t h e l a s e r was b u i l t a s i t was i n c l u d i n g t h e r e l e v a n t mode s e l e c t i o n t e c h n i q u e s t h a t w e r e c o n s i d e r e d ; ( 3 ) an e v a l u a t i o n o f t h e l a s e r o s c i l -l a t o r p e r f o r m a n c e ; ( 4 ) a n e v a l u a t i o n o f t h e l a s e r a m p l i f i e r p e r f o r m a n c e ; ( 5 ) a d i s c u s s i o n o f t h e c o m p l e t e l a s e r . T h e d e t e c t i o n s y s t e m i s e x p l a i n e d i n C h a p t e r 3 a n d c o n s i s t s o f f i v e p h o t o m u l t i p i i e r s e a c h r e c o r d i n g a s e p a r a t e s e g m e n t o f t h e d e s i r e d s c a t t e r e d l i g h t s p e c t r u m . F i b e r o p t i c s b u n d l e s t r a n s m i t l i g h t f r o m t h e o u t p u t o f a m o n o c h r o m a t e r t o t h e p h o t o m u l t i p i i e r s w h i c h a r e g a t e d o n f o r 1 0 0 n s e c d u r i n g t h e l a s e r p u l s e . T h e p u l s e s a r e d i s p l a y e d s e q u e n t i a l l y t o g i v e an i n t e n s i t y v s . w a v e l e n g t h p r o f i l e o n a n o s c i l l o s c o p e s c r e e n . T h e s e c t i o n s i n C h a p t e r 3 d e a l w i t h t h e c o n s t r u c t i o n o f t h e f i b e r o p t i c s b u n d l e s , t h e m e t h o d u s e d t o p u l s e t h e p h o t o m u l t i p i i e r s a n d an e v a l u a t i o n o f t h e d e t e c t i o n s y s t e m as a w h o l e . A p p e n d i x I r e v i e w s t h e t e c h n i q u e s t h a t h a v e b e e n u s e d b y o t h e r s t o p u l s e p h o t o m u l t i p i i e r s . A l l o f t h e s e a r e s h o w n t o h a v e s e r i o u s d r a w b a c k s i f a p p l i e d t o t h i s s y s t e m . A p p e n d i x I I d e t a i l s t h e o p e r a t i o n o f t h e new p u l s i n g c i r c u i t s t h a t w e r e d e v e l o p e d t o o v e r c o m e t h e s e d i f f i c u l t i e s . P A R T I C h a p t e r 2 THE LASER SYSTEM 2.1 L a s e r A m p l i f i e r T h e o r y S o much h a s b e e n w r i t t e n i n r e c e n t y e a r s a b o u t t h e t h e o r y o f l a s e r o p e r a t i o n t h a t a n y a t t e m p t h e r e t o d i s c u s s i t i n d e t a i l w o u l d b e f u t i l e . I n s t e a d a s i m p l i f i e d t r e a t m e n t , d u e l a r g e l y t o S t e e l e a n d D a v i s [ 1 ] , w i l l b e p r e s e n t e d t o p r o v i d e t h e n e c e s s a r y b a c k g r o u n d f o r t h e a n a l y s i s i n t h e s e c t i o n o n A m p l i f i e r P e r f o r m a n c e . I n p a r t i c u l a r t h e t h e o r y w i l l y i e l d an e x p r e s s i o n f o r t h e s i n g l e p a s s g a i n o f t h e l a s e r a m p l i f i e r a n d t h e f u n c t i o n a l f o r m o f t h e p o p u l a t i o n i n v e r s i o n w i t h pump e n e r g y . T h e p r o b l e m i s f o r m u l a t e d a s a t w o l e v e l a p p r o x i -m a t i o n t o t h e b a s i c t h r e e l e v e l s y s t e m f o r r u b y . I n t h i s a p p r o x i m a t i o n i t i s a s s u m e d t h a t t h e n o n - r a d i a t i v e t r a n s i -t i o n b e t w e e n t h e pump b a n d a n d t h e u p p e r l a s e r l e v e l i s much f a s t e r t h a n a l l o t h e r t r a n s i t i o n r a t e s f o r t h e s y s t e m . D e f i n i n g : N x , t h e e l e c t r o n p o p u l a t i o n d e n s i t y i n t h e g r o u n d s t a t e ; N 2 , t h e e l e c t r o n p o p u l a t i o n d e n s i t y i n t h e u p p e r 3 4 exci ted s t a t e ; N 3 , the e lectron population in the pump l e v e l s ; N 0 = Ni + N 2 + N 3 , the to ta l e lect ron population dens i ty ; the two level approximation assumes N 3 = 0 so that No = Ni + N 2 . The standard energy balance equations for the rates of change of e lect ron density in the two laser leve ls are given as: = N 2acp - Nicrcp = ac p(N 2 - N j d N z - N l 0-cp - N 2acp = -ac p(N 2 - Ni) dt where: p = photon density causing st imulated emission [photons - cm" 3] o = rad ia t ion absorption c r o s s - s e c t i o n for C + 3 [cm* 2 ] r c = v e l o c i t y of l i g h t [ c m - s e c - 1 ] Defining n = N 2 - Nx as the e lect ron population invers ion density the two rate equations g ive : 5 In order to derive an expression for the gain i t is necessary to determine the photon density as a function of position and time. Using: = rate of change of photon density in volume dv canp = rate at which photons are generated by transitions in dv rate at which photons leave dv gives an equation of continuity: St = c a n P For a square photon pulse of length x 0 and density p 0 incident on the laser material at x = 0 and t = 0 6 B e l l m a n , B i r n b a u m and Wagner [2] have shown t h a t t h e s o l u -t i o n o f t h e a b o v e e q u a t i o n s f o r t h e p h o t o n d e n s i t y i s : p ( x , t ) = po * 1 - £ - e - a n ° ^ | e x p £ - 2 a P o c ( t - x / c f j • -1 w h e r e n 0 i s t h e i n v e r t e d e l e c t r o n p o p u l a t i o n d e n s i t y a t t i m e t = 0 . The t o t a l e n e r g y g a i n G i n a l a s e r m a t e r i a l o f l e n g t h L w i l l be g i v e n by t h e r a t i o o f e m e r g i n g p h o t o n d e n s i t y t o i n c i d e n t p h o t o n d e n s i t y : • CO G = 1 PO To p ( L , t ) d t S u b s t i t u t i n g f o r p ( L , t ) and i n t e g r a t i n g y i e l d s G = 1 2ap 0 C T 0 1 + £ e x p ( 2 a p 0 C T 0 ) - fj e x p ( n 0 a L ) (2.1) 7 To determine the dependence of population inver-sion density n 0 on the flashlamp energy E , neglect the thermal population of the upper laser level and assume the electron density population of the ground state to have the form: Ni = No e " B E 3 = KT to give n 0 = N 2 - N 0 e~^E Since the upper laser level in ruby is real ly a double level only one-half of the electrons pumped into i t are available for laser action. Accordi ngly: N 2 = Jj(N 0 - Ni ) = %N 0 - ^ (N 0 e" B E) And f i n a l l y : n 0 fcNoO - 3e"3 E ) (2.2) 8 In t h e s e c t i o n on A m p l i f i e r P e r f o r m a n c e e q u a t i o n s (2.1) and (2.2) w i l l be u s e d t o e v a l u a t e t h e p a r t i c u l a r c a s e o f a s i x i n c h r u b y r o d . 2.2 O s c i l l a t o r D e s i g n As o u t l i n e d i n t h e I n t r o d u c t i o n t h e l a s e r r e q u i r e d f o r t h e p r o p o s e d s c a t t e r i n g e x p e r i m e n t s had t o hav e t h e f o l l o w i n g s p e c i f i c a t i o n s : o 1) s p e c t r a l width < 0.5 A 2) power output > 75 MW 3) reproduceable pulses ii) accurate syncronization to other equipment U s i n g t h e s e c r i t e r i a i t was d e c i d e d t h a t t h e l a s e r s h o u l d c o n s i s t o f a t h r e e i n c h r u b y r o d i n a low power (10 - 20 MW) o s c i l l a t o r f o l l o w e d by a s i x i n c h r u b y r o d as a power a m p l i f i e r . The r e a s o n i n g w h i c h l e d t o t h i s c h o i c e i s d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n and c a n b e s t be u n d e r -s t o o d by f i r s t c o n s i d e r i n g r e q u i r e m e n t s one and two and l a t e r r e q u i r e m e n t s t h r e e and f o u r . 2.2.1 S p e c t r a l W i d t h and Power O u t p u t When a l a s e r u s e s a s i n g l e r u b y r o d as a h i g h power o s c i l l a t o r , h e a t i n g e f f e c t s s e v e r e l y l i m i t t h e s p e c t r a l w i d t h 9 t h a t can be a t t a i n e d . S e v e r a l a u t h o r s [ 3 , 4 , 5 ] d e a l w i t h t h e e f f e c t s o f thermal gradients. These g r a d i e n t s , caused by t he f l a s h l a m p p u m p i n g , can p r o d u c e : a) v a r i a t i o n s i n o p t i c a l path length b) b i r e f r i n g e n c e due to s t r e s s e s W e l l i n g et al. [5 ] a n a l y s e the changes o f o p t i c a l pa th l e n g t h w i t h i n the pumping p e r i o d u s i n g s t r e a k and f r a m i n g camera p i c t u r e s o f i n t e r f e r e n c e p a t t e r n s . Sims et al. [4] c o n s i d e r t he p rob lem o f b i r e f r i n g e n c e and i t s e f f e c t on the o u t p u t p o l a r i z a t i o n o f the r o d . They show t h a t c o n s i d e r a b l e e f f e c t i s seen up to 18 s e c o n d s a f t e r the f l a s h l a m p p u l s e i s o v e r and t h a t b i r e f r i n g e n c e e f f e c t s a re reduced w i t h s h o r t e r r o d s . As w e l l as c a u s i n g p h y s i c a l changes i n the ruby r o d , h e a t i n g d u r i n g the pumping p r o c e s s can a l s o be r e s p o n -s i b l e f o r v a r i a t i o n s i n the ene rgy l e v e l scheme f o r r u b y . In 1916 G i b s o n [6 ] d i s c o v e r e d t h a t the peak o f the a b s o r p -t i o n l i n e s i s t e m p e r a t u r e d e p e n d e n t . S i n c e the c o n d i t i o n f o r l a s e r a c t i o n i s b e s t f u l f i l l e d a t the peak o f the f l u o r e s c e n c e l i n e , t he ruby o s c i l l a t i o n s s h o u l d e x h i b i t t he same d e p e n d e n c e . More r e c e n t l y I z a t t et al. [ 3 ] have v e r i f i e d t h i s a s s u m p t i o n w i t h F a b r y - P e r o t i n t e r f e r e n c e p a t t e r n s show ing the t h e r m a l dependence o f the l a s e r e m i s s i o n . 10 S m a l l e r , l o w e r power r o d s h a v e s e v e r a l o t h e r a d v a n -t a g e s b e s i d e s t h e r e d u c e d p r o b l e m s o f h e a t i n g . B r a d l e y et al. [ 7 ] d e s c r i b e m e a s u r e m e n t s o f f r e q u e n c y s h i f t s i n r u b y p u l s e s w h i c h a r e d e p e n d e n t on i n t e n s i t y . T h i s e f f e c t i s r e d u c e d w i t h l o w e r e n e r g y o s c i l l a t o r o u t p u t s . The mode s t r u c t u r e i n a l a s e r c a v i t y i s s t r o n g l y d e p e n d e n t on t h e c o n d i t i o n o f t h e o p t i c a l s u r f a c e s w i t h i n t h e c a v i t y , and t h e o p t i c a l q u a l i t y o f t h e r u b y r o d . I n a d d i t i o n t o a c h a n g e i n t h e mode s t r u c t u r e d e g r a d a t i o n o f r u b y q u a l i t y a l s o c a u s e s i n c r e a s e d p u l s e l e n g t h , d e c r e a s e d p ower and i n c r e a s e d beam d i v e r g e n c e [ 8 ] . A t l o w e r p o w e r s t h e o p t i c a l s u r f a c e s as w e l l h a v e l o n g e r u s e f u l l i v e s . 2.2.2 R e p r o d u c i b i l i t y and S y n c r o n i z a t i o n The r e q u i r e m e n t s o f h a v i n g r e p r o d u c e a b l e p u l s e s f r o m t h e l a s e r and a c c u r a t e s y n c r o n i z a t i o n w i t h e x t e r n a l e l e c t r o n i c s p l a c e a g r e a t d e a l o f i m p o r t a n c e on t h e c h o i c e and o p e r a t i o n o f t h e Q - s w i t c h t o be u s e d . S i n c e t h e t e c h -n i q u e was f i r s t d e s c r i b e d by M c C l u n g and H e l l w a r t h i n 1961 [ 9 ] and l a t e r t r e a t e d m a t h e m a t i c a l l y by Wagner and L e n g y e l [ 1 0 ] a l a r g e number o f m e t h o d s h a v e been d e v i s e d . F o r t h i s s y s t e m a P o c k e l s C e l l was c h o s e n as t h e Q - s w i t c h i n g e l e m e n t . The r e a s o n s f o r t h i s c h o i c e and t h e o p e r a t i o n o f t h e c e l l a r e o u t l i n e d b e l o w . 11 For the Pockels effect, the birefringence of a material varies linearly with applied electric f i e l d . Uni-axial crystals such as KDP, KD*P and ADP are generally used with their optical axis along the light beam. Effects such as Raman scattering are not observed and the control voltage for a KDP cell is typically only 7 kv. Syncronization of other equipment can be achieved with a high degree of reproduci b i 1 i ty. Figure (1) shows the basic configuration for quarter wave operation of the Pockels Cell. Seven kilovolts on the KDP CRYSTAL REAR POLARIZER RUBY REFLECTOR F i g u r e 1 Pockels C e l l C o n f i g u r a t i o n f o r % Wave Operation. i FRONT REFLECTOR plates is sufficient to rotate the plane of polarization of the ruby emission by 45°. After reflection from the 99% rear mirror and a further rotation of 45° the light is blocked by the crossed polarizer on its return path. To re-establish a high cavity Q a thyratron drops the 12 retardation voltage to zero in 20 nsec and thus allows laser action at a controlled time. The voltage is allowed to increase to 7 kv again to prevent multiple pulsing but this increase can proceed relat ively slowly (~ several micro-seconds) since the recovery time of the laser after emitting a giant pulse is on the order of 10 usee [11]. The only major disadvantage of the Pockels Cell is the damage that occurs to the crystal at high laser powers. However, by using the cel l in a low power o s c i l l a t o r , this problem is avoided. In this system more power is obtained by following the osc i l la tor with a six inch ruby rod as an amp!i f i er. 2.2.3 Mode Selection As mentioned ear l ier some methods must be used to control the resonant modes of the laser cavity in order to produce the desired output. These modes are c lass i f ied in two general categories: (a ) transverse modes w h i c h c o r r e s p o n d t o v a r i o u s g e o m e t r i c a l " r o u n d t r i p " c o n f i g u r a t i o n s , (b ) axial ( l o n g i t u d i n a l ) modes w h i c h c o r r e s p o n d t o t h e r e s o n a n c e c o n d i t i o n s f o r a F a b r y -P e r o t e t a I o n . The following is a consideration of some of the most common mode control techniques and their appl icabi l i ty to this system. 13 Apertures: By p l a c i n g a s m a l l a p e r t u r e i n s i d e t h e l a s e r c a v i t y , t h e F r e s n e l number c an be r e d u c e d t o a l l o w o n l y low o r d e r t r a n s v e r s e modes t o e x i s t . T h i s r e d u c t i o n i n mode v o l u m e i s a c c o m p a n i e d by a r e d u c t i o n i n v o l u m e o f a c t i v e m a t e r i a l u s e d and h e n c e by a r e d u c t i o n i n e n e r g y o u t p u t . F o r t h i s r e a s o n a p e r t u r e s w e r e n o t u s e d . Mirror Separation: I n c r e a s i n g t h e m i r r o r s e p a r a -t i o n w i l l a l s o d e c r e a s e t h e F r e s n e l number and l i m i t t h e number o f t r a n s v e r s e modes. B u t b e c a u s e t h e a x i a l modes a r e s p a c e d by AX = A 2 / 2 L w h e r e L = o p t i c a l l e n g t h o f c a v i t y , i n c r e a s i n g L a l s o i n c r e a s e s t h e number o f a x i a l modes. S i n c e i t i s d e s i r a b l e t o h a v e t h e f e w e s t number o f a x i a l modes i n s i d e t h e f l u o r e s c e n t l i n e w i d t h as p o s s i b l e , t h e u s u a l p r a c t i c e i s t o h a v e t h e s h o r t e s t p o s s i b l e c a v i t y and r e s t r i c t t r a n s v e r s e modes i n o t h e r ways. Pump Power: Ross [ 1 2 ] shows t h a t d u r i n g t h e f l a s h -lamp p u m p i n g p u l s e t h e p o p u l a t i o n a t t h e c e n t r e o f t h e r u b y r o d i s i n v e r t e d f i r s t . As t h e low o r d e r t r a n s v e r s e modes a r e r e s t r i c t e d t o t h e c e n t r e o f t h e r o d , t h e s e w i l l b e g i n t o l a s e f i r s t . H i g h e r pump p o w e r s r e s u l t i n a l a r g e r c r o s s -s e c t i o n o f t h e r o d b e i n g i n v e r t e d . T h u s i f pump power i s 14 k e p t nea r t h r e s h o l d o n l y t he l o w e s t o r d e r t r a n s v e r s e modes w i l l l a s e . T h i s t e c h n i q u e was s u c c e s s f u l l y used i n the f i n a l d e s i g n as e x p l a i n e d i n the s e c t i o n " O s c i 1 1 a t o r  P e r f o r m a n c e . " L o w e r i n g t he pumping power a l s o l o w e r s the l a s e r o u t p u t wh i ch i n t h i s case e x t e n d s the l i f e o f the o p t i c a l s u r f a c e s . Etalon F i l t e r s : C o l l i n s and Wh i te [13 ] f i r s t s u g g e s t e d p l a c i n g a t i l t e d F a b r y - P e r o t e t a l o n i n s i d e the l a s e r c a v i t y t o a c t as a transmission mode s e l e c t o r . T h i s second r e s o n a n t c a v i t y w i t h s h o r t e r l e n g t h must have a s m a l l e r mode d e n s i t y t han the main c a v i t y . O s c i l l a t i o n s can o n l y o c c u r a t t h o s e f r e q u e n c i e s w h i c h s a t i s f y t h e r e s o n a n t c o n -d i t i o n f o r bo th c a v i t i e s . Use o f two e t a l o n s , t i l t e d i n two p e r p e n d i c u l a r p l a n e s o f f e r s bo th a x i a l and t r a n s v e r s e s e l e c t i o n . However , f o r r e a s o n a b l e f i n e s s e t he p l a t e s must be c o a t e d f o r a h i g h r e f l e c t i v i t y (60-80%) wh ich r e s u l t s i n a d r a s t i c r e d u c t i o n o f o u t p u t power [ 1 4 ] . T h i s t e c h n i q u e was no t t r i e d . Resonant Reflectors: An e t a l o n can a l s o be used as a reflection mode s e l e c t o r i f i t forms the o u t p u t m i r r o r 15 o f t he c a v i t y [ 1 8 ] . In the case o f s e v e r a l uncoa ted f l a t s s t a c k e d t o g e t h e r the r e f l e c t i v i t y becomes: R = 1 - d / n )2 N ^ 2 1 + ( l / n ) 2 N where N = number o f p l a t e s n = i n d e x o f r e f r a c t i o n o f p l a t e The r e s o n a t o r Q o f the main l a s e r c a v i t y i s e f f e c t i v e l y modu-l a t e d by t he mode s t r u c t u r e o f the e t a l o n , so a g a i n the o n l y o s c i l l a t i o n s t h a t w i l l o c c u r a re t hose t h a t s a t i s f y a l l the r e s o n a n t c o n d i t i o n s . For a homogeneous ly b roadened s y s t e m such as ruby power i s no t r e j e c t e d i n unwanted modes bu t r a t h e r c h a n n e l l e d i n t o the s e l e c t e d w a v e l e n g t h s [ 1 9 ] . T h i s method was r e j e c t e d i n the i n t e r e s t s o f s i m p l i c i t y and ease o f o p e r a t i o n because the r e f l e c t o r must be t e m p e r a t u r e tuned to g i v e maximum l a s e r o u t p u t [ 1 4 ] . However , a m o d i f i c a t i o n on the e t a l o n t e c h n i q u e was s u c c e s s f u l l y e m p l o y e d . By u s i n g a ruby rod w i t h p l a n e p a r a l l e l ends and no a n t i - r e f 1 e c t i o n c o a t i n g s a r e s o n a n t c a v i t y c o u l d be s e t up between the o u t p u t m i r r o r and one o f the ends o f the r o d . T u n i n g t h i s c a v i t y by a d j u s t i n g the t i l t o f the ruby p roved s i m p l e and r e l i a b l e . 16 Saturable Absorbers: A s u i t a b l y a b s o r b i n g dye when p l a c e d i n t he l a s e r c a v i t y w i l l b l e a c h and become t r a n s -p a r e n t a t some c r i t i c a l power d e n s i t y . As e x p l a i n e d by Sooy [ 15 ] modes w i t h f r e q u e n c y c l o s e s t t o the c e n t r e of t he ruby f l u o r e s c e n c e l i n e w i l l r e a c h t h r e s h o l d f i r s t , b l e a c h t h e dye and grow e x p o n e n t i a l l y . In t h i s l a s e r a dye c e l l was used s u c c e s s f u l l y as a mode s e l e c t o r i n c o n j u n c t i o n w i t h a P o c k e l s C e l l Q - s w i t c h i n the same c a v i t y . Spherical Resonators: R e p l a c i n g e i t h e r o r bo th o f t he p l a n e c a v i t y end m i r r o r s w i t h s p h e r i c a l m i r r o r s can p r o d u c e c o n f i g u r a t i o n s i n w h i c h a l l the t r a n s v e r s e modes a r e d e g e n e r a t e . The h e m i c o n c e n t r i c case [12 ] i s i n s e n s i t i v e t o m e c h a n i c a l v i b r a t i o n s but the e x c e s s i v e i n t e n s i t y a t the f o c a l s p o t damages the p l a n e m i r r o r . A c o n f o c a l r e s o -n a t o r was t r i e d i n t h i s o s c i l l a t o r w i t h m i r r o r s w h i c h had e q u a l r a d i i o f c u r v a t u r e o f 50 cm. As shown i n the nex t s e c t i o n the r e s u l t s were not p a r t i c u l a r l y good . One s h o u l d a l s o no te t h a t even w i t h p l a n e m i r r o r s d e f e c t s and t h e r m a l g r a d i e n t s can a c t as l e n s e s and cause t he r e s o n a t o r t o o s c i l l a t e i n a s e m i - c o n f o c a l mode. T h i s p o i n t i s a l s o p e r s u e d more f u l l y i n the nex t s e c t i o n . 17 2 . 3 O s c i l l a t o r P e r f o r m a n c e 2 . 3 . 1 S p e c t r a l Wid th The s p e c t r a l o u t p u t of the l a s e r o s c i l l a t o r was examined u s i n g the a r rangement i n F i g u r e ( 2 ) . A f t e r r e f l e c -t i o n f rom a g l a s s p l a t e the a t t e n u a t e d l a s e r beam was LIGHT DUMP GLASS PLATE LASER NEGATIVE LENS ALUMINUM DIFFUSING SCREEN D=3 400 mm F.L. LENS WRATTEN NO. 29 FABRY-PEROT CAMERA N.D. 0.3 FILTER F i g u r e 2. Optics f o r A n a l y s i s of S p e c t r a l C h a r a c t e r i s t i c s of Laser O s c i l -- l a t o r Output. expanded w i t h a n e g a t i v e l e n s and r e f l e c t e d on to the F a b r y -P e r o t p l a t e s w i t h an a luminum d i f f u s i n g s c r e e n . A 400 mm f o c a l l e n g t h l e n s imaged the i n t e r f e r e n c e p a t t e r n on the p l a n e o f a n e u t r a l d e n s i t y f i l t e r ( d e n s i t y 0 . 3 ) . The p a t t e r n and f i l t e r were then pho tog raphed on T R I - X (ASA 400) f i l m u s i n g a c l o s e - u p l e n s and a #29 W r a t t e n f i l t e r t o r e d u c e s t r a y l i g h t . As seen i n the f o l l o w i n g p r i n t s the n e u t r a l 18 d e n s i t y f i l t e r c o v e r e d o n l y h a l f the r i n g p a t t e r n . T h i s t e c h n i q u e g r e a t l y s i m p l i f i e d the measurements o f the h a l f -w i d t h s o f t he r i n g s because no H-D cu rve i s r e q u i r e d f o r the f i l m . U s i n g a m i c r o d e n s i t o m e t e r the h a l f - i n t e n s i t y p o i n t s on a p a r t i c u l a r r i n g a re found s i m p l y by c o m p a r i s o n w i t h the o t h e r h a l f o f the p a t t e r n . Wi th a p l a t e s e p a r a t i o n o f 5 mm the free spectral o o range a t 6943 A was A X g R = 0 . 4 8 A. The f i n e s s e o f the p l a t e s was a p p r o x i m a t e l y 30 so t h a t the chromatic resolving o power was a p p r o x i m a t e l y AXX = 0 .016 A. In o r d e r to e s t i m a t e the e r r o r s due to i n s t r u m e n t b r o a d e n i n g t he t r e a t m e n t used by Cooper and G r e i g [ 16 ] p r o v e s h e l p f u l . I f t r u e s o u r c e w i d t h measured s o u r c e w i d t h t h e n 1) t he minimum e f f e c t o f the i n s t r u m e n t i s z e r o b r o a d e n i n g , i . e . AX T = AX M 2) the w o r s t e f f e c t o f i n s t r u m e n t b r o a d -e n i n g i s AX.. = AX^ + AX, 3 M T H. AXr AX M 19 Assuming t h a t the t r u e w i d t h A X T l i e s a t the mean between t h e s e two l i m i t s g i v e s : A X S R A X T = A X M - ( 1 ± 1 } Now f o r a c c u r a c y , A X M / A X g R must be as l a r g e as p o s s i b l e S i n c e s u c c e s s i v e o r d e r s o f i n t e r f e r e n c e p a t t e r n must no t o v e r l a p , the maximum v a l u e o f A A „ / A A „ „ i s l i m i t e d t o : M * - T -hence A X T = A A M (1 - 1 /F ± 1 /F) o For the measurements p r e s e n t e d here A X M ^ 0.1 A and F == 3 0 , g i v i n g A X T = 0.1 (1 - .03 ± .03 ) T a k i n g the ave rage between t h e s e l i m i t s A X T = .0965 ± .0035 Thus t he i n s t r u m e n t b r o a d e n i n g e r r o r f o r a measured w i d t h o f 0.1 A i s o f the o r d e r o f 4%. 20 Added to the e r r o r s i n the d e n s i t o m e t e r r e a d i n g s one must c o n c l u d e t h a t the f o l l o w i n g r e s u l t s a re a c c u r a t e to about 15% a t b e s t . F o l l o w i n g i s a l i s t o f the v a r i o u s mode c o n t r o l methods t h a t were t r i e d and the s u c c e s s o b t a i n e d w i t h e a c h . S P H E R I C A L MIRRORS: F i g u r e (3) shows t h e o s c i l l a t o r o u t p u t f o r a c o n f o c a l c o n f i g u r a t i o n u s i n g s p h e r i c a l m i r r o r s o f 50 cm r a d i i and r e f l e c t i v i t i e s o f 30% ( f r o n t ) and 99.9% ( r e a r ) . Many modes a re seen to be l a s i n g ove r the who le f r e e s p e c t r a l range o f the i n s t r u m e n t . No amount o f m i r r o r a d j u s t m e n t c o u l d reduce t h e s e s i g n i f i c a n t l y and hence no q u a n t i t a t i v e a n a l y s i s was c a r r i e d o u t . PLANE MIRRORS: R e p l a c i n g the s p h e r i c a l m i r r o r s w i t h p l a n e m i r r o r s (75% f r o n t ) gave the o u t p u t i n F i g u r e ( 4 ) . A t r i p l e s e t o f o modes are r e s o l v e d w i t h a t o t a l h a l f - w i d t h o f 0 .24 A . T h i s o u t p u t i s much n a r r o w e r t han f o r the c o n f o c a l case and as p o i n t e d ou t i n the s e c t i o n on mode c o n t r o l , may be a t t r i b u t e d to d e f e c t s and t h e r m a l g r a d i e n t s l i m i t i n g the number o f o s c i l l a t i n g modes. F i g u r e H. O s c i l l a o t r Output: (75% f r o n t ) . P l a n e M i r r o r s 22 In a t t e m p t i n g t o nar row the o u t p u t l i n e w i d t h f u r t h e r two changes were made t o g e t h e r : a) output mirror changed to 30$ r e f l e c t i v i t y b) ruby face aligned p a r a l l e l to output mirror. In a d d i t i o n , the s p a c i n g between the c a v i t y end m i r r o r s was g r e a t e r than 50 cm. S i n c e the ruby i s o n l y 3" x 3 / 8 " d i a m e t e r , t r a n s v e r s e modes a re s t r o n g l y d i f f r a c t i o n l i m i t e d . F i g u r e s 5 (a) and (b) show the o u t p u t u s i n g the 30% f r o n t m i r r o r a l i g n e d w i t h the ruby f a c e . They show two c o n s e c u t i v e s h o t s w i t h a one m inu te i n t e r v a l b e t w e e n . In F i g u r e 5 (a) a s i n g l e l i n e i s seen w i t h a h a l f - w i d t h o f o 0 .042 A . The nex t p h o t o g r a p h , F i g u r e 5 (b) , shows an o u t -pu t w i t h two c o m p o n e n t s , each w i t h a w i d t h equa l t o the o i n s t r u m e n t a l r e s o l v i n g power (~ .02 A ) . The t o t a l w i d t h o f bo th components t o g e t h e r i s o .084 A . The l a s e r power o u t p u t was 20 MW. These pho tog raphs a l s o show the t e m p e r a t u r e d e p e n -den t f r e q u e n c y s h i f t d i s c u s s e d e a r l i e r . As the ruby warms up the f r i n g e p a t t e r n moves o u t w a r d , i n d i c a t i n g a s h i f t toward l o n g e r w a v e l e n g t h s . 23 Figure 6 . O s c i l l a t o r Output: Plane M i r r o r s and Dye C e l l . 24 D Y E C E L L : As the l a s t mode s e l e c t i n g t e c h n i q u e a c e l l o f c r y p t o c y a n i n e d i s s o l v e d i n methano l was p l a c e d i n the l a s e r c a v i t y between the f r o n t m i r r o r and the r u b y . The dye c o n -c e n t r a t i o n was a d j u s t e d e x p e r i m e n t a l l y t o g i v e a r e a s o n a b l e compromise between mode s t r u c t u r e and power o u t p u t . " F i g u r e (6) shows the l i n e w i d t h f o r a dye c o n c e n t r a t i o n g i v i n g 10 MW o u t p u t a t the same pump ene rgy as i n the o t h e r c a s e s . The h a l f - w i d t h i s .074 A . From t h i s i t i s p o s s i b l e t o e s t i m a t e the number o f a x i a l modes t h a t a re l a s i n g . The a x i a l mode s p a c i n g i s o g i v e n by AX = X 0 2 / 2 d . For X 0 = 6943 A and an o p t i c a l c a v i t y l e n g t h d = 50 cm: AX = .0048 A Thus the number o f l a s i n g modes must be a t l e a s t ( . 0 7 4 / . 0 0 4 8 ) = 1 5 . T h i s i s i n agreement w i t h McClung and We ine r [17 ] who found the dye no t e n t i r e l y r e l i a b l e i n s u r p r e s s i n g a l l unwanted modes a t h i g h power . A l t h o u g h the dye does l i m i t the number o f modes , i t s p r i m a r y advan tage appears to be i n s t a b i l i z i n g the l a s e r o u t p u t . The g i a n t p u l s e s a re smoother than w i t h the P o c k e l s 25 C e l l a l o n e . Magyar [14] s u g g e s t s t h i s i s due to the absence o f modes w h i c h bea t w i t h one a n o t h e r and modu la te the p u l s e . C l o s e l y a l l i e d to the s p e c t r a l w i d t h o f the o u t p u t i s the coherence l e n g t h I = c/Av ; where c = speed o f l i g h t and Av = f r e q u e n c y s p r e a d o f l a s e r o u t p u t . For t h i s c a s e : AX - .08 A t h e r e f o r e Av « 5 GHz and the c o h e r e n c e l e n g t h I = 6 cm. 2 . 3 . 2 P u l s e C h a r a c t e r i s t i c s In a d d i t i o n to the s p e c t r a l w i d t h o f t he l a s e r o u t p u t the s i z e and shape o f the p u l s e s a re a l s o i m p o r t a n t . Fo r the f o l l o w i n g the p u l s e l e n g t h w i l l be d e f i n e d as the f u l l w i d t h a t h a l f maximum h e i g h t as measured by a PIN p h o t o d i o d e and a T e k t r o n i x 519 o s c i l l o s c o p e . The ene rgy i s the t o t a l ene rgy i n a s i n g l e g i a n t p u l s e measured w i t h a TRG Model 101 t h e r m o p i l e . The most s i g n i f i c a n t component a f f e c t i n g p u l s e shape i s the p o l a r i z e r a s s o c i a t e d w i t h the P o c k e l s C e l l . T h i s i s a s t a c k o f t h i n q u a r t z p l a t e s t h a t p o l a r i z e by m u l t i p l e r e f l e c t i o n . T y p i c a l l y w i t h the maximum number o f 26 p l a t e s ( e i g h t ) the o u t p u t p u l s e was abou t 25 nsec w ide w i t h an ene rgy o f 0 . 5 j o u l e s , i . e . 20 MW. Removing p l a t e s f rom the p o l a r i z e r has the e f f e c t o f i n c r e a s i n g bo th p u l s e ene rgy and w i d t h i n r o u g h l y the same r a t i o . Fo r example w i t h no p o l a r i z e r the g i a n t p u l s e l a s t s about 70 nsec and c o n t a i n s 1.4 j o u l e s o f e n e r g y , a g a i n g i v i n g about 20 MW o f power . In g e n e r a l o p e r a t i o n the o u t p u t p u l s e i s about 0 .6 j o u l e s i n 30 n s e c . The u n c e r t a i n t y i n o u t p u t power v a r i e s abou t ±15%, most o f w h i c h i s v a r i a t i o n i n p u l s e w i d t h . For an o u t p u t o f 0 .6 j o u l e s the pump ene rgy i s 1600 j o u l e s , g i v i n g an e f f i c i e n c y o f 0 .4%. 2 . 3 . 3 Summary From the F a b r y - P e r o t pho tog raphs one can see t h a t the l a s e r o s c i l l a t o r , u s i n g p l a n e m i r r o r s , a P o c k e l s C e l l Q - s w i t c h and c a r e f u l a l i g n m e n t o f the ruby r o d , i s c a p a b l e o o f p r o d u c i n g a s p e c t r a l o u t p u t .08 A (5 GHz) w ide a t 20 o M e g a w a t t s . Powers up to 30 MW and 0.1 A w i d t h s can be p roduced bu t more r a p i d d e g r a d a t i o n o f the o p t i c a l s u r f a c e s r e s u l t s . W i th a c e l l o f c r y p t o c y a n i n e and methano l i n the c a v i t y the o u t p u t w i d t h n a r r o w s , the power d rops a c c o r d i n g 27 to the dye c o n c e n t r a t i o n and the p u l s e e n v e l o p e becomes s m o o t h e r . The c o h e r e n c e l e n g t h o f the l a s e r o u t p u t f o r a s p e c t r a l w i d t h o f .08 A i s 6 cm. 2 . 4 A m p l i f i e r P e r f o r m a n c e Fo r t h i s sys tem the most i m p o r t a n t a m p l i f i e r p a r -ameter i s the gain a c h i e v e d w i t h a p a r t i c u l a r ene rgy i n p u t to the f l a s h l a m p s . E f f i c i e n c y , and p o s s i b l e p u l s e s h o r t e n -i n g a re o t h e r u s e f u l c h a r a c t e r i s t i c s t h a t were s t u d i e d , bu t the m a j o r i t y o f t he f o l l o w i n g a n a l y s i s d e a l s w i t h t he d e -pendence o f the p o p u l a t i o n i n v e r s i o n and g a i n on i n p u t e n e r g y . 2 . 4 . 1 Ene rgy G a i n In S e c t i o n 2.1 an e x p r e s s i o n f o r the ene rgy g a i n i n a l a s e r a m p l i f i e r o f l e n g t h L was d e r i v e d to b e : o G = 28 To s i m p l i f y t h i s f o r the e x p e r i m e n t a l case we use the f o l 1 o w i ng : p 0 c x 0 = number o f p h o t o n s / u n i t a r e a i n c i d e n t on f a c e o f l a s e r rod d u r i n g p u l s e o f t ime T 0 T o t a l i n c i d e n t p o w e r / u n i t a r e a = P / A = hv ( n o . o f p h o t o n s ) A T 0 = h v p 0 c Now, i f t he a m p l i f i e r i s to o p e r a t e w e l l w i t h i n i t s u n s a t u r a t e d r e g i o n , the i n p u t pho ton f l u x must be s m a l l so t h a t 2 a p 0 C T 0 << 1 c T h i s c o n d i t i o n can be shown to e x i s t by c o n s i d e r i n g the o u t -put o f the l a s e r o s c i l l a t o r and by c a l c u l a t i n g 29 2 a p 0 c x 0 = ^ . P. . .081 w h e r e : a = r a d i a t i o n a b s o r p t i o n c r o s s s e c t i o n f o r chromium = 2 .5 x 1 0 ~ 2 0 c m + 2 T 0 = 30 x 1 0 " 9 s e c v = 4 . 3 x 1 0 1 * HZ P = 2 x 10' w a t t s A = 1.3 cm 2 Wi th the above r e s t r i c t i o n , the e x p r e s s i o n f o r the ene rgy g a i n r e d u c e s t o : G = e n 0 a L A l s o i n S e c t i o n 2.1 the f r a c t i o n a l p o p u l a t i o n i n v e r s i o n was found to have the f o l l o w i n g f u n c t i o n dependence 30 on the f l a s h l a m p ene rgy E : He. _ v 1 - 3 e " e E where B> d e f i n e d as the pumping c o e f f i c i e n t , i s a measure o f t h e e f f i c i e n c y o f the p a r t i c u l a r pumping c o n f i g u r a t i o n ( i n t h i s c a s e , two l i n e a r w a t e r c o o l e d f l a s h l a m p s w i t h i n a d o u b l e e l l i p t i c a l c a v i t y w i t h the ruby a t the common f o c i ) . To d e t e r m i n e B, e x p e r i m e n t a l d a t a shows t h a t the a m p l i f i e r gave a u n i t y g a i n f o r an i n p u t ene rgy o f 2100 j o u l e s . U s i n g e q u a t i o n ( 2 . 2 ) w i t h no = 0 and E = 2100 j o u l e s g i v e s : B = 5 .23 x 10" 1 * j o u l e s " 1 S u b s t i t u t i o n o f t h i s v a l u e o f B i n t o e q u a t i o n ( 2 . 2 ) g i v e s the fo rm o f the f r a c t i o n a l p o p u l a t i o n i n v e r s i o n shown i n F i g u r e ( 7 ) . Here the i n v e r s i o n i s e x p r e s s e d as a p e r c e n t a g e ( q 0 / N 0 x 100%) and the maximum energy i s l i m i t e d by the s t o r a g e o f the c a p a c i t o r bank . 31 32 U s i n g the dependence i n F i g u r e ( 7 ) , t he s i m p l i f i e d g a i n e x p r e s s i o n o f e q u a t i o n ( 2 . 1 ) and the v a l u e N 0 = 8 . 8 x 1 0 1 8 c m - 3 , the c a l c u l a t e d g a i n v s . f l a s h l a m p ene rgy c u r v e i s shown i n F i g u r e ( 8 ) , a l o n g w i t h the e x p e r i m e n t a l p o i n t s f o r the s i x i n c h a m p l i f i e r r o d . The agreement i s seen to be r e a s o n a b l e c o n s i d e r i n g t h a t the t h e o r e t i c a l t r e a t m e n t was done f o r an i d e a l s q u a r e p u l s e i n p u t t o the a m p l i f i e r . 2 . 4 . 2 P u l s e D i s t o r t i o n S a t u r a t i o n i n a l a s e r a m p l i f i e r o c c u r s when the i n c i d e n t pho ton f l u x i s l a r g e enough to c o m p l e t e l y empty t he upper e x c t e d l a s e r l e v e l i n the a c t i v e m a t e r i a l . Fo r a p u l s e i n p u t t h i s r e s u l t s i n d i s t o r t i o n o f the p u l s e shape due t o h i g h e r a m p l i f i c a t i o n o f the l e a d i n g edge than o f the t r a i l i n g e d g e . The o u t p u t p u l s e i s thus s h o r t e n e d by t h e s a t u r a t i o n e f f e c t and the power i s c o r r e s p o n d i n g l y i n c r e a s e d . However , the a n a l y s i s f o r the g a i n o f the a m p l i -f i e r i n the p r e c e d i n g s e c t i o n r e q u i r e d t h a t t h e r e be no s a t u r a t i o n , i . e . 2 a p 0CT 0 << 1 . T h i s was c a l c u l a t e d t o be t r u e and the e x p e r i m e n t a l v e r i f i c a t i o n i s shown i n F i g u r e ( 9 ) , wh i ch g i v e s t y p i c a l i n p u t and o u t p u t p u l s e s t o the a m p l i f i e r . I t i s seen t h a t t h e r e i s no p u l s e shape 34 d i s t o r t i o n and t h a t the power g a i n o f the a m p l i f i e r i s equa l to i t s energy g a i n . (a ) (b) INPUT OUTPUT Figure 9 . Laser A m p l i f i e r Pulses Normalized to the Same Energy. 2 . 5 P i s c u s s i on F i g u r e (10) shows the assemb ly o f the comp le te l a s e r s y s t e m . The SYNC OUT p u l s e f rom the P o c k e l s C e l l D r i v e r i s used to t r i g g e r the d e t e c t i o n sys tem p r e s e n t e d i n the nex t c h a p t e r . D u r i n g the t ime o f the f l a s h l a m p pumping the l a s e r i t s e l f i s t r i g g e r a b l e ove r a range o f s e v e r a l hundred m i c r o s e c o n d s w i t h an u n c e r t a i n t y o f about one m i c r o s e c o n d . The p r i m a r y s o u r c e o f t h i s j i t t e r i s the y rear / reflector ' / / p.c. polarizer 30% 3 in. ruby dye 7kv T P O C K E L S C E L L driver f lashlamps A oscillator capacitor bank X 6 in. ruby •v_ , —^ light out ^ flashlamps amplifier capacitor bank sync, out to detection system laser trigger in from plasma electronics -O pulse from plasma electronics to fire flashlamps (advanced 800/Jsec w.r.t. laser trigger pulse) F i g u r e 10. L a s e r O p e r a t i o n . co en 36 t h y r a t r o n i n the P o c k e l s C e l l D r i v e r . However , the t r i g g e r p u l s e f o r the d e t e c t i o n sys tem i s t a k e n d i r e c t l y f rom the c a b l e d r i v i n g the P o c k e l s C e l l so the s y n c r o n i z a t i o n between the l a s e r and the d e t e c t i o n e l e c t r o n i c s i s a c c u r a t e to s e v e r a l n a n o s e c o n d s . As shown i n the s e c t i o n on O s c i l l a t o r P e r f o r m a n c e the s p e c t r a l o u t p u t o f the l a s e r i s r e a s o n a b l y nar row even though none o f the o p t i c a l components were o f p a r t i c u -l a r l y good q u a l i t y . In f a c t , as the e a r l y t e s t i n g was c a r r i e d ou t a t h i g h powers (> 50 MW) c o n s i d e r a b l e damage o c c u r r e d t o some o f t he o p t i c a l s u r f a c e s . C o n s i d e r i n g t h i s , b e t t e r s p e c t r a l w i d t h i s t o be e x p e c t e d i f a new ruby rod and KDP c r y s t a l were t o be used i n the o s c i l l a t o r . The w i d t h c o u l d a l s o be nar rowed f u r t h e r w i t h e t a l o n f i l t e r s and a p e r t u r e s i n s i d e the c a v i t y but any such sys tem wou ld r e q u i r e a t l e a s t one more a m p l i f i e r s t a g e to m a i n t a i n the power o u t p u t a t i t s p r e s e n t l e v e l . P A R T I I C h a p t e r 3 THE DETECTION SYSTEM 3.1 G e n e r a l O u t l i n e o f P o l y c h r o m a t o r The pu rpose o f any m u l t i c h a n n e l s y s t e m . i s to a l l o w s e v e r a l segments o f a s p e c t r u m to be o b s e r v e d s i m u l t a n e o u s l y . S e p a r a t e p h o t o m u l t i p i i e r s a re g e n e r a l l y used to m o n i t o r the d e s i r e d w a v e l e n g t h segments i n the o u t p u t o f a s p e c t r o -g raph . In t h i s s ys tem the e x i t s l i t o f a monochromator i s r e p l a c e d w i t h a package o f f i v e i n d i v i d u a l s l i t s c o n -s t r u c t e d f rom g l a s s f i b e r s . The f i b e r s c a r r y l i g h t to f i v e p h o t o m u l t i p i i e r t u b e s . To d i s p l a y the o u t p u t o f the p h o t o -m u l t i p l i e r s , the most s t r a i g h t f o r w a r d way would be to use f i v e s e p a r a t e o s c i l l o s c o p e s w i t h r e c o r d i n g c a m e r a s . W h i l e t h i s has been d o n e , the o b v i o u s d i s a d v a n t a g e i s the c o s t o f the equ ipment i n v o l v e d . I n s t e a d o f t h i s , a method was d e v i s e d to d i s p l a y a l l the p h o t o m u l t i p i i e r o u t p u t s on a s i n g l e o s c i l l o s c o p e t r a c e . As w i t h any p u l s e d s c a t t e r i n g e x p e r i m e n t the p h o t o -m u l t i p l i e r s a re r e q u i r e d to d e t e c t a s h o r t p u l s e o f s c a t t e r e d 37 38 l a s e r l i g h t . T h i s l i g h t p u l s e a r r i v e s s i m u l t a n e o u s l y a t each p h o t o c a t h o d e and hence a l l the o u t p u t p u l s e s come f rom the p h o t o t u b e s a t the same t ime ( n e g l e c t i n g t r a n s i t t ime v a r i a t i o n s f rom tube to t u b e ) . By s i m p l y e l e c t r o n i c a l l y d e l a y i n g each p u l s e i t i s seen t h a t they can be s e q u e n t i a l l y d i s p l a y e d on a s i n g l e o s c i l l o s c o p e t r a c e . T h i s has the f u r t h e r advan tage t h a t w i t h p r o p e r c a l i b r a t i o n the o s c i l -l o s c o p e r e c o r d i n g i s a d i r e c t measure o f i n t e n s i t y v s . w a v e l e n g t h . T h i s s i m p l e method has one v e r y b a s i c p r o b l e m . The i n f o r m a t i o n d i s p l a y e d on the scope i s a c t u a l l y the sum o f a l l t he p h o t o m u l t i p i i e r o u t p u t s , each d e l a y e d a g a i n s t one a n o t h e r . But i n a d d i t i o n t o the l a s e r p u l s e , each p h o t o t u b e a l s o sees the backg round l i g h t f rom the p lasma wh ich appears as a DC l e v e l i n the o u t p u t o f the t u b e . So on the scope t r a c e , as a l l the PM o u t p u t s were added the f i n a l b a s e l i n e wou ld appear as a n o i s e l e v e l f i v e t imes g r e a t e r than f rom one tube a l o n e . To p r e v e n t t h i s , each p h o t o m u l t i p i i e r i s p u l s e d on f o r an i n t e r v a l e x a c t l y as l o n g as the d e l a y between t u b e s . T h i s e l i m i n a t e s the o v e r l a p p i n g o f the backg round l e v e l s by e f f e c t i v e l y d i s p l a y i n g o n l y one tube a t a t i m e . The f o l l o w i n g s e c t i o n s d e s c r i b e the f i b e r o p t i c s used f o r the s l i t p a c k a g e s , the d e s i g n o f the p h o t o m u l t i p i i e r c i r c u i t s and the p e r f o r m a n c e o f the f i n a l s y s t e m . 39 3 . 2 F i b e r O p t i c s F i g u r e (11) shows the s l i t package t h a t i s p l a c e d a t the e x i t p l a n e o f the monochromato r . S i n g l e l a y e r s o f . 003 i n c h g l a s s f i b e r s (Edmonds #41,225) a re s t a c k e d between s p a c e r s h e e t s o f . 003 i n c h s t a i n l e s s s t e e l and the who le assemb ly cemented w i t h c l e a r e p o x y . Wi th a monochromator o o f 10 A/mm d i s p e r s i o n t h e s e s l i t s g i v e f i v e s p e c t r a l p o i n t s o o each s e p a r a t e d by 0 .75 A w i t h a r e s o l u t i o n o f 0 . 7 5 A . The t r a n s m i s s i o n c h a r a c t e r i s t i c s o f the f i b e r o p t i c s b u n d l e s a re s t r o n g l y dependen t on the q u a l i t y o f the end p o l i s h i n g and to a l e s s e r e x t e n t on the q u a l i t y o f g l a s s [ 2 0 ] . I d e a l l y , l i g h t s h o u l d l e a v e a f i b e r a t the same a n g l e a t wh ich i t e n t e r e d , i . e . p r e s e r v a t i o n o f s o l i d a n g l e . But d e f e c t s i n p o l i s h i n g the ends o f t he f i b e r s l e a v e i r -r e g u l a r i t i e s wh i ch randomly s c a t t e r the l i g h t . Added to the normal r e f l e c t i o n l o s s a t a g l a s s - a i r i n t e r f a c e the t o t a l l i g h t l o s t per end i s t y p i c a l l y .15-20%. A b s o r p t i o n i n the g l a s s adds a n o t h e r 7% pe r f o o t o f f i b e r so f o r t h e s e b u n d l e s the e s t i m a t e d t r a n s m i s s i o n i s about 70%. L o s s e s f rom one end are n e g l e c t e d here because o f the method used to i n t r o d u c e the l i g h t i n t o the p h o t o m u l t i p i i e r s . T h i s i s e l a b o r a t e d on i n the s e c t i o n e n t i t l e d P h o t o m u l t i p i i e r s :  D e s i g n C o n s i d e r a t i o n s . F i g u r e 11. F i b e r O p t i c s S l i t P a c k a g e . 41 3 . 3 P h o t o m u l t i p i i e r s 3 . 3 . 1 D e s i g n C o n s i d e r a t i o n s Quantum Efficiency Enhancement: In 1965 G u n t e r , E r i c s o n and G r a n t [ 2 1 , 2 2 ] showed e x p e r i m e n t a l l y t h a t the s e n s i t i v i t y o f a p h o t o m u l t i p i i e r c o u l d be i n c r e a s e d by t o t a l i n t e r n a l r e f l e c t i o n o f l i g h t i n the end window o f the t u b e . Under optimum c o n d i t i o n s t hey showed t h a t n e a r l y a 5 x i n c r e a s e i n quantum e f f i c i e n c y (QE) was p o s s i b l e w i t h red l i g h t . A m a t h e m a t i c a l model c o n s i d e r e d by S i z e l o v e and Love [23 ] p r e d i c t e d s i m i l a r r e s u l t s . A s i m i l a r t e c h n i q u e was used i n t h i s s ys tem to c o u p l e l i g h t f rom the f i b e r o p t i c b u n d l e s i n t o the p h o t o -m u l t i p l i e r s (EMI 9558A, S -20 ) s u r f a c e . The f i n a l a r r a n g e -ment i s shown i n F i g u r e ( 1 2 ) . L i g h t f rom the g l a s s f i b e r s B E A M F i g u r e 1 2 . P h o t o m u l t i p l i e r End Window Showing Method of T o t a l I n t e r n a l R e f l e c t i o n . 42 e n t e r s the end window a t 45° t h rough a drop o f index m a t c h -i n g p a r a f f i n o i l . The l i g h t i s t r a p p e d by t o t a l i n t e r n a l r e f l e c t i o n between the p h o t o c a t h o d e and the tube f a c e and r e l e a s e s p h o t o e l e c t r o n s on each b o u n c e . In agreement w i t h J e n n i n g s et al. [24 ] b l u e and g r e e n l i g h t i s c o m p l e t e l y a b s o r b e d i n t h i s p r o c e s s as seen by o b s e r v i n g the l i g h t wh i ch f i n a l l y e s c a p e s from the edge o f the w indow. The quantum e f f i c i e n c y o f a p h o t o m u l t i p l i e r i s g i ven by [25] : QE = S x 1 2 3 x 9 ' 5 x 100% w h e r e : X = w a v e l e n g t h i n n a n o m e t e r s S = c a t h o d e r a d i a n t s e n s i t i v i t y Camp/wat t ] ] = c u r r e n t l e a v i n g p h o t o c a t h o d e i n c i d e n t r a d i a n t power I f the w a v e l e n g t h and pho ton f l u x o f the i n c i d e n t r a d i a t i o n a re kep t c o n s t a n t a long w i t h the g a i n o f the dynode s t a g e s o f the tube i t i s seen t h a t the anode c u r r e n t i s d i r e c t l y p r o p o r t i o n a l to the Q E . Thus the i n c r e a s e i n QE by any enhancement t e c h n i q u e i s e a s i l y found d i r e c t l y by m e a s u r i n g the anode c u r r e n t . 43 To measure the enhancement the method o f F i g u r e (12) was compared to a s i m i l a r a r rangement i n wh i ch l i g h t f rom the f i b e r s e n t e r e d the tube normal to the s u r f a c e at the c e n t r e o f the p h o t o c a t h o d e . Us ing a g a l l i u m - a r s e n i d e o l i g h t - e m i t t i n g d i o d e as the l i g h t s o u r c e (X ~ 6600 A) the quantum e f f i c i e n c y was found to i n c r e a s e by a f a c t o r o f 3 . T h i s i s s l i g h t l y l o w e r than t h a t c l a i m e d by G u n t h e r , E r i c s o n and G r a n t [21] bu t i d e n t i c a l t o the enhancement a c h i e v e d by Oke and S c h i l d [26] who a l s o i n v e s t i g a t e d S20 s u r f a c e s i n the red end o f the s p e c t r u m . A c c e p t i n g the EMI s p e c i f i c a t i o n s t h a t the t y p i c a l QE o f an S20 s u r f a c e a t 6943 A i s about 3.5% i t i s seen t h a t the enhanced QE i s about 10% a t t h i s w a v e l e n g t h . Phot omul t i p l i e v Pulsing: The b a s i c o p e r a t i o n o f the d e t e c t i o n s y s t e m , as b r i e f l y o u t l i n e d e a r l i e r , i s as f o l l o w s : a l a s e r l i g h t p u l s e a r r i v e s s i m u l t a n e o u s l y a t each p h o t o m u l t i p i i e r ; each anode p u l s e i s s u i t a b l y d e l a y e d and d i s p l a y e d s e q u e n t i a l l y on a s i n g l e o s c i l l o s c o p e . As the l a s e r p u l s e i s a p p r o x i m a t e l y 30 nsec l o n g , the d e l a y between p h o t o m u l t i p i i e r s was chosen as 100 nsec and a c h i e v e d by e q u a l l e n g t h s o f d e l a y c a b l e . Tha t i s , the s i g n a l f rom the f i r s t tube a r r i v e s a t the scope w i t h no d e l a y , the second 44 t u b e ' s p u l s e i s d e l a y e d by 100 n s e c , the t h i r d tube by 200 nsec e t c . u n t i l a l l f i v e s i g n a l s a re d i s p l a y e d . To a v o i d a d d i n g the backg round l e v e l s , the PM's must a l l be p u l s e d on f o r 100 nsec d u r i n g the l a s e r p u l s e and then t u r n e d o f f a g a i n . A p p e n d i x I r e v i e w s the p o s s i b l e c o n f i g u r a t i o n s used f o r p u l s i n g photomul t i p i i e r s , and the r e s t o f t h i s s e c t i o n d e a l s s p e c i f i c a l l y w i t h the two methods t r i e d . One o f the methods was s u c c e s s f u l and an e x p l a n a t i o n i s g i v e n f o r the f a i l u r e o f the o t h e r . In g e n e r a l t h e r e a re two c h o i c e s f o r p u l s i n g pho tomu1 t i p i i e r s : (a ) s w i t c h i n g t h e t u b e i t s e l f on and o f f , o r (b) e l e c t r o n i c a l l y g a t i n g t h e o u t p u t s i g n a l Method (a) was i n v e s t i g a t e d f i r s t because f a s t t r a n s m i s s i o n g a t e s were no t a v a i l a b l e w i t h the p r o p e r s p e c i f i c a t i o n s , and because by p u l s i n g the tube an i n c r e a s e i n the g a i n can be a c h i e v e d under the p r o p e r c o n d i t i o n s [ 2 7 ] . PULSING THE PHOTOMULTIPLIER-TUBE S p e c i f i c a l l y , the ca thode was p u l s e d n e g a t i v e w i t h r e s p e c t to the f i r s t dynode . A p u l s e g e n e r a t o r u s i n g a mercu ry 45 w e t t e d r e l a y and a d e l a y c a b l e gave a 150 v o l t p u l s e , 100 nsec w i d e , w i t h r i s e and f a l l t imes o f l e s s than one nano-second bu t was d i s c a r d e d because i t had a j i t t e r on the o r d e r of 1 u s e e . A n o t h e r g e n e r a t o r u s i n g a h i g h v o l t a g e t r a n s i s t o r d r i v e n by a o n e - s h o t m u l t i v i b r a t o r a l s o gave f a s t s q u a r e p u l s e s w i t h j i t t e r s l e s s than one n a n o s e c o n d . But the o u t p u t o f the p h o t o m u l t i p i i e r l a s t e d much l o n g e r when i l l u m i n a t e d w i t h a DC l i g h t s o u r c e than 100 n s e c , and a l a r g e s p i k e was p r e s e n t i n the o u t p u t a t the b e g i n n i n g o f the ga te p u l s e . T h i s s p i k e was a t t r i b u t e d to a ca thode space cha rge and cu red by b i a s i n g the ca thode ten v o l t s p o s i t i v e w i t h r e s p e c t to the f i r s t dynode . Nex t a n e g a t i v e s i n e - s h a p e d p u l s e was a p p l i e d to the ca thode i n s t e a d o f the squa re p u l s e . Wi th t h i s t h e r e was even l e s s n o i s e i n the o u t p u t bu t i n a l l c a s e s the anode c u r r e n t p u l s e was c o n s i d e r a b l y l o n g e r than 100 n s e c , an u n s a t i s f a c t o r y c o n d i t i o n because o f the d e l a y l i n e s . An e x p l a n a t i o n f o r the l e n g t h o f the anode p u l s e i s g i v e n by F a r i n e l l i and Ma lvano [ 2 8 ] . They c o n s i d e r t h a t the ca thode i s a t h i n l a y e r o f s e m i c o n d u c t i n g m a t e r i a l and t h a t a p u l s e a p p l i e d to i t s c i r c u m f e r e n c e t a k e s a r e l a t i v e l y l o n g t ime to t r a v e l a c r o s s the s u r f a c e . By s o l v i n g the r e l e v a n t p r o p a g a t i o n e q u a t i o n they can e s t i m a t e the t r a n s i t t ime o f a p u l s e t h rough the p h o t o c a t h o d e . A s i m i l a r e f f e c t i s no ted by De M a r t i n i and Wacks [29 ] and by S u e m a t s u , 46 Normura and Tom i t a [30] who p l o t the e q u i d e l a y l i n e s on the p h o t o c a t h o d e o f an RCA 7102 . A l l t h i s p o i n t s out the f a c t t h a t p u l s i n g the p h o t o c a t h o d e i s an u n a c c e p t a b l e method because o f the f undamen ta l l i m i t a t i o n o f the tube i t s e l f . G A T I N G PM O U T P U T The s u c c e s s f u l p u l s i n g t e c h n i q u e uses a t r a n s m i s s i o n ga te on the o u t p u t c i r c u i t o f each p h o t o m u l t i p l i e r . The t ubes a re run c o n t i n u o u s l y and the o u t p u t s sampled f o r 100 nsec d u r i n g t he l a s e r p u l s e . Space cha rge p rob lems a s s o c i a t e d w i t h p u l s i n g t he tubes t h e m s e l v e s a re e l i m i n a t e d because o f t he c o n t i n u o u s o p e r a t i o n . A d e t a i l e d d e s c r i p t i o n o f the o p e r a t i o n o f the t r a n s m i s s i o n g a t e s , a l o n g w i t h the c i r c u i t s i n v o l v e d i s g i v e n i n Append i x I I . 3 . 3 . 2 P e r f o r m a n c e To e v a l u a t e the p e r f o r m a n c e o f the comp le te d e t e c -t i o n s y s t e m , a l i g h t - e m i t t i n g d i o d e (LED) was used as a p u l s e d l i g h t s o u r c e i n the c i r c u i t o f F i g u r e ( 1 3 ) . The l i g h t o u t p u t f o l l o w s the shape o f the d r i v i n g p u l s e and the LED has a r i s e t i m e o f l e s s than f i v e n s e c . 47 Q +3v t o — ( Datapulse + out MLED630 N O T E : Peak emission wavelength of LED is 6600 A F i g u r e 13. C i r c u i t of LED D r i v e r . P r o v i s i o n was made i n the w i r i n g o f the p h o t o -m u l t i p l i e r s to v a r y the v o l t a g e o f dynodes D4 and D5 o f each tube between 33% and 100% o f the ave rage i n t e r d y n o d e v o l t a g e . Fo r a Venetian b l i n d s t r u c t u r e such as p r e s e n t i n the EMI 9558 t h i s v o l t a g e v a r i a t i o n g i v e s a g a i n a d j u s t m e n t o f about 30% f rom the maximum [ 3 1 ] . T h i s a l l o w s a l l f i v e c h a n n e l s t o be a d j u s t e d f o r the same absolute s e n s i t i v i t y by compen-sat ing f o r i nd iv idua l v a r i a t i o n s i n tube gain and op t ica l f i b e r t r a n s m i s s i o n . 48 Each tube as w e l l has a t r a n s i t t ime d e l a y between the a r r i v a l of t he l i g h t p u l s e and the anode c u r r e n t p u l s e . S i n c e a l l the tubes have t he same o v e r a l l b i a s on t h e i r dynode c h a i n s , the t r a n s i t t imes are a l l r e a s o n a b l y equa l (~ 50 n s e c ) and no p r o v i s i o n i s made to compensate f o r d i f f e r e n c e s . F i g u r e (14) shows the n o i s e i n the o u t p u t when no l i g h t i s p r e s e n t . The o s c i l l o s c o p e i s a T e c k t r o n i x 7704 w i t h 150 MHz b a n d w i d t h . T h i s n o i s e r e s u l t s f rom d i f f e r e n c e s i n r i s e and f a l l t imes o f the p o s i t i v e and n e g a t i v e ga te p u l s e s r e q u i r e d by the t r a n s m i s s i o n g a t e s . I t s peak v a l u e i s l e s s t han 50 mV. T h i s n o i s e l e v e l i s much l e s s than t h a t a c h i e v e d by De Marco and Penco [27 ] who c l a i m to have d e v e l o p e d a v e r y low n o i s e p u l s i n g t e c h n i q u e . F i g u r e s (15a) and (15b) show the o u t p u t o f the sys tem f o r a DC l i g h t l e v e l and a 30 nsec l i g h t p u l s e r e s p e c -t i v e l y f rom the LED s o u r c e . A l t h o u g h the l i g h t p u l s e has r i s e and f a l l t imes o f f i v e nsec i t i s seen t h a t the PM p u l s e s a re c o n s i s t e n t w i t h the s p e c i f i c a t i o n s o f 15 nsec r i s e t i m e f o r t h e s e t u b e s . The v a r i a t i o n s i n a m p l i t u d e o f the p u l s e s i n F i g u r e (15b) i s a s t a t i s t i c a l f l u c t u a t i o n due to the e x t r e m e l y low l i g h t o u t p u t o f the LED s o u r c e at t h i s p u l s e w i d t h . In bo th p h o t o g r a p h s t he l e n g t h o f t he PM g a t i n g p u l s e s has been made s l i g h t l y s h o r t e r than the 100 nsec v e r t i c a l s c a l e : 200 mV/division Fi g u r e 14. Noise i n PM Gating E l e c t r o n i c s . Figure 1 5 . P h o t o m u l t i p l i e r Outputs. 50 i n t e r - c h a n n e l d e l a y , thus a l l o w i n g the i n d i v i d u a l c h a n n e l s to be e a s i l y i d e n t i f i e d . 3 .4 P i s c u s s i on The c o m p l e t e sys tem i s shown i n F i g u r e ( 1 6 ) . A f t e r an a p p r o p r i a t e d e l a y t o a l l o w f o r the t r a n s i t t ime o f the l a s e r l i g h t and the i n t e r n a l t r a n s i t t ime o f the p h o t o m u l -t i p l i e r s , the ga te p u l s e g e n e r a t o r ( F i g u r e A - 2 , A p p e n d i x I I ) i s t r i g g e r e d by a s y n c p u l s e f rom the P o c k e l s C e l l . I t i n t u r n t r i g g e r s t he scope and opens the t r a n s m i s s i o n g a t e s ( F i g u r e A - l , A p p e n d i x I I ) t o a l l o w the PM p u l s e s t o pass i n t o the d e l a y c a b l e s . Each p u l s e i s t hen d e l a y e d by a d i f f e r e n t amount and d i s p l a y e d s e q u e n t i a l l y on the s c o p e . Bo th ends o f t h e t o t a l d e l a y l i n e a re t e r m i n a t e d by the l i n e ' s c h a r -a c t e r i s t i c impedance to p r e v e n t r e f l e c t i o n s . The method o f u s i n g t r a n s m i s s i o n g a t e s to p u l s e t he PM o u t p u t s appea rs s u p e r i o r to p u l s i n g the tubes them-s e l v e s . As e x p l a i n e d i n A p p e n d i x I the b e s t t e c h n i q u e s r e p o r t e d to da te have g a i n r i s e t i m e s o f 20 nsec and s p u r i o u s n o i s e b u r s t s o f a t l e a s t 100 mV. In the sys tem p r e s e n t e d he re t he g a i n r i s e t i m e i s f i v e nsec and a l l e l e c t r i c a l n o i s e i n the o u t p u t i s be low 50 mV. In a d d i t i o n the c u t o f f e f f i c -i e n c y o f the g a t e s i s v e r y h i g h , i n c o n t r a s t to one o f the b e s t p u l s e d tube methods [ 2 7 ] . light inputs from monochromator via fiber optics photomultipliers —jtlftJUb—1—JlxJUib—1—<SlMSlSLs 47 to all gates variable delay delay cables (100 nsec""^ each) tout gate pulse generator sync transmission gates signal in 6 sync, pulse from POCKELS CELL driver F i g u r e 16 . D e t e c t i o n System O p e r a t i o n . C h a p t e r 4 CONCLUSION In p r e p a r a t i o n f o r f u t u r e l a s e r l i g h t s c a t t e r i n g e x p e r i m e n t s on p lasmas a p u l s e d ruby l a s e r has been d e v e l o p e d i n c o n j u n c t i o n w i t h a m u l t i c h a n n e l s p e c t r a l a n a l y s e r f o r d e t e c t i o n o f the s c a t t e r e d l i g h t . The l a s e r , c o n s i s t i n g o f s e p a r a t e o s c i l l a t o r and o a m p l i f i e r r o d s , has a s p e c t r a l l i n e w i d t h o f 0 .08 A a t powers up to 150 M e g a w a t t s . The use o f a P o c k e l s C e l l as the Q-s w i t c h p e r m i t s a c c u r a t e s y n c r o n i z a t i o n w i t h the s p e c t r a l a n a l y s e r and a l l e x t e r n a l e l e c t r o n i c s . Fo r the m u l t i c h a n n e l d e t e c t i o n sys tem the o u t p u t s o f f i v e g a t e d p h o t o m u l t i p l i e r s a re s e q u e n t i a l l y d i s p l a y e d to g i v e an i n t e n s i t y v s . w a v e l e n g t h p r o f i l e on an o s c i l l o -scope s c r e e n . The a b i l i t y to r e c o r d f i v e d a t a p o i n t s s i m u l -t a n e o u s l y g r e a t l y s i m p l i f i e s the measurements o f s c a t t e r e d p r o f i l e s and r e d u c e s the e r r o r i n v o l v e d i n p l o t t i n g them. F u t u r e improvements to the sys tem s h o u l d i n c l u d e b e t t e r q u a l i t y o p t i c a l components i n the l a s e r and more 52 53 deve lopmen t o f the methods i n v o l v e d i n mak ing the g l a s s f i b e r s l i t s . B e t t e r o p t i c a l components would r e s u l t i n a n a r r o w e r , more s t a b l e mode p a t t e r n f o r the l a s e r . S m a l l e r g l a s s f i b e r s and pe rhaps b e t t e r s t a c k i n g t e c h n i q u e s c o u l d r e s u l t i n more a c c u r a t e s l i t s p a c i n g and l e s s l i g h t l o s s to the p h o t o m u l t i p i i e r s . REFERENCES S t e e l e , E . L . and W.C. D a v i s . 1965. " L a s e r A m p l i f i e r s J . A p p i . P h y s . , 3_6(2): 3 4 8 - 3 5 1 . B e l l m a n , R. , G. B i rnbaum and W.G. Wagner. 1963 . " T r a n s m i s s i o n o f Monoch roma t i c R a d i a t i o n i n a Two-L e v e l M a t e r i a l . " J . A p p i . P h y s . , 3 4 ( 4 ) : 780 . I z a t t , J . R . , R . C . M i t c h e l l and H. A . Daw. 1966. "The rma l Dependence o f Ruby L a s e r E m i s s i o n . " J . A p p i . P h y s . , 3 7 ( 4 ) : 1 5 5 8 - 6 2 . S imms, S . D . , A . S t e i n and C. R o t h . 1967 . "Rods Pumped by F l a s h l a m p s . " A p p i . O p t . , 6^(3): 5 7 9 - 5 8 0 . W e l l i n g , H . , C . J . B i c k a r t and H .G . A n d r e s e n . "Change o f O p t i c a l Pa th Leng th i n L a s e r Rods w i t h i n the Pumping P e r i o d . " IEEE J . Quan t . E l e c t r o n i c s , Q E - 1 ( 5 ) : 2 2 3 - 4 . G i b s o n , K . S . 1916. "The E f f e c t of Tempera tu re upon the A b s o r p t i o n Spec t rum o f a S y n t h e t i c R u b y . " P h y s . R e v . , 8 : 3 8 . B r a d l e y , D . J . , G. Magyar and M .C . R i c h a r d s o n . 1966. " I n t e n s i t y Dependent F requency S h i f t i n Ruby L a s e r G i a n t P u l s e s . " N a t u r e , 212 : 6 3 - 4 . B r a d l e y , D . J . , A .W. M c C u l l o u g h and P . D . S m i t h . 1966 . " I n t e r n a l S e l f - D a m a g e i n a 25 MW Ruby L a s e r O s c i l l a t o r B r i t . J . A p p i . P h y s . , 1 7.( 1 9 ) : 1 221 - 2 2 . M c C l u n g , F . J . and R.W. H e l l w a r t h . 1963 . " C h a r a c t e r s t i c s o f G i a n t O p t i c a l P u l s a t i o n s from R u b y . " P r o c . I E E E , 5 1 ( 1 ) : 4 6 - 5 3 . 54 55 [10 ] Wagner , W.G. and B . A . L e n g y e l . 1963 . " E v o l u t i o n of the G i a n t P u l s e i n a L a s e r . " J . A p p l . P h y s . , 34(7) : 2 0 4 0 - 4 6 . [11 ] L e n g y e l , B e l a A . 1962 . L a s e r s . John W i l e y & Sons I n c . New Y o r k . [12 ] R o s s , D. 1966. L a s e r s , L i g h t A m p l i f i e r s and O s c i l l a t o r s . Academic P r e s s , London and New Y o r k . [13 ] C o l l i n s , S . A . and G . R . W h i t e . 1963 . " I n t e r f e r o m e t e r L a s e r Mode S e l e c t o r . " A p p l . O p t . , 2: 448 . [14 ] M a g y a r , G. 1969 . "Mode S e l e c t i o n T e c h n i q u e s f o r S o l i d - S t a t e L a s e r s . " O p t i c s & L a s e r T e c h n o l o g y , 1 ( 5 ) : 2 3 1 - 3 9 . [15 ] S o o y , W. 1965 . "The N a t u r a l S e l e c t i o n o f Modes i n a P a s s i v e Q - S w i t c h e d L a s e r . " A p p l . P h y s . L e t t . , 7_: 36 . [ 16 ] C o o p e r , J . and J . R . G r e i g . A R a p i d S c a n n i n g F a b r y -P e r o t S p e c t r o m e t e r . I m p e r i a l C o l l e g e , London . [17 ] M c C l u n g , F . J . and D. W e i n e r . 1965 . " L o n g i t u d i n a l Mode C o n t r o l i n G i a n t P u l s e L a s e r s . " IEEE J . Quan t . E l e c t r o n i c s , 1 : 9 4 - 9 9 . [18 ] M a g y a r , G. 1967. " S i m p l e G i a n t P u l s e Ruby L a s e r o f H igh S p e c t r a l B r i g h t n e s s . " Rev . S c i . I n s t r . , 38_: 517 . [19 ] S t e i n , A . 1967. "Mode S e l e c t i o n f o r G i a n t P u l s e Ruby L a s e r s . " A p p l . O p t . , 6 ( 1 2 ) : 2 1 9 3 - 4 . [20 ] C o r n i n g G l a s s Works . 1969 . End F i n i s h i n g C o n s i d e r -a t i o n s . [21 ] G u n t e r , W.D. J r . , E . F . E r i c k s o n and G . R . G r a n t . 1965. "Enhancement o f P h o t o m u l t i p i i e r S e n s i t i v i t y by T o t a l I n t e r n a l R e f l e c t i o n . " A p p l . O p t . , 4 ( 4 ) : 512 . 56 [22] G r a n t , G . R . , W.D. G u n t e r , J r . and E . F . E r i c k s o n . 1965 . " H i g h A b s o l u t e P h o t o c a t h o d e S e n s i t i v i t y . " Rev . S c i . I n s t r . , 36_: 1 5 1 1 - 1 2 . [23 ] S i z e l o v e , J . R . and J . A . Love I I I . 1967 . " A n a l y s i s o f a M u l t i p l e R e f l e c t i v e T r a n s l u c e n t P h o t o c a t h o d e . " A p p i . Op t . , 6 (3 ) : 4 4 3 - 6 . [24 ] J e n n i n g s , R . J . , W.D. G u n t e r , J r . and G . R . G r a n t . Quantum E f f i c i e n c i e s G r e a t e r than 50% f rom C o m m e r c i a l l y A v a i l a b l e P h o t o m u l t i p i i e r s . Ames R e s e a r c h C e n t r e , NASA, M o f f e t t F i e l d , C a l i f . , 94035 . [25 ] RCA. T e c h n i c a l Manual P T - 6 0 . P h o t o t u b e s and P h o t o c e l 1 s . [26 ] O k e , J . B . and R . E . S c h i l d . 1968. "A P r a c t i c a l M u l -t i p l e R e f l e c t i o n T e c h n i q u e f o r Imp rov ing the Quantum E f f i c i e n c y o f P h o t o m u l t i p i i e r T u b e s . " A p p i . O p t . , 7 ( 4 ) : 6 1 7 - 2 2 . [27 ] De M a r c o , F. and E. P e n c o . 1969 . " P u l s e d P h o t o -m u l t i p l i e r s . " Rev . S c i . I n s t r . , 40_(9): 1 1 5 8 - 6 0 . [28 ] F a r i n e l l i , U and R. M a l v a n o . 1958 . " P u l s i n g o f P h o t o -m u l t i p l i e r s . " Rev . S c i . I n s t r . , 29^(8): 6 9 9 - 7 0 1 . [29 ] De M a r t i n i , F. and K . P . Wacks . 1967. " P h o t o m u l t i p i i e r Gate f o r S t i m u l a t e d Spon taneous L i g h t S c a t t e r i n g D i s c r i m i n a t i o n . " Rev . S c i . I n s t r . , 38 . (7 ) : 8 6 6 - 6 8 . [30 ] S u e m a t s u , Y . , K. Normura and E. T o m i t a . 1968. "Measurement o f D e l a y Time D i f f e r e n c e s on the P h o t o -ca thode S u r f a c e o f a P h o t o m u l t i p i i e r . " P r o c . I E E E , 5 6 ( 8 ) : 1 4 0 5 - 6 . [31 ] [32 ] EMI . An I n t r o d u c t i o n to the P h o t o m u 1 t i p 1 i e r . P o s t , R . F . 1952 . "The P e r f o r m a n c e o f P u l s e d P h o t o -m u l t i p l i e r s . " N u c l e o n i c s , 1_0: 4 6 . 57 [33 ] S i n g e r , S . , L . K . Neher and R. R u e h l e . " P u l s e d P h o t o -m u l t i p l i e r s f o r F a s t S c i n t i l l a t i o n C o u n t i n g . " Rev . S c i . I n s t r . , 27 : 4 0 . [34 ] E l p h i c k , B . L . 1959 . "A Method of A p p l y i n g an A v a l a n c h e T r a n s i s t o r G e n e r a t e d 70 ns G a t i n g P u l s e to a Focused P h o t o m u l t i p l i e r . " J . S c i . I n s t r u m . ( J . P h y s . E) , 2: 9 5 3 - 5 5 . APPENDIX I COMPARISON OF PHOTOMULTIPLIER PULSING TECHNIQUES In t he c o u r s e o f d e v e l o p i n g a s u i t a b l e p h o t o m u l -t i p l i e r g a t e , a s u r v e y was done o f the e x i s t i n g t e c h n i q u e s . T h i s i s p r e s e n t e d be low and t he r e l e v a n t t e r m i n o l o g y i s as fo l1ows : gain visetime : l e n g t h o f t i m e r e q u i r e d t o s w i t c h g a i n o f t u b e f r o m 10$ t o 90% o f i t s f u l l v a l u e . cutoff e f f i c i e n c y : r a t i o o f o u t p u t f o r t u b e ON t o o u t p u t f o r t u b e O F F . gain a m p l i f i c a t i o n : i n c r e a s e in g a i n o f t u b e o v e r n o r m a l n o n - p u l s e d o p e r a t i o n . As w e l l , K r e f e r s t o the ca thode and D n to the n t t L dynode o f a t u b e . 58 59 P u l s i n g Whole R e s i s t o r C h a i n In normal o p e r a t i o n the g a i n o f a p h o t o m u l t i p i i e r i s l i m i t e d by the maximum i n t e r - d y n o d e v o l t a g e t h a t can be a p p l i e d b e f o r e a r c i n g o c c u r s . However , i f the v o l t a g e to the dynode c h a i n i s s u p p l i e d as a s u f f i c i e n t l y s h o r t p u l s e , t h i s maximum v o l t a g e may be s a f e l y exceeded owing to the low m o b i l i t y o f i o n s r e l a t i v e to e l e c t r o n s [ 3 2 ] . As shown by P o s t [32 ] o v e r v o l t i n g the tube i n such a manner can l e a d to a s u b s t a n t i a l a m p l i f i c a t i o n o f g a i n . P u l s i n g a 931A w i t h a 4 kv p u l s e 2 . 5 usee l o n g r e s u l t e d i n an o v e r a l l g a i n o f 1 0 9 , an i n c r e a s e o f 2 x 1 0 3 o v e r the normal g a i n f o r t h i s t u b e . S i n g e r et al. [ 33 ] have r e p o r t e d s i m i l a r r e s u l t s f rom the same t y p e o f t u b e , and De Marco and Penco [27 ] r e p o r t a g a i n a m p l i f i c a t i o n o f 1 0 3 by d r i v i n g an RCA 7265 w i t h a 6 kv p u l s e . The g a i n r i s e t i m e i n t h i s l a t t e r case i s c l a i m e d to be 10 n s e c . T h i s i s i n d i r e c t c o n t r a s t to the work o f F a r i n e l l i and Ma lvano [28 ] who show t h a t f a s t r i s e s s h o u l d be s e v e r e l y l i m i t e d by the ca thode r e s i s t i v i t y . In a d d i t i o n to the advan tage o f g a i n a m p l i f i c a -t i o n p u l s i n g the who le dynode c h a i n a l s o has an e x c e l l e n t c u t o f f e f f i c i e n c y , t y p i c a l l y 1 0 6 [ 2 7 ] . D e s p i t e t h e s e advan tages t h i s method has s e v e r a l s e v e r e d i s a d v a n t a g e s wh ich make the t e c h n i q u e g e n e r a l l y u n a c c e p t a b l e : (a) the p u l s e g e n e r a t o r must s u p p l y f a s t , 60 w e l l - f o r m e d h i g h v o l t a g e p u l s e s w i t h n e g l i g i b l e j i t t e r i n t o a low impedance l o a d ; (b) the g a i n r i s e t i m e i s l i m i t e d by t he ca thode r e s i s t i v i t y [ 2 8 ] ; ( c ) l a r g e n o i s e s i g n a l s a re p r e s e n t i n the o u t p u t [ 2 7 ] ; (d) a t h i g h o v e r v o l t a g e s some tubes have a t endency t o o s c i l l a t e [ 3 3 ] . P u l s i n g F o c u s i n g E l e c t r o d e F a r i n e l l i and Ma lvano [28 ] r e p o r t t h i s the most c o n v e n i e n t method f o r them when u s i n g a Dumont 6292 p h o t o -t u b e . No i n d i c a t i o n o f the n o i s e i n t r o d u c e d i n the o u t p u t i s g i v e n and the c u t o f f e f f i c i e n c y seems v e r y g o o d . E l p h r i c k [34] p u l s e s the f o c u s o f an RCA 6810 w i t h a s p e c i a l a v a l a n c h e t r a n s i s t o r c i r c u i t and shows g a i n r i s e t i m e s o f 10 n s e c . He p o i n t s out t h a t b i a s i n g the f o c u s e l e c t r o d e to p h o t o -c a t h o d e p o t e n t i a l and then p u l s i n g i t p o s i t i v e r e s u l t s i n l a r g e n o i s e t r a n s i e n t s due to space cha rge a c c u m u l a t i o n s . I n s t e a d , he b i a s e s the f o c u s i n g e l e c t r o d e to the second dynode p o t e n t i a l and p u l s e s i t n e g a t i v e l y . No n o i s e m e a s u r e -ments a re g i v e n and a g a i n the c u t o f f e f f i c i e n c y seems v e r y g o o d . P u l s i n g C a t h o d e - F i r s t Dynode S t a g e Two p o s s i b i l i t i e s a re p r e s e n t h e r e : 61 1. P u l s i n g K Negative with Respect to DI. A l t h o u g h an e x c e l l e n t c u t o f f e f f i c i e n c y can be a c h i e v e d and the n o i s e i n the o u t p u t can be made v e r y l o w , t h i s method i s not a d v i s a b l e because of i t s poor g a i n r i s e -t i m e . As e x p l a i n e d by F a r i n e l l i and Ma lvano [28 ] and c o n -f i r m e d by De M a r t i n i and Wacks [29 ] and the work i n t h i s t h e s i s , a v o l t a g e p u l s e r e q u i r e s c o n s i d e r a b l e t ime to t r a v e l a c r o s s the p h o t o c a t h o d e , wh i ch i s a t h i n l a y e r o f s e m i c o n -d u c t i n g m a t e r i a l . In a d d i t i o n , the tube g a i n i s most s e n s i t i v e t o f l u c t u a t i o n s i n the K-D l s t a g e w h i c h s h o u l d p r e f e r a b l y be v o l t a g e s t a b i l i z e d . 2. P u l s i n g DI P o s i t i v e with Respect to K. De M a r t i n i and Wacks [29] r e p o r t a method o f p u l s i n g an RCA 7265 d e t e c t o r w i t h a p o s i t i v e s q u a r e p u l s e a p p l i e d t o the f i r s t dynode w h i c h i s b i a s e d n e g a t i v e w i t h r e s p e c t to the c a t h o d e . An e x c e l l e n t c u t o f f e f f i c i e n c y i s a c h i e v e d and the g a i n r i s e t i m e i s c l a i m e d to be b e t t e r t han 20 n s e c . However , no n o i s e measurements a re g i v e n , no m e n t i o n i s made o f space cha rge t r a n s i e n t s and the ga te p u l s e w i d t h i s v e r y l o n g (100 y s e c ) . 62 P u l s i n g C a t h o d e and F i r s t Two D y n o d e s As found by De Marco and Penco [27J p u l s i n g K-D1-D2 w i t h a n e g a t i v e p u l s e has the same c h a r a c t e r i s t i c s as t h o s e o u t l i n e d above f o r p u l s i n g the ca thode a l o n e . The c u t o f f e f f i c i e n c y i s v e r y good and the g a i n r i s e t i m e i s p o o r . E l p h i c k [34 ] a l s o r e p o r t s a method i n wh i ch D2 i s b i a s e d to the p o t e n t i a l o f DI and then p u l s e d p o s i t i v e . He c o n -s i d e r s t h i s u n s a t i s f a c t o r y because o f poor g a i n r i s e t i m e . P u l s i n g L a s t Dynode S t a g e s De Marco and Penco [27 ] recommend a c i r c u i t t hey d e v e l o p e d to p u l s e dynodes D12 and D13 o f an RCA 7265 t u b e . They show g a i n r i s e t i m e s o f 20 nsec and ga te p u l s e s 300 nsec l o n g . However the d i s a d v a n t a g e s a re as f o l l o w s : a) t h e O N / O F F r a t i o f o r c u t o f f e f f i c i e n c y i s o n l y a b o u t 3 0 0 , b) e v e n w i t h e l a b o r a t e s h i e l d i n g t h e o u t -p u t n o i s e i s g r e a t e r t h a n 200 mV, c ) t h e method r e q u i r e s a 400 v o l t s q u a r e p u l s e o f a p p r o p r i a t e w i d t h . 63 C o u p l i n g Anode S i g n a l T h r o u g h T r a n s m i s s i o n G a t e I n s t e a d of s w i t c h i n g the PM on and o f f a d i f f e r e n t app roach i s to ga te the anode s i g n a l f o r the d e s i r e d t i m e . T h i s l e a v e s t he tube r u n n i n g c o n t i n u o u s l y and e l i m i n a t e s the d i f f i c u l t i e s of t r a n s i e n t n o i s e and g a i n r i s e t i m e s a s s o -c i a t e d w i t h the p r e v i o u s t e c h n i q u e s . However , the method i s not u s e f u l f o r s i t u a t i o n s where the PM must be p u l s e d to p r e v e n t o v e r l o a d i n g . F a r i n e l l i and Ma lvano [28 ] men t i on e l e c t r o n i c g a t i n g and d i s c o u n t i t as b e i n g i m p r a c t i c a l . S i n c e then improved t e c h n o l o g y has made the method f a r more a t t r a c t i v e . U s i n g the g a t e s d e v e l o p e d f o r t h i s t h e s i s (Append i x I I ) t he p u l s e d p h o t o m u l t i p i i e r s have the f o l l o w i n g c h a r a c t e r i s t i c s : a) c u t o f f e f f i c i e n c y i s as good as f o u n d by p u l s i n g p h o t o c a t h o d e ( p r o b a b l y g r e a t e r t h a n \0h), b) t h e e f f e c t i v e g a i n r i s e t i m e i s l e s s t h a n f i v e n s e c . T h i s i s t h e t u r n - o n t i m e f o r t h e g a t e s , c ) o u t p u t n o i s e i s l e s s t h a n 50 mV w h i c h i s c o n s i d e r a b l y l e s s t h a n f o r any o t h e r s y s t e m p r e s e n t e d i n t h e l i t e r a t u r e . APPENDIX I I PHOTOMULTIPLIER ELECTRONICS Each s e t o f dynode c h a i n r e s i s t o r s i s w i r e d as a s t a n d a r d c o n f i g u r a t i o n w i t h two m ino r e x c e p t i o n s . F i r s t l y , t he p o t e n t i a l o f t he f o u r t h dynode (D4) w i t h r e s p e c t to D5 i s v a r i a b l e f rom 33% to 100% of t he ave rage i n t e r d y n o d e v o l t a g e . T h i s a l l o w s a 30% v a r i a t i o n i n the g a i n o f each t u b e . S e c o n d l y , t he anode l o a d r e s i s t a n c e i s p a r t o f t he t r a n s m i s s i o n ga te c i r c u i t and i s a d j u s t a b l e . Wi th a b i a s o f - 1500 v o l t s the c h a i n c u r r e n t pe r tube i s about 1 ma. A l l the dynode c h a i n s a re c o n n e c t e d i n p a r a l l e l to the same h i g h v o l t a g e s u p p l y . The anode o f each tube i s c o n n e c t e d to a s e p a r a t e t r a n s m i s s i o n ga te c i r c u i t shown i n F i g u r e ( A - l ) . A l l f i v e g a t e s a re i d e n t i c a l and a l l a re d r i v e n i n p a r a l l e l by a p u l s e g e n e r a t o r d e s c r i b e d l a t e r . The h e a r t o f t he ga te i s a h i g h s p e e d , u n i t y g a i n b u f f e r d e s i g n a t e d LH0033 by the m a n u f a c t u r e r , N a t i o n a l S e m i c o n d u c t o r C o r p o r a t i o n . The c h a r -a c t e r i s t i c s t h a t make i t p a r t i c u l a r l y a t t r a c t i v e a re as f o l l o w s : i n p u t impedance of 1 0 1 1 ohms, o u t p u t impedance 64 F i g u r e A - l . T r a n s m i s s i o n Gate C i r c u i t . 66 o f 6 ohms, bandw id th o f TOO MHz. W h i l e not o r i g i n a l l y i n -tended to be used w i t h a p u l s e d power s u p p l y , i t pe r f o rms v e r y w e l l i n the c i r c u i t g i v e n . I t s o n l y i n c o n v e n i e n c e i s the r e q u i r e m e n t o f two s u p p l y v o l t a g e s p o s i t i v e and n e g a t i v e w i t h r e s p e c t to g r o u n d . Thus t u r n i n g the ga te on f o r 100 nsec r e q u i r e s two s i m u l t a n e o u s s q u a r e c u r r e n t p u l s e s each 100 nsec l o n g and o f o p p o s i t e p o l a r i t y . R e f e r r i n g to F i g u r e ( A - l ) the IK p o t e n t i o m e t e r between i n p u t and g round s e r v e s as the anode l o a d o f the PM. T h i s a d j u s t m e n t v a r i e s the o u t p u t o f each ga te and to some e x t e n t the r i s e t i m e . The d r i v i n g p u l s e s e n t e r t h r o u g h p r o p e r l y t e r m i n a t e d c o a x i a l c a b l e s f rom the p u l s e g e n e r a t o r t o p i n s 10 and 12 o f the b u f f e r . Damping r e s i s t o r s o f 100 ohms each c o n n e c t t h e s e i n p u t s to p i n s 9 and 1 r e s p e c t i v e l y to a l l o w the b u f f e r to d r i v e l o n g c o a x i a l c a b l e s . The 100 ohm po t between p i n s 7 and 10 p r o v i d e s a d j u s t m e n t f o r o f f s e t n u l l . Ou tpu t i s t h r o u g h an i s o l a t i o n r e s i s t o r to a s t a n d a r d c o a x i a l c o n n e c t o r . The o u t p u t f rom the b u f f e r i s a c o m b i n a t i o n o f the i n p u t s i g n a l and the a l g e b r a i c a d d i t i o n o f the two s u p p l y v o l t a g e s . N o r m a l l y the s u p p l y v o l t a g e s would be DC and any v a r i a t i o n between them wou ld appear as a DC l e v e l i n the o u t p u t , wh i ch c o u l d be removed w i t h the o f f s e t a d j u s t . But i n t h i s a p p l i c a t i o n the s u p p l i e s a re p u l s e d so i t i s e s s e n t i a l 67 t h a t the two o p p o s i t e g o i n g p u l s e shapes be as n e a r l y i d e n -t i c a l as p o s s i b l e . Any v a r i a t i o n s i n the t ime o f a r r i v a l or i n the r i s e t i m e s o f d r i v i n g p u l s e s w i l l show up as n o i s e s p i k e s i n the o u t p u t . In a d d i t i o n , each b u f f e r r e q u i r e s t h a t the p u l s e s have an a m p l i t u d e o f a t l e a s t ±5 v o l t s and s i n c e a l l a re d r i v e n i n p a r a l l e l t h i s means t h a t the g e n -e r a t o r must work i n t o a s o u r c e impedance o f 10 ohms. The c i r c u i t o f the g a t i n g p u l s e g e n e r a t o r t h a t meets t h e s e s p e c i f i c a t i o n s i s shown i n F i g u r e ( A - 2 ) . H igh speed t r a n s i s t o r s a re used i n an a v a l a n c h e mode to d i s c h a r g e c a b l e s o f 50 nsec e l e c t r i c a l l e n g t h . Two t r a n s i s t o r s p r o -duce the p o s i t i v e and n e g a t i v e g o i n g p u l s e s w h i l e a t h i r d i s used w i t h a c a p a c i t o r to p r o v i d e a sync p u l s e o u t p u t . The two d r i v i n g p u l s e s a re a d j u s t e d to be e q u a l i n w i d t h by v a r y i n g the l e n g t h s o f the d i s c h a r g e c a b l e s . The o v e r a l l c h a r a c t e r i s t i c s o f the g e n e r a t o r a re as f o l l o w s : I n p u t t r i g g e r I eve I : O u t p u t p u l s e a m p l i t u d e s O u t p u t p u l s e w i d t h s : Outp ut r i s e t i mes : O u t p u t f a l l t i m e s : S y n c p u l s e a m p l i t u d e : S y n c p u l s e w i d t h : S y n c p u l s e r i s e t i m e : 300 mV ± 5 V i n t o 10 ohm Ioad 100 ns I e s s t h a n 2 n s e c 5 n s e c + 10 V i n t o 50 ohm Ioad a p p r o x . I y s e c 5 n s e c F i g u r e A-2. A v a l a n c h e T r a n s i s t o r Gate P u l s e G e n e r a t o r . 69 F i g u r e (A -3 ) shows the power s u p p l y f o r the ga te p u l s e g e n e r a t o r . The v o l t a g e s a re a d j u s t e d to g i v e r e l i -a b l e a v a l a n c h e o p e r a t i o n and equa l o u t p u t p u l s e a m p l i t u d e s . F i g u r e A - 3 . Power Supp l y f o r Gate P u l s e G e n e r a t o r . o