DEVELOPMENT OF A LASER AMPLIFIER COMBINATION OSCILLATORAND A M U L T I - CHANNEL SPECTRAL DETECTION FOR LIGHT SCATTERING SYSTEM EXPERIMENTS by GARY GEORGE B.Sc, A THESIS University SUBMITTED ALBACH of Waterloo, IN PARTIAL THE REQUIREMENTS 1970 FULFILMENT OF FOR THE DEGREE OF MASTER OF SCIENCE in t h e Department of PHYSICS We accept required this thesis as c o n f o r m i n g tothe standard THE U N I V E R S I T Y OF B R I T I S H July, 1972 COLUMBIA In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s thesis f o r s c h o l a r l y purposes may by h i s representatives. be granted by the Head of my Department or I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y o,f B r i t i s h Columbia Vancouver 8, Canada ABSTRACT In 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 been developed a pulsed ruby in conjunction with a multichannel a n a l y s e r f o r d e t e c t i o n of the s c a t t e r e d The l a s e r has spectral light. l a s e r , c o n s i s t i n g of separate oscillator and o a m p l i f i e r r o d s has up t o 100 Q-switch a s p e c t r a l line width Megawatts. permits a n a l y s e r and The o f .08 A a t p o w e r s use o f a P o c k e l s Cell accurate synchronization with as the the s p e c t r a l a l l external electronics. For the m u l t i c h a n n e l o p t i c s s l i t bundles transmit detection system f i v e l i g h t from the output fiber of a monochromater to f i v e p h o t o m u l t i p i i e r tubes, which are on f o r 100 nsec d u r i n g the l a s e r p u l s e . The pulses gated are 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 profile 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 Chapter 1 INTRODUCTION . 1 •. 3 PART I 2 THE LASER SYSTEM 2.1 Laser A m p l i f i e r Theory 3 2.2 O s c i l l a t o r Design 8 2.2.1 S p e c t r a l W i d t h 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 2.2.3 Mode S e l e c t i o n 2.3 O s c i l l a t o r Performance . 10 12 . 17 2.3.1 S p e c t r a l Width . 17 2.3.2 Pulse C h a r a c t e r i s t i c s 25 2.3.3 Summary iii . 26 Chapter Page 2.4 2.5 A m p l i f i e r Performance 2.4.1 Energy 2.4.2 Pulse Gain . Distortion . 34 II THE DETECTION SYSTEM 37 3.1 General Outline 3.2 Fiber Optics. 39 3.3 Photomultipliers. 41 3.3.1 Design Considerations 41 3.3.2 Performance 46 3.4 4 27 32 Discussion PART 3 27 of Polychrometer. . . . . . 37 Discussion 50 DISCUSSION 52 REFERENCES • • 54 . 58 APPENDICES I II COMPARISON OF PHOTOMULTIPLIER PULSING TECHNIQUES PHOTOMULTIPLIER ELECTRONICS iv 64 LIST OF FIGURES Figure 1. Page Pockels Cell Configuration Operation. . f o r \ Wave 11 2. Optics f o r Analysis of Spectral Charact e r i s t i c s of Laser O s c i l l a t o r Output 17 3. O s c i l l a t o r Output: Confocal 21 4. O s c i l l a t o r Output: (75% f r o n t ) Plane 5. O s c i l l a t o r Output: Plane Mirrors ( 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 Output: and Dye C e l l 23 7. Fractional Population a)s,t>) Plane Resonator Mirrors Mirrors 21 Inversion vs. Pumping E n e r g y , 31 8. A m p l i f i e r Gain v s . Pumping E n e r g y 9. L a s e r A m p l i f i e r P u l s e s , a) b) O u t p u t , N o r m a l i z e d Input, t o Same E n e r g y 10. Laser Operation 11. F i b e r Optics S l i t Package 33 34 . 35 v 40 Fi gure 12. Page Photomultipiier of Total End-Window Showing Method Internal Reflection 41 13. C i r c u i t of LED Driver . 47 14. Noise in PM Gating Electronics . 49 15. Photomultipiier Outputs: a) DC Light Input, b) 30 nsec Light Pulse. 16. Detection System Operation . . . A-l Transmission Gate C i r c u i t A-2 Avalanche Transistor Gate Pulse Generator. A-3 Power Supply for Gate Pulse Generator vi 49 51 . . . . 65 68 70 ACKNOWLEDGEMENTS I wish t o thank Dr. J . Meyer 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 work. 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 Plasma Physics Group I would technical during discussion. 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 a s s i s t a n c e o f Mr. D. S i e b e r g development I would for f o r many h o u r s o f u s e f u l a n d Mr. J . Z a n g a n e h of the detection e l e c t r o n i c s . a l s o l i k e t o acknowledge Shari Haller a fine job i n the typing of this thesis. Financial Council a s s i s t a n c e from the National is gratefully acknowledged. T h i s work i s s u p p o r t e d by a g r a n t Atomic Energy Control Research Board o f Canada. vi i from t h e Chapter 1 INTRODUCTION In any l a s e r l i g h t essential itself The s c a t t e r i n g experiment the two pieces o f d i a g n o s t i c equipment are the l a s e r and the d e t e c t i o n system f o r the s c a t t e r e d work presented here i s the development and o p e r a t i o n of a complete system, c o n s i s t i n g of a pulsed c o n j u n c t i o n with light. a multichannel ruby l a s e r i n spectral analyser, f o r plasma d i a g n o s t i c s . As o r i g i n a l l y used to study dictated pulsed proposed, the apparatus was t o be plasmas i n magnetic f i e l d s . t h a t the l a s e r have the f o l l o w i n g 1) high 2) reproduceible 3) accurate synchronization t o external electronics Chapter 2 describes This goal characteristics: spectral brightness pulses the design of the l a s e r system. c o n s i d e r a t i o n s and performance The f i n a l design inch ruby rod i n a low power o s c i l l a t o r 1 c o n s i s t s of a t h r e e with a Pockels Cell 2 Q-switch. This amplifier. The (1) the is followed various relevant theory (2) as i n c l u d i n g the lator performance; performance; (5) The and consists separate Fiber (4) detection an with nsec during an construction of the detection Appendix u s e d by others desired II d e t a i l s that were developed The laser as a amplifier a spectrum. output which are The 3 in Chapter from the sections fiber oscil- laser. light of a gated pulses are on displayed profile in Chapter 3 optics bundles, photomultipiiers I reviews the techniques i n t e n s i t y vs. wavelength and an deal the evaluation whole. the techniques photomultipiiers. shown to have s e r i o u s Appendix light built each r e c o r d i n g scattered laser pulse. the to pulse laser laser system i s explained of the system of the complete photomultipiiers to give the with: l a s e r was of the of the transmit the method used to pulse are evaluation oscilloscope screen. the evaluation photomultipiiers optics bundles sequentially on an segment of the 100 an a discussion of f i v e the power deal to evaluate reasons a r e l e v a n t mode s e l e c t i o n (3) monochromater to the for the as 2 of Chapter necessary performance; t h a t were c o n s i d e r e d ; a s i x inch rod sections amplifier i t was by drawbacks operation that A l l of i f applied of the to overcome these new have been these to t h i s pulsing difficulties. system. circuits P A R T I Chapter 2 THE LASER SYSTEM 2.1 Laser Amplifier Theory So much h a s b e e n w r i t t e n the theory discuss o f laser operation i t i ndetail treatment, t o provide analysis form pass gain a simplified [ 1 ] , w i l l be background for the will yield an e x p r e s s i o n inversion with problem i s formulated t o the basic three approximation In f o r the o f the laser amplifier and the functional o f the population mation Instead and Davis the necessary the theory The tion would be f u t i l e . about here t o i n the s e c t i o n on A m p l i f i e r Performance. particular single that any attempt due l a r g e l y t o Steele presented i nrecent years level i t i s assumed t h a t pump energy. as a two l e v e l system f o r ruby. the non-radiative approxiIn this transi- 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 Defining: state; a l l other t r a n s i t i o n rates N , the electron population x N , the electron population 2 3 f o r the system. density i n the ground density i n the upper 4 excited s t a t e ; N 0 = Ni + N N , the e l e c t r o n 3 + N , 2 the t o t a l 3 the two l e v e l approximation p o p u l a t i o n in the pump l e v e l s ; electron assumes N population 3 density; = 0 so t h a t No = Ni + N . 2 The standard energy balance equations for r a t e s of change of e l e c t r o n d e n s i t y i n the two l a s e r the levels are given a s : = N acp - Nicrcp = ac p ( N 2 - Nj N -cp - N acp = - a c p ( N 2 - Ni) 2 d N - z dt where: l0 2 p = photon d e n s i t y c a u s i n g emission [photons o = radiation for C r + 3 n = N d e n s i t y the 2 - N x cm" ] 3 absorption cross-section [cm* ] 2 c = v e l o c i t y of l i g h t Defining - stimulated [cm-sec ] as the e l e c t r o n two r a t e equations give: - 1 population inversion 5 In order to derive an expression for the gain i t i s necessary to determine the photon density as a function of position and time. Using: = rate of change of photon density i n volume dv canp = rate at which photons are generated by t r a n s i t i o n s 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 incident on the laser material at x 0 x = 0 and density and t = 0 p 0 6 B e l l m a n , B i r n b a u m a n d Wagner [2] have shown t h a t t h e s o l u t i o n o f t h e above equations f o r the photon density i s : -1 1 p ( x , t ) = po * where time n - £ - e - a n exp£-2a °^| P o c(t - x/cfj i s the inverted electron population 0 • density at t = 0 . The length L total will energy gain be g i v e n G i n a laser material of by t h e r a t i o o f e m e r g i n g d e n s i t y t o i n c i d e n t photon photon density: • CO G = Substituting for G = 1 2ap 0 CT p(L,t) 1 0 1 PO + To p(L,t) dt and i n t e g r a t i n g y i e l d s £exp(2ap CT ) 0 0 - fj exp(n aL) 0 (2.1) 7 To determine the dependence of population inversion 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 " to give n = N 0 2 - N 0 B E 3 e~^ = KT E Since the upper laser level in ruby is r e a l l y a double level only one-half of the electrons pumped into i t are available for laser a c t i o n . Accordi ngly: N 2 = Jj(N = %N - ^(N 0 - Ni) 0 e" ) BE 0 And f i n a l l y : n fcNoO - 3 e " ) 3E 0 (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 (2.1) a n d (2.2) w i l l be u s e d t o e v a l u a t e equations theparticular case o f a s i x inch ruby r o d . 2.2 O s c i l l a t o r Design 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 thelaser required 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 have t h e following specifications: o 1) s p e c t r a l width < 0.5 2) power output > 75 3) reproduceable pulses i) a c c u r a t e s y n c r o n i z a t i o n t o other equipment i Using should these stood by a s i x inch ruby rod as The reasoning which l e d t o t h i s choice i n t h e f o l l o w i n g s e c t i o n a n d c a n b e s t be u n d e r - by f i r s t considering l a t e r requirements three 2.2.1 that the laser i n c h ruby 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 is described MW c r i t e r i a i t was d e c i d e d consist o f a three a power a m p l i f i e r . A Spectral r e q u i r e m e n t s one a n d two a n d and four. W i d t h a n d 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 a s a h i g h o s c i l l a t o r , heating effects severely l i m i t the s p e c t r a l power width 9 that the c a n be a t t a i n e d . effects by t h e b i r e f r i n g e n c e due t o [5] a n a l y s e the pumping of period interference problem of s e e n up t o of the changes of patterns. rod. effects path and f r a m i n g effect They show t h a t the optical S i m s et al. and i t s 18 s e c o n d s a f t e r birefringence caused stresses using streak birefringence with produce: b) the and t h a t can deal These g r a d i e n t s , gradients. pumping, [3,4,5] v a r i a t i o n s i n o p t i c a l path l e n g t h polarization is authors a) et al. pictures the thermal flashlamp Welling within of Several camera [4] c o n s i d e r on t h e output considerable flashlamp length pulse are reduced w i t h effect is over shorter rods. As w e l l rod, heating sible In lines laser the variations is pumping in the is line, best the same d e p e n d e n c e . verified this patterns showing fulfilled the More r e c e n t l y thermal the at the should Fabry-Perot dependence of absorp- condition peak o f Izatt ruby. the S i n c e the ruby respon- scheme f o r peak o f oscillations assumption with the that level dependent. ruby changes i n p r o c e s s can a l s o be energy discovered temperature action fluorescence the during 1916 G i b s o n [ 6 ] tion for for as c a u s i n g p h y s i c a l the exhibit et al. [3] have interference the laser emission. 10 S m a l l e r , lower tages besides the reduced power rods have s e v e r a l o t h e r problems o f heating. [7] d e s c r i b e measurements o f frequency advan- B r a d l e y et al. s h i f t s i n ruby pulses 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 with lower energy oscillator outputs. T h e 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 surfaces within the 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 ruby r o d . In 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 ruby q u a l i t y a l s o causes increased pulse length, p o w e r a n d i n c r e a s e d beam d i v e r g e n c e [8]. decreased At lower 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 2.2.2 lives. 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 from requirements of having reproduceable 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 l e c t r o n i c s place a great deal o f importance and powers o p e r a t i o n o f the Q-switch t o be u s e d . pulses external on t h e c h o i c e Since the tech- n i q u e was f i r s t d e s c r i b e d b y M c C l u n g a n d 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 methods have been d e v i s e d . system The a Pockels reasons 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 For this element. 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 are o u t l i n e d below. 11 For the Pockels e f f e c t , the birefringence of a material varies l i n e a r l y with applied e l e c t r i c f i e l d . Uni- axial crystals such as KDP, KD*P and ADP are generally used with t h e i r optical axis along the l i g h t beam. Effects such as Raman scattering are not observed and the control for voltage a KDP c e l l i s t y p i c a l l y 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 f o r quarter wave operation of the Pockels C e l l . Seven k i l o v o l t s on the KDP CRYSTAL REAR REFLECTOR Figure 1 POLARIZER RUBY i FRONT REFLECTOR Pockels C e l l Configuration f o r % Wave O p e r a t i o n . plates i s s u f f i c i e n t to rotate the plane of p o l a r i z a t i o n of the ruby emission by 45°. After r e f l e c t i o n from the 99% rear mirror and a further rotation of 45° the l i g h t i s blocked by the crossed p o l a r i z e r on i t s return path. re-establish a high cavity Q a thyratron drops the To 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 increase can proceed r e l a t i v e l y pulsing but this slowly (~ several micro- seconds) since the recovery time of the laser after a giant pulse is on the order of 10 usee emitting [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 c e l l in a low power o s c i l l a t o r , problem is avoided. In this system more power is this obtained by following the o s c i l l a t o r with a six inch ruby rod as an amp!i f i er. 2.2.3 Mode Selection As mentioned e a r l i e r some methods must be used to control the resonant modes of the laser cavity in order to produce the desired output. two general (a) categories: transverse modes w h i c h geometrical (b) axial to These modes are c l a s s i f i e d in the Perot "round correspond trip" various configurations, ( l o n g i t u d i n a l ) modes w h i c h resonance to conditions for correspond a Fabry- eta I on. The following is a consideration of some of the most common mode control techniques and their applicability to this system. 13 By p l a c i n g a s m a l l a p e r t u r e Apertures: inside 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 a n b e r e d u c e d o n l y l o w o r d e r t r a n s v e r s e modes t o e x i s t . This t o allow reduction i n mode v o l u m e i s a c c o m p a n i e d b y a r e d u c t i o n i n v o l u m e o f active material output. used and hence by a r e d u c t i o n i n energy For t h i s reason Mirror tion will a p e r t u r e s were n o t used. Separation: also decrease b y AX = A / 2 L 2 separa- t h e F r e s n e l number a n d l i m i t t h e number o f t r a n s v e r s e modes. are spaced Increasing the mirror B u t b e c a u s e t h e a x i a l modes where 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 have t h e fewest 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 the usual and number o f as p o s s i b l e , p r a c t i c e i s t o have the s h o r t e s t p o s s i b l e c a v i t y 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 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 rod i s i n v e r t e d f i r s t . ways. a t the c e n t r e o f t h e ruby As t h e l o w o r d e r t r a n s v e r s e modes are r e s t r i c t e d t o the centre o f the r o d , these w i l l begin to lase f i r s t . Higher 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 - section o f the rod being i n v e r t e d . T h u s i f pump p o w e r i s 14 kept near t h r e s h o l d will lase. final This Lowering the in this cavity to second resonant mode d e n s i t y at act pumping power a l s o l o w e r s the life of cavity than for with shorter the main c a v i t y . both c a v i t i e s . Use o f planes o f f e r s However, f o r coated for a high in a drastic reduction the the first inside the mode s e l e c t o r . This l e n g t h must have a s m a l l e r O s c i l l a t i o n s can the only resonant con- two e t a l o n s , t i l t e d both a x i a l and in transverse reasonable f i n e s s e the p l a t e s reflectivity of [13] Fabry-Perot etalon as a transmission be (60-80%) o u t p u t power which [14]. must results This technique tried. Resonant as the "Osci11ator those f r e q u e n c i e s which s a t i s f y selection. not section C o l l i n s and W h i t e Filters: two p e r p e n d i c u l a r was the case extends suggested p l a c i n g a t i l t e d dition t r a n s v e r s e modes surfaces. Etalon occur order t e c h n i q u e was s u c c e s s f u l l y u s e d i n output which optical laser lowest d e s i g n as e x p l a i n e d i n Performance." laser only the a reflection An e t a l o n Reflectors: mode s e l e c t o r if it forms can a l s o be used the output mirror 15 of the cavity [18]. stacked together In the the reflectivity R = where N = number o f n = index of The r e s o n a t o r lated only all Q of the resonant rather This d / n ) 1 + (l/n) refraction tuned to of that w i l l the channelled into in the not tilt of the ruby rod. is effectively modu- the satisfy in interests reflector of up b e t w e e n t h e Tuning t h i s and [19]. simplicity must be and temperature [14]. on t h e etalon By u s i n g a r u b y proved simple u n w a n t e d modes s e l e c t e d wavelengths the because the c o u l d be s e t the plate rejected ends and no a n t i - r e f 1 e c t i o n the 2 e t a l o n , so a g a i n g i v e maximum l a s e r o u t p u t ends o f ^ For a homogeneously broadened power i s was s u c c e s s f u l l y e m p l o y e d . cavity N 2 N of However, a m o d i f i c a t i o n parallel 2 occur are those t h a t conditions. operation flats plates method was r e j e c t e d ease of uncoated becomes: 1 - by t h e mode s t r u c t u r e oscillations several the main l a s e r c a v i t y s y s t e m s u c h as r u b y but case of rod w i t h coatings and one by a d j u s t i n g reliable. plane a resonant output mirror cavity technique the of 16 Saturable when p l a c e d i n parent Sooy at [15] the modes w i t h ruby the dye and grow the same the in to mechanical spot equal radii of the is in by centre of first, bleach was u s e d s u c c e s s f u l l y a Pockels Cell with spherical in which this curvature should and t h e r m a l resonator point the Replacing either all The h e m i c o n c e n t r i c results One defects with end m i r r o r s but damages t h e was t r i e d the to reach threshold a dye c e l l vibrations nator section will Resonators: configurations focal closest conjunction cavity are degenerate. the As e x p l a i n e d trans- as Q-switch cavity. plane produce line dye b l e a c h and become power d e n s i t y . laser Spherical of will absorbing exponentially. this a mode s e l e c t o r cavity frequency fluorescence In in laser some c r i t i c a l the A suitably Absorbers: to were the case plane m i r r o r . of 50 cm. not gradients in that insensitive at reso- which the had next good. even w i t h p l a n e mirrors as l e n s e s and c a u s e a semi-confocal in can modes A confocal As shown i n can a c t a l s o p e r s u e d more f u l l y is with mirrors particularly a l s o note mirrors transverse [12] both excessive intensity oscillator oscillate the or the next mode. section. This 17 2.3 2.3.1 Oscillator Performance Spectral Width The s p e c t r a l examined u s i n g tion the output arrangement from a g l a s s p l a t e LIGHT DUMP the in laser Figure attenuated oscillator (2). 400 mm F.L. L E N S FABRYPEROT Figure 2. l a s e r beam was CAMERA lens plates with an a l u m i n u m focal length lens imaged the plane of using a close-up stray light. WRATTEN NO. 29 N.D. 0.3 FILTER Perot were reflec- Optics f o r Analysis 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. a negative a neutral After was LASER D=3 ALUMINUM DIFFUSING SCREEN and f i l t e r the GLASS PLATE NEGATIVE LENS expanded w i t h of density then lens and r e f l e c t e d diffusing onto screen. interference the A 400 mm pattern on The filter (density 0.3). photographed on T R I - X (ASA 400) and a #29 As s e e n i n the Wratten following filter prints Fabry- to the the pattern film reduce neutral 18 density filter covered only h a l f technique greatly widths the of the f i l m . points simplified rings half With ring of the a plate at the This the required halffor half-intensity a r e f o u n d s i m p l y by c o m p a r i s o n pattern. s e p a r a t i o n of o range pattern. the measurements of Using a microdensitometer other ring b e c a u s e no H-D c u r v e i s on a p a r t i c u l a r w i t h the the 5 mm t h e free spectral o 6943 A was AX g R p l a t e s was a p p r o x i m a t e l y = 0 . 4 8 A. 30 so t h a t The f i n e s s e o f the t h e chromatic resolving o power was a p p r o x i m a t e l y In order broadening the proves helpful. A. the e r r o r s due t o instrument used by C o o p e r and G r e i g true r source [16] width measured s o u r c e M t h e minimum e f f e c t is 2) = 0.016 If AX 1) X estimate treatment AX then to AX of width the zero broadening, i . e . instrument AX T = the worst e f f e c t of i n s t r u m e n t e n i n g i s AX.. = AX^ + AX, M T H. 3 AX M broad- 19 Assuming t h a t these two the limits true width AX A T = accuracy, Since successive overlap, A X AX /AX M orders ( 1 of AX = AA T the measurements M ± 1 mean between } must be as l a r g e g R interference of as pattern AA„/AA„„ is possible must limited not to: * -T- M For the S R X - M t h e maximum v a l u e hence at gives: A X Now f o r lies T (1 - presented 1/F ± here 1/F) AX M ^ 0.1 o A and F == 3 0 , giving AX Taking the T = 0.1 of 0.1 instrument A is - .03 ± average between t h e s e AX Thus t h e (1 of the T = .0965 ± broadening order of 4%. .03) limits .0035 error for a measured w i d t h 20 Added t o the errors in one must conclude that the to 15% a t Following about mode c o n t r o l with best. methods the densitometer following t h a t were is results a list tried are of readings accurate the various and t h e s u c c e s s obtained each. SPHERICAL MIRRORS: Figure confocal radii (3) shows t h e configuration using s p h e r i c a l and r e f l e c t i v i t i e s Many modes a r e s e e n t o range of could the oscillator of instrument. over No amount reduce these s i g n i f i c a n t l y analysis was c a r r i e d mirrors 30% ( f r o n t ) be l a s i n g output for of and 99.9% the whole of mirror and hence no a 50 cm (rear). free spectral adjustment quantitative out. PLANE M I R R O R S : Replacing (75% f r o n t ) the gave t h e spherical output in mirrors Figure with plane (4). A triple mirrors set of o modes a r e r e s o l v e d w i t h a total output than pointed to is much n a r r o w e r out defects oscillating in the section and t h e r m a l modes. for half-width the of confocal on mode c o n t r o l , gradients limiting 0.24 A. c a s e and as may be the This attributed number of Figure H. O s c i l l a o t r Output: (75% front). Plane Mirrors 22 In two attempting c h a n g e s were made a) to narrow the output linewidth further together: output m i r r o r changed t o 30$ reflectivity b) ruby f a c e a l i g n e d p a r a l l e l t o output m i r r o r . In addition, greater than the s p a c i n g between the 50 cm. Since the t r a n s v e r s e modes a r e s t r o n g l y Figures 30% f r o n t mirror 5 (a) F i g u r e 5 (a) ruby a one m i n u t e line is end m i r r o r s only 3" x 3 / 8 " was diameter, limited. show t h e a l i g n e d w i t h the a single is diffraction and (b) two c o n s e c u t i v e s h o t s w i t h In ruby cavity output face. using They show interval seen w i t h the between. a half-width of o 0.042 A. put with The n e x t photograph, F i g u r e 5 (b) , shows an two c o m p o n e n t s , e a c h w i t h a width equal to out- the o i n s t r u m e n t a l r e s o l v i n g power (~ . 0 2 A ) . The t o t a l w i d t h o f b o t h components together is o .084 A. The l a s e r power o u t p u t These photographs dent frequency s h i f t was 20 MW. a l s o show t h e discussed e a r l i e r . up t h e fringe pattern moves o u t w a r d , toward longer wavelengths. temperature As t h e indicating depen- r u b y warms a shift 23 Figure 6. O s c i l l a t o r Output: and Dye C e l l . Plane M i r r o r s 24 DYE CELL: As t h e cryptocyanine cavity last d i s s o l v e d i n methanol between the centration mode s e l e c t i n g front mirror was a d j u s t e d shows t h e experimentally MW o u t p u t a t linewidth the The h a l f - w i d t h is axial ruby. to give laser The dye con- a reasonable a dye c o n c e n t r a t i o n the of the and power o u t p u t . " same pump e n e r g y as i n Figure giving other 10 cases. .074 A. From t h i s of for a cell was p l a c e d i n and t h e c o m p r o m i s e b e t w e e n mode s t r u c t u r e (6) technique modes t h a t it is p o s s i b l e to are l a s i n g . estimate The a x i a l the number mode s p a c i n g is o g i v e n by cavity AX = X length 2 0 /2d . For = 15. number o f This who f o u n d is the in Although primary output. = 6943 A and an optical .0048 A l a s i n g modes must be a t high the entirely reliable (.074/.0048) in [17] surpressing all power. dye d o e s l i m i t a d v a n t a g e a p p e a r s t o be i n The g i a n t least a g r e e m e n t w i t h M c C l u n g and W e i n e r dye n o t u n w a n t e d modes a t its 0 d = 50 cm: AX = Thus t h e X p u l s e s are smoother the number o f m o d e s , stabilizing the than w i t h the laser Pockels 25 Cell of alone. Magyar [14] modes w h i c h b e a t w i t h Closely is suggests this the light coherence one a n o t h e r allied to the is due t o the absence and m o d u l a t e spectral width of the pulse. the output I = c / A v ; where c = s p e e d length and A v = f r e q u e n c y s p r e a d of laser output. of For this case: AX - .08 A therefore Av « 5 GHz and t h e 2.3.2 coherence length Pulse In output For the the full photodiode is the Characteristics addition size to the and s h a p e o f following width I = 6 cm. the at h a l f pulse energy i n a TRG Model 101 the length w i l l This is multiple the a single the laser important. be d e f i n e d as the as m e a s u r e d by a PIN giant The e n e r g y p u l s e measured w i t h thermopile. polarizer a stack of 519 o s c i l l o s c o p e . The most s i g n i f i c a n t shape i s width p u l s e s are a l s o maximum h e i g h t and a T e k t r o n i x total spectral of a s s o c i a t e d w i t h the thin reflection. component a f f e c t i n g quartz plates Typically with that Pockels polarize pulse Cell. by t h e maximum number of 26 plates with (eight) the an e n e r g y o f from the 1.4 roughly the joules joules varies of general in about width. 2.3.3 effect the of wide Removing p l a t e s i n c r e a s i n g both same r a t i o . pulse lasts operation the pulse For example about output The u n c e r t a i n t y ±15%, most o f w h i c h i s F o r an o u t p u t 70 n s e c about and 20 MW of output variation 0.6 j o u l e s an e f f i c i e n c y in pulse is the of in about power pulse pump e n e r g y is 0.4%. Summary From t h e F a b r y - P e r o t photographs laser oscillator, Q-switch of 20 MW. energy, again g i v i n g 30 n s e c . 1600 j o u l e s , g i v i n g the giant i.e. 25 n s e c power. In 0.6 has t h e in w i t h no p o l a r i z e r contains p u l s e was a b o u t 0.5 j o u l e s , polarizer e n e r g y and w i d t h of output and c a r e f u l producing using plane m i r r o r s , alignment a spectral output of the ruby one can s e e a Pockels rod, is o . 0 8 A (5 GHz) w i d e that Cell capable at 20 o Megawatts. Powers up t o 30 MW and 0.1 p r o d u c e d b u t more r a p i d d e g r a d a t i o n of A widths the can be optical surfaces results. With cavity the a cell output of width cryptocyanine narrows, and m e t h a n o l t h e power d r o p s in the according 27 to the dye c o n c e n t r a t i o n and t h e p u l s e e n v e l o p e becomes smoother. The c o h e r e n c e l e n g t h spectral 2.4 width of Amplifier the s y s t e m t h e most t h e gain achieved with to flashlamps. are other the m a j o r i t y pendence of l a s e r output for a 6 cm. For t h i s is ing .08 A i s of Performance ameter the o useful of the the Efficiency, important a particular population energy and p o s s i b l e p u l s e characteristics following amplifier parinput shorten- t h a t were s t u d i e d , a n a l y s i s deals w i t h the inversion and g a i n on but de- input energy. 2.4.1 Energy Gain In S e c t i o n 2.1 in a laser G = amplifier of an e x p r e s s i o n f o r length L the was d e r i v e d energy to be: gain 28 To s i m p l i f y this for the e x p e r i m e n t a l c a s e we use the f o l 1 o w i ng : p cx 0 0 = number o f incident photons/unit on f a c e o f rod d u r i n g p u l s e of Total incident power/unit = of its so time T 0 photons) A T Now, i f laser area = P / A hv ( n o . = area 0 hvp c 0 the amplifier u n s a t u r a t e d r e g i o n , the is input to operate well photon f l u x within must be s m a l l that 2ap CT 0 0 << 1 c This put condition of the can be shown t o laser oscillator exist and by by c o n s i d e r i n g t h e calculating out- 29 2ap cx 0 = ^ 0 . P. . .081 where: a = radiation for absorption 0 section chromium = 2.5 x 1 0 ~ T cross = 30 x 1 0 " 2 0 cm + 2 sec 9 v = 4 . 3 x 1 0 * HZ 1 P = 2 x 10' A = 1.3 With energy gain cm the watts 2 above r e s t r i c t i o n , reduces in = e the n aL S e c t i o n 2.1 i n v e r s i o n was f o u n d expression for to: G Also the to 0 the have t h e fractional following population function dependence 30 on t h e flashlamp energy E: He. _ v w h e r e B> d e f i n e d of the (in as t h e efficiency this case, two double e l l i p t i c a l amplifier joules. of 3 e" the particular l i n e a r water cavity with To d e t e r m i n e B, gave a u n i t y gain for form of Figure (7). x 0 storage a measure configuration at the common foci). d a t a shows t h a t an i n p u t e n e r g y o f no = 0 and the 2100 E = 2100 gives: Substitution 0 ruby with 1 (q /N pumping experimental (2.2) is cooled flashlamps w i t h i n a the B = 5 . 2 3 x 10" * the e E pumping c o e f f i c i e n t , Using equation joules 1 - of the the value fractional Here the 100%) of this of joules" B into population inversion is equation bank. (2.2) gives i n v e r s i o n shown in e x p r e s s e d as a p e r c e n t a g e and t h e maximum e n e r g y i s capacitor 1 limited by the 31 32 Using the gain e x p r e s s i o n of 10 cm 1 8 is - 3 , the shown i n for the six dependence i n equation calculated Figure inch (8), 2.4.2 Pulse the photon upper a pulse due t o that the value flashlamp the N = 8.8 x 0 energy experimental The a g r e e m e n t the simplified theoretical square pulse input to is curve points seen to treatment the amplifier. Distortion Saturation incident rod. (7), and t h e gain vs. amplifier an i d e a l (2.1) along with be r e a s o n a b l e c o n s i d e r i n g was done f o r Figure flux excted input a laser amplifier is large laser this higher in level results trailing edge. the saturation in in amplification the enough t o the active the and t h e power is material. of thus is the empty the For pulse l e a d i n g edge t h a n The o u t p u t p u l s e effect completely distortion of o c c u r s when shape of shortened by correspondingly i ncreased. However, the fier in the saturation, be t r u e Figure the preceding section i.e. and t h e (9), analysis 2ap CT 0 It gives is the required << 1 . experimental which amplifier. 0 for gain of that the there ampli- be no T h i s was c a l c u l a t e d verification is shown to in typical input and o u t p u t p u l s e s seen t h a t there is no p u l s e s h a p e to 34 distortion to its and t h a t energy t h e power g a i n o f the amplifier (b) INPUT OUTPUT Figure 9. Laser A m p l i f i e r Pulses Normalized t o t h e Same Energy. P i s c u s s i on Figure laser system. Driver is in next the several (10) shows t h e assembly of The SYNC OUT p u l s e f r o m used t o pumping t h e one equal gain. (a) 2.5 is trigger chapter. the detection During the laser i t s e l f is time of triggerable hundred microseconds w i t h microsecond. the The p r i m a r y the complete Pockels system the presented flashlamp over a range an u n c e r t a i n t y source of Cell this of jitter of about is the 30% y p.c. polarizer 3 in. ruby dye 6 in. ruby rear / reflector ' / •v_ —^ / 7kv P O C K E L S C E L L driver light out ^ flashlamps f lashlamps T , A oscillator capacitor bank sync, out to detection system laser trigger in from plasma electronics amplifier capacitor bank X -O pulse from plasma electronics to fire flashlamps (advanced 800/Jsec w.r.t. laser trigger pulse) co en F i g u r e 10. Laser Operation. 36 thyratron in pulse the for the cable driving between to the several Pockels detection this, ruby laser and t h e detection output the of so t h e good q u a l i t y . In out at high t o some o f better section the the rod optical fact, powers the spectral and a p e r t u r e s at directly trigger from the syncronization electronics is accurate inside least power o u t p u t a t the on O s c i l l a t o r is as t h e (> to to cavity present early testing surfaces. particuwas Considering be e x p e c t e d i f be u s e d i n the further with but one more a m p l i f i e r its narrow 50 MW) c o n s i d e r a b l e damage is were reasonably Performance components were o f optical width and KDP c r y s t a l laser The w i d t h c o u l d a l s o be n a r r o w e d require taken Cell none o f occurred However, the nanoseconds. even though carried system i s Pockels spectral larly Driver. the As shown i n the Cell level. a new oscillator. etalon any s u c h s y s t e m stage filters would to m a i n t a i n the P A R T I I Chapter 3 THE DETECTION SYSTEM 3.1 General Outline of Polychromator The p u r p o s e o f several segments of Separate the any m u l t i c h a n n e l a spectrum to photomultipiiers system.is to allow be o b s e r v e d s i m u l t a n e o u s l y . are g e n e r a l l y d e s i r e d w a v e l e n g t h segments i n the used t o output of monitor a spectro- graph . In is this replaced with structed system the a package of from g l a s s f i b e r s . photomultipiier multipliers, exit tubes. five t h e most s t r a i g h t f o r w a r d this has been d o n e , t h e devised single equipment to oscilloscope As w i t h multipliers carry slits con- light to output of recording the Instead the cameras. of this, photomultipiier the five photo- way w o u l d be t o obvious disadvantage i s involved. display all a monochromator individual To d i s p l a y t h e separate oscilloscopes with the of The f i b e r s five of slit use While cost a method was outputs on a trace. any p u l s e d s c a t t e r i n g are r e q u i r e d to detect 37 experiment a short pulse of the photo- scattered 38 laser light. This light each p h o t o c a t h o d e from the time variations and h e n c e a l l phototubes at from the tube d e l a y i n g each p u l s e i t displayed pulse arrives on a s i n g l e is the By s i m p l y seen t h a t they advantage t h a t w i t h proper loscope recording a direct sequentially This calibration measure of transit electronically can be oscilloscope trace. at p u l s e s come (neglecting tube). further is output same t i m e to simultaneously has the the oscil- intensity vs. wavelength. This s i m p l e method has one v e r y The i n f o r m a t i o n of all the phototube which But i n a l s o sees addition the scope t r a c e , each d e l a y e d the in the the than from one t u b e an i n t e r v a l tubes. This this, exactly eliminates by e f f e c t i v e l y from output PM o u t p u t s used f o r the slit circuits and t h e against of the the plasma tube. were added five So the times alone. each p h o t o m u l t i p i i e r is as l o n g as the delay between overlapping the the displaying The f o l l o w i n g t h e sum l a s e r p u l s e , each background l i g h t as a l l To p r e v e n t levels to problem. actually b a s e l i n e w o u l d a p p e a r as a n o i s e l e v e l greater on f o r outputs, a p p e a r s as a DC l e v e l on t h e final d i s p l a y e d on t h e s c o p e i s photomultipiier one a n o t h e r . basic only of one t u b e s e c t i o n s d e s c r i b e the p a c k a g e s , the performance of background at a time. fiber design of the the system. final pulsed optics photomultipiier 39 3.2 Fiber Optics Figure at the exit .003 inch (11) plane of shows t h e o 10 A/mm d i s p e r s i o n these s l i t s With give and t h e The t r a n s m i s s i o n end p o l i s h i n g a monochromator five spectral [20]. Ideally, at which it in regularities the normal total in the bundles from to the which Design of the the on i n dependent scatter p e r end i s of the the section fiber quality at the solid of foot is of light. fiber about of the same a n g l e leave ir- Added to the so f o r these Losses method photomultipiiers. entitled glass Absorption 70%. the the angle. fibers here because of into A. quality t y p i c a l l y .15-20%. transmission Considerations. points a glass-air interface 7% p e r neglected the on t h e ends o f l o s s at light on t h e leave a f i b e r the randomly estimated introduce characteristics preservation adds a n o t h e r one end a r e elaborated i.e. polishing lost glass 0.75 should reflection light of a lesser extent light entered, But d e f e c t s a resolution are s t r o n g l y and t o between o e a c h s e p a r a t e d by 0 . 7 5 A w i t h bundles of whole o optics placed are s t a c k e d steel c l e a r epoxy. is Single layers (Edmonds # 4 1 , 2 2 5 ) .003 i n c h s t a i n l e s s assembly cemented w i t h of package t h a t the monochromator. glass fibers spacer sheets of slit used This Photomulti p i i ers : is Figure 11. Fiber Optics S l i t Package. 41 3. 3 Photomulti p i i ers 3.3.1 Design C o n s i d e r a t i o n s Quantum Ericson and G r a n t sensitivity internal of increase Love in a photomultipiier light conditions couple light from model the end window fiber Figure that the of nearly the total tube. a 5 x (QE) was p o s s i b l e w i t h red results. t e c h n i q u e was u s e d i n the Gunter, c o n s i d e r e d by S i z e l o v e and optic (EMI 9 5 5 8 A , S - 2 0 ) shown i n 1965 c o u l d be i n c r e a s e d by t h e y showed t h a t predicted similar multipliers is in quantum e f f i c i e n c y A similar ment of A mathematical [23] In Enhancement: [ 2 1 , 2 2 ] showed e x p e r i m e n t a l l y reflection Under optimum light. Efficiency (12). bundles surface. Light this system into the The f i n a l from the to photoarrange- glass 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 o f T o t a l I n t e r n a l R e f l e c t i o n . fibers 42 enters ing the end window a t paraffin reflection oil. The l i g h t between the photoelectrons Jennings et al. [ 2 4 ] absorbed in which f i n a l l y is a drop of index match- t r a p p e d by t o t a l photocathode releases this 45° t h r o u g h and t h e on e a c h b o u n c e . tube In b l u e and g r e e n l i g h t internal f a c e and agreement is completely p r o c e s s as s e e n by o b s e r v i n g t h e escapes from t h e edge o f The quantum e f f i c i e n c y of the with light window. a photomultiplier is g i ven by [25] : QE = where: X = wavelength S = cathode = If radiation 2 3 9 x in ' 5 constant the tube proportional it to sensitivity anode Camp/watt]] photocathode power and p h o t o n f l u x along with is the QE by any e n h a n c e m e n t t e c h n i q u e by m e a s u r i n g t h e 100% x nanometers radiant the wavelength are kept directly 1 current leaving incident radiant dynode s t a g e s o f is S x current. is the the g a i n of seen that QE. of the Thus t h e anode incident the current increase e a s i l y found in directly 43 To m e a s u r e t h e (12) was compared t o from the the fibers centre of a similar entered the enhancement the method of the arrangement tube normal photocathode. to Figure in which the light surface at Using a g a l l i u m - a r s e n i d e o light-emitting d i o d e as t h e quantum e f f i c i e n c y This is slightly and G r a n t [21] was f o u n d lower but than the r e d end o f the Accepting QE o f the detection follows: the 10% a t a laser light the first lengths tube of this the 3.5% i t outlined is typical seen 30 earlier, is simultaneously on a s i n g l e approximately that that The b a s i c o p e r a t i o n e a c h anode p u l s e is suitably as at delayed oscilloscope. nsec l o n g , the of As delay was c h o s e n as 100 n s e c and a c h i e v e d delay cable. arrives S20 s u r f a c e s wavelength. pulse arrives between p h o t o m u l t i p i i e r s by e q u a l 3. enhancement a c h i e v e d about Pulsing: s y s t e m , as b r i e f l y laser pulse is the 6943 A i s and d i s p l a y e d s e q u e n t i a l l y the of c l a i m e d by G u n t h e r , E r i c s o n EMI s p e c i f i c a t i o n s about each p h o t o m u l t i p i i e r ; the spectrum. an S20 s u r f a c e a t e n h a n c e d QE i s i n c r e a s e by a f a c t o r that to s o u r c e (X ~ 6600 A) who a l s o i n v e s t i g a t e d Phot omul t i p l i e v the to identical by Oke and S c h i l d [26] in light at the That i s , the scope with signal no d e l a y , t h e from second 44 tube's pulse is nsec e t c . until adding the on f o r five background Appendix I pulsing deals all signals the reviews third P M ' s must a l l of the In To by 200 avoid be p u l s e d off the used possible configurations the rest two methods of this tried. 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 failure tube l a s e r p u l s e and t h e n t u r n e d p h o t o m u l t i p i i e r s , and the s p e c i f i c a l l y with the are d i s p l a y e d . l e v e l s , the 100 n s e c d u r i n g again. for d e l a y e d by 100 n s e c , is section One o f the given for the other. general there a r e two choices for pulsing photomu1ti p i i e r s : Method (a) (a) switching off, or the (b) electronically signal was i n v e s t i g a t e d g a t e s were n o t the tube proper itself gating first a v a i l a b l e w i t h the b e c a u s e by p u l s i n g t h e a c h i e v e d under tube the on and output because f a s t proper s p e c i f i c a t i o n s , and an i n c r e a s e i n conditions transmission the gain can be [27]. PULSING THE PHOTOMULTIPLIER-TUBE Specifically, respect to the first the dynode. c a t h o d e was p u l s e d n e g a t i v e A pulse generator with using a mercury 45 wetted relay and a d e l a y c a b l e gave a 150 v o l t nsec w i d e , w i t h rise and f a l l times s e c o n d b u t was d i s c a r d e d b e c a u s e i t of 1 usee. driven Another generator of the with a DC l i g h t present This in the first applied this the source than output at to there the gate pulse. positive with respect a thin time propagation the l a y e r of long travel because of its length of equation a pulse through n o t e d by De M a r t i n i the the With all than they anode p u l s e photocathode. and Wacks [ 2 9 ] a that and relatively By s o l v i n g can e s t i m a t e nsec, lines. semiconducting material surface. cases 100 They c o n s i d e r circumference takes a c r o s s the in the d e l a y and M a l v a n o [ 2 8 ] . a p u l s e a p p l i e d to to but p u l s e was c o n s i d e r a b l y l o n g e r for to s i n e - s h a p e d p u l s e was output that is the the cathode i s of of was e v e n l e s s n o i s e i n condition output illuminated square p u l s e . g i v e n by F a r i n e l l i time But the a c a t h o d e s p a c e c h a r g e and c u r e d Next a n e g a t i v e An e x p l a n a t i o n relevant square the anode c u r r e n t the transistor cathode i n s t e a d of an u n s a t i s f a c t o r y is to order and a l a r g e s p i k e was beginning cathode ten v o l t s dynode. a l s o gave f a s t 100 n s e c , the on t h e voltage one n a n o s e c o n d . 100 one n a n o - had a j i t t e r l a s t e d much l o n g e r when s p i k e was a t t r i b u t e d by b i a s i n g t h e the l e s s than photomultipiier l e s s than using a high by a o n e - s h o t m u l t i v i b r a t o r pulses with j i t t e r s of pulse, the A similar the transit effect and by S u e m a t s u , 46 Normura and T o m i t a photocathode of that the pulsing because of the GATING PM [30] who p l o t an RCA 7 1 0 2 . All photocathode fundamental is on t h e tubes output the circuit of continuous tubes out of the on the the fact method tube itself. technique uses a t r a n s m i s s i o n each p h o t o m u l t i p l i e r . outputs The sampled f o r Space charge problems themselves are e l i m i n a t e d 100 associated because of operation. A detailed description transmission g a t e s , along with i n Appendi x II. 3.3.2 points an u n a c c e p t a b l e and t h e laser pulse. w i t h p u l s i n g the the this lines OUTPUT are run c o n t i n u o u s l y nsec d u r i n g equidelay limitation The s u c c e s s f u l p u l s i n g gate the of the the operation circuits of the involved is given Performance To e v a l u a t e t h e performance tion system, a light-emitting pulsed light source in output follows has a r i s e t i m e the of the shape of less diode circuit the than f i v e of the complete detec- (LED) was used as a of Figure driving nsec. (13). The light p u l s e and t h e LED 47 Q +3v t o — ( Datapulse + out MLED630 NOTE: Peak emission wavelength of LED is 6600 A Figure 13. Circuit o f 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 multipliers tube For to o f d y n o d e s D4 and D5 o f e a c h b e t w e e n 33% and 100% o f t h e a v e r a g e i n t e r d y n o d e a Venetian b l i n d 9558 t h i s 30% to vary the v o l t a g e of the photo- voltage structure variation f r o m t h e maximum [ 3 1 ] . be a d j u s t e d sating fiber s u c h as p r e s e n t allows f o r t h e same absolute f o r individual transmission. i n t h e EMI gives a gain adjustment This variations a l l five sensitivity i n tube voltage. of about channels by c o m p e n - gain and o p t i c a l 48 Each tube the arrival Since all dynode (~ of the the light tubes chains, 50 n s e c ) as w e l l the has a t r a n s i t pulse have t h e transit and t h e time delay anode c u r r e n t same o v e r a l l times and no p r o v i s i o n is are between all made t o bias on pulse. their reasonably compensate equal for di f f e r e n c e s . Figure no l i g h t with in rise present. and f a l l required less that than system f o r tively 50 mV. of noise from the low the positive noise are risetime the these In level of five with tubes. Figure extremely pulsewidth. pulses in and ( 1 5 b ) low output from Its peak value much l e s s than [27] who c l a i m to have technique. show t h e nsec i t the output is light seen t h a t specifications The v a r i a t i o n s (15b) is light o u t p u t of has been made s l i g h t l y gate is Although in a statistical both photographs 7704 differences and n e g a t i v e and a 30 n s e c l i g h t the when a Tecktronix gates. level pulsing LED s o u r c e . consistent for pulses noise (15a) times the results transmission This a DC l i g h t and f a l l pulses to This by t h e a very in The o s c i l l o s c o p e i s times Figures the noise a c h i e v e d by De M a r c o and P e n c o developed rise shows t h e 150 MHz b a n d w i d t h . pulses is is (14) the shorter the of of pulse respec- pulse has t h e PM 15 n s e c amplitude fluctuation LED s o u r c e length than the of the the at of due this PM g a t i n g 100 n s e c vertical F i g u r e 14. scale: 200 mV/division N o i s e i n PM G a t i n g E l e c t r o n i c s . F i g u r e 15. P h o t o m u l t i p l i e r Outputs. 50 inter-channel delay, to identified. be e a s i l y 3.4 thus allowing an a p p r o p r i a t e laser light tipliers, the triggered (Figure the delay to and t h e gate internal pulse the shown i n for transit generator cables. II) to is channels of total delay line The method the PM o u t p u t s As e x p l a i n e d reported to bursts here the the iency best date of gain output of the pulsed of 100 mV. is is 50 mV. below gates tube is to very methods five by t h e It II) in gates pass into B o t h ends line's I In the of the gates the best In char- pulse tubes them- techniques system addition in to 20 n s e c and n s e c and a l l high, [27]. Cell. scope. pulsing risetimes risetime A - 2 , Appendix d e l a y e d by a d i f f e r e n t on t h e in Appendix least the photomul- PM p u l s e s t o using transmission have g a i n at the After reflections. appears s u p e r i o r selves. noise prevent the transmission are t e r m i n a t e d impedance to (16). time of Pockels then and d i s p l a y e d s e q u e n t i a l l y acteristic time of (Figure allow Each p u l s e Figure transit from the amount the the s c o p e and opens t h e A - l , Appendix delay allow by a s y n c p u l s e turn triggers in individual P i s c u s s i on The c o m p l e t e s y s t e m i s is the spurious presented electrical the contrast to cutoff one o f noise efficthe light inputs from monochromator via fiber optics photomultipliers transmission gates —jtlftJUb— —JlxJUib— —<SlMSlSLs 1 1 delay cables (100 nsec""^ each) 47 to all gates tout gate pulse generator variable delay signal in sync 6 sync, pulse from POCKELS CELL Figure driver 16. Detection System Operation. Chapter 4 CONCLUSION In experiments in preparation future laser light on p l a s m a s a p u l s e d r u b y conjunction detection for of with the a multichannel scattered The l a s e r , scattering l a s e r has been spectral developed analyser for light. consisting of separate o s c i l l a t o r and o amplifier up t o rods, has a s p e c t r a l 150 M e g a w a t t s . switch permits analyser The use o f accurate and a l l external five gated p h o t o m u l t i p l i e r s to give an i n t e n s i t y profiles Future better quality the record profile Q- spectral five error involved to components 52 the in oscillo- data points in of simul- scattered plotting system should the outputs displayed on an the measurements improvements optical powers as t h e system the are s e q u e n t i a l l y to simplifies and r e d u c e s Cell w i t h the detection vs. wavelength The a b i l i t y greatly a Pockels 0.08 A at electronics. of taneously width of syncronization For the m u l t i c h a n n e l scope s c r e e n . line them. include l a s e r and more 53 development slits. of Better t h e methods optical mode p a t t e r n and p e r h a p s better slit i n making the components would more s t a b l e accurate involved for stacking the laser. techniques s p a c i n g and l e s s light result in glass a narrower, Smaller glass could l o s s to result the fiber in fibers more photomultipiiers. 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"The E f f e c t o f T e m p e r a t u r e upon the A b s o r p t i o n Spectrum of a S y n t h e t i c R u b y . " Phys. R e v . , 8: 38. 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 D e p e n d e n t F r e q u e n c y S h i f t i n Ruby L a s e r Giant 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. "Characters t i c s of 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 . " Proc. IEEE, 51(1): 46-53. 54 55 [10] W a g n e r , W . G . and B . A . L e n g y e l . 1963. the G i a n t P u l s e i n a L a s e r . " J . Appl. 2040-46. [11] L e n g y e l , Bela A. Inc. New Y o r k . [12] R o s s , D. 1966. 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E r i c k s o n and G . R . G r a n t . 1965. " E n h a n c e m e n t 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 Internal Reflection." A p p l . O p t . , 4 ( 4 ) : 512. 1962. Lasers. 1969. " E v o l u t i o n of P h y s . , 34(7) : John W i l e y & Sons "Interferometer 448. S e l e c t i o n o f Modes i n A p p l . P h y s . L e t t . , 7_: "Longitudinal IEEE J . Q u a n t . G i a n t P u l s e Ruby L a s e r R e v . S c i . I n s t r . , 38_: for Giant 2193-4. End F i n i s h i n g of 517. Pulse Consider- 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. "High Absolute Photocathode S e n s i t i v i t y . " R e v . 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 . L o v e I I I . 1967. "Analysis of 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 . Opt. , 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 t h a n 50% f r o m C o m m e r c i a l l y Available Photomultipiiers. 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 . , 9 4 0 3 5 . [25] RCA. T e c h n i c a l Photocel1s. [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 I m p r o v i n g t h e Quantum E f f i c i e n c y of P h o t o m u l t i p i i e r T u b e s . " Appi. Opt., 7(4): 617-22. [27] De M a r c o , F. multipliers." [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 of multipliers." R e v . 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 . W a c k s . 1967. "Photomultipiier Gate f o r S t i m u l a t e d Spontaneous L i g h t S c a t t e r i n g Discrimination." 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. " M e a s u r e m e n t o f D e l a y Time D i f f e r e n c e s on t h e P h o t o cathode S u r f a c e of a P h o t o m u l t i p i i e r . " Proc. IEEE, 56(8): 1405-6. [31] EMI. [32] Post, R.F. 1952. "The P e r f o r m a n c e multipliers." N u c l e o n i c s , 1_0: 4 6 . Manual PT-60. Phototubes and and E . P e n c o . 1969. "Pulsed PhotoR e v . S c i . I n s t r . , 40_(9): 1 1 5 8 - 6 0 . An I n t r o d u c t i o n to the Photo- Photomu1tip1ier. of Pulsed Photo- 57 [33] S i n g e r , S . , L . K . N e h e r and R. R u e h l e . "Pulsed Photom u l t i p l i e r s for Fast 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: 40. [34] Elphick, B.L. 1959. "A Method o f 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 . Instrum. ( J . P h y s . E) , 2: 9 5 3 - 5 5 . APPENDIX I COMPARISON OF PHOTOMULTIPLIER PULSING TECHNIQUES In t h e c o u r s e o f d e v e l o p i n g tiplier This as gate, photomul- a s u r v e y was done o f t h e e x i s t i n g i s presented below and t h e r e l e v a n t techniques. terminology is fol1ows : gain visetime switch full As w e l l , gain : length of tube of time from required 10$ t o 90% o f to its value. cutoff of a suitable efficiency: ratio tube ON t o o u t p u t gain amplification: tube over K refers normal of output for f o r tube OFF. increase in gain of non-pulsed operation. t o the cathode a tube. 58 and D n to the n t t L dynode 59 Pulsing Whole In is limited Resistor normal operation arcing occurs. dynode c h a i n i s this g a i n of maximum v o l t a g e ions by P o s t overvolting a substantial voltage However, i f relative the to 10 , tube amplification an i n c r e a s e o f 9 this tube. from the report with is in of c l a i m e d to t h e work of tube, cutoff be 10 n s e c . addition efficiency, gain. of 10 risetime This is the typically in (a) pulse, in As shown gain by t h e 6 lead gain for similar results [27] an RCA 7265 this in direct latter case contrast to who show t h a t cathode gain fast resistivity. amplificaexcellent [27]. these advantages t h i s the the an o v e r a l l normal advantage of 10 to P u l s i n g a 931A by d r i v i n g 3 and M a l v a n o [ 2 8 ] to [32]. have r e p o r t e d d i s a d v a n t a g e s w h i c h make t h e unacceptable: short t h e w h o l e dynode c h a i n a l s o has an Despite severe voltage s u c h a manner can over the s h o u l d be s e v e r e l y l i m i t e d pulsing can be and De M a r c o and Penco The g a i n Farinelli In tion of 3 [33] a gain a m p l i f i c a t i o n a 6 kv p u l s e . rises 2 x 10 S i n g e r et al. same t y p e that the electrons w i t h a 4 kv p u l s e 2 . 5 u s e e l o n g r e s u l t e d of photomultipiier may be s a f e l y e x c e e d e d o w i n g t o of [32] a s u p p l i e d as a s u f f i c i e n t l y low m o b i l i t y to the by t h e maximum i n t e r - d y n o d e applied before the Chain method has s e v e r a l technique pulse generator generally must s u p p l y fast, 60 well-formed high a low i m p e d a n c e the cathode present tubes in voltage load; output Focusing tube. [34] method and t h e pulses transistor He p o i n t s cathode large the noise of focus that high efficiency limited overvoltages by are some [33]. report this the output avalanche due t o it of 10 nsec. electrode to photo- positive space charge focusing it risetimes focus pulsing and a g a i n t h e First photo- an RCA 6810 w i t h a s p e c i a l and p u l s e s - most Elphrick b i a s i n g the seems v e r y the 6292 in electrode negatively. cutoff Dynode Two p o s s i b i l i t i e s to the in second No n o i s e m e a s u r e - efficiency Stage are present results accumulations. good. Cathode into good. and t h e n dynode p o t e n t i a l Pulsing jitter noise signals noise introduced and shows g a i n transients given at oscillate the of he b i a s e s t h e are (d) large is them when u s i n g a Dumont cutoff potential Instead, ments for circuit out to (c) risetime and M a l v a n o [ 2 8 ] No i n d i c a t i o n given [28]; gain negligible Electrode Farinelli convenient the [27]; have a t e n d e n c y Pulsing is (b) resistivity the pulses with here: seems very 61 1. P u l s i n g K Negative with Although achieved this and t h e method is an e x c e l l e n t noise in not the cutoff output As e x p l a i n e d by F a r i n e l l i firmed by De M a r t i n i across a voltage the sensitive to preferably 2. pulse requires In fluctuations be v o l t a g e an RCA 7265 d e t e c t o r to the first to the cathode. the However, in the tube K-Dl poor gain layer rise- and in con- this to travel of s e m i c o n - gain i s most stage which should Respect t o K. and Wacks [ 2 9 ] with a positive An e x c e l l e n t is report a method o f applied biased negative with respect cutoff c l a i m e d to efficiency be b e t t e r is than achieved 20 no n o i s e m e a s u r e m e n t s a r e g i v e n , no m e n t i o n space charge t r a n s i e n t s is long (100 y s e c ) . pulsing square pulse made o f very the low, stabilized. dynode w h i c h i s gain risetime its and t h e work a thin P u l s i n g DI P o s i t i v e w i t h De M a r t i n i and can be made v e r y c o n s i d e r a b l e time is addition, can be and M a l v a n o [ 2 8 ] and Wacks [ 2 9 ] photocathode, which ducting material. efficiency a d v i s a b l e because of time. thesis, Respect t o DI. and t h e gate pulse nsec. is width 62 Pulsing Cathode and As f o u n d with a negative outlined Elphick to the is pulse siders this Pulsing of DI Last the Dynode nsec long. a) is b) c) risetimes However t h e the ON/OFF only even i n which as The is poor gain K-D1-D2 those cutoff poor. D2 i s pulsed p o s i t i v e . [27] biased He c o n - risetime. noise is the method recommend a c i r c u i t D12 and D13 o f 20 n s e c for a r e as cutoff pulses 300, greater requires appropriate than a 400 200 volt width. the out- mV, square tube. 300 follows: efficiency shielding they an RCA 7265 and g a t e disadvantages elaborate put of alone. risetime because of of ratio about with pulse pulsing Stages p u l s e dynodes They show g a i n gain a method De Marco and Penco d e v e l o p e d to cathode and t h e n unsatisfactory [27J same c h a r a c t e r i s t i c s good and t h e also reports potential Dynodes has t h e pulsing very [34] Two by De Marco and Penco above f o r efficiency First 63 Coupling Anode Signal Instead approach i s to of gate Through switching the leaves the tube running the difficulties of transient is not to prevent the useful previous for Using the gates a different desired and time. eliminates risetimes However, the where t h e it as b e i n g PM must be cutoff asso- method pulsed far thesis good photocathode more as five time for output gain found (probably nsec. the noise greater system This is is the less turn-on gates, is considerably other risetime less than 50 less than for any the literature. presented in mV II) the characteristics: h effective then attractive. (Appendix following as Since \0 ), than is this is electronic impractical. have t h e efficiency pulsing the mention has made t h e method developed for than c) the n o i s e and g a i n and M a l v a n o [ 2 8 ] technology by b) for techniques. pulsed photomultipiiers a) PM on and o f f overloading. and d i s c o u n t improved Gate continuously situations Farinelli gating the anode s i g n a l This ciated with Transmission which APPENDIX II PHOTOMULTIPLIER ELECTRONICS E a c h s e t o f dynode standard the is configuration potential variable voltage. tube. from This exceptions. (D4) w i t h a 30% v a r i a t i o n gate voltage circuit respect chains i n t h e g a i n o f each gate circuit gates are i d e n t i c a l pulse generator speed, manufacturer, follows: a bias i s about 1 ma. t o t h e same supply. transmission acteristics p e r tube of the With are connected i n p a r a l l e l The anode o f e a c h t u b e a high t o D5 interdynode and i s a d j u s t a b l e . the chain current as a Firstly, S e c o n d l y , t h e anode l o a d r e s i s t a n c e i s p a r t t h e dynode high i s wired 33% t o 100% o f t h e a v e r a g e allows -1500 volts All w i t h two m i n o r o f t h e f o u r t h dynode transmission of chain r e s i s t o r s shown i n F i g u r e and a l l a r e d r i v e n described later. unity gain buffer National (A-l). All in parallel The h e a r t 64 by a is d e s i g n a t e d LH0033 by t h e particularly impedance of 1 0 five of the gate Semiconductor Corporation. t h a t make i t input i s connected to a separate 1 1 attractive ohms, o u t p u t The c h a r a r e as impedance Figure A-l . Transmission Gate Circuit. 66 of 6 ohms, bandwidth tended very the to well be used w i t h in the requirement with respect to of two 100 n s e c and o f PM. input This properly to pins the terminated 10 and 12 o f allow ohm p o t offset the standard buffer Output coaxial input voltages. variation signal between in this the (A-l) the to c a b l e s from buffer. drive gate IK and is negative on f o r 100 pulses each potentiometer anode l o a d o f o u t p u t of The d r i v i n g inputs each gate to pins long pulses enter the pulse 9 and 1 coaxial and 10 p r o v i d e s is through the and generator of 100 respectively cables. The adjustment an i s o l a t i o n to through Damping r e s i s t o r s 7 from and t h e Normally o u t p u t , which positive the performs inconvenience square current the coaxial it in- 100 for resistor to a connector. The o u t p u t the varies originally polarity. Figure these between p i n s null. Thus t u r n i n g opposite to only supply voltages risetime. ohms e a c h c o n n e c t to Its not supply, and g r o u n d s e r v e s as t h e adjustment some e x t e n t given. simultaneous Referring While a p u l s e d power ground. two long TOO MHz. circuit nsec r e q u i r e s between of the the buffer algebraic supply them w o u l d the a combination addition voltages of would the supplies are the offset of two supply be DC and any a p p e a r as a DC l e v e l c o u l d be removed w i t h application is in adjust. p u l s e d so i t is the But essential 67 that the two opposite g o i n g p u l s e s h a p e s be as n e a r l y tical as p o s s i b l e . Any v a r i a t i o n s or the of in spikes that in the since risetimes the output. In are d r i v e n must work in into The c i r c u i t meets cables of duce t h e is parallel positive used w i t h by v a r y i n g of the is of this lengths of show up as n o i s e least requires ±5 v o l t s means t h a t the Figure gen- that (A-2). High discharge Two t r a n s i s t o r s going pulses while provide a sync pulse to and 10 ohms. an a v a l a n c h e mode t o length. the arrival pulse generator p u l s e s are a d j u s t e d The o v e r a l l at shown i n and n e g a t i v e to of each b u f f e r gating are used i n a capacitor the time a s o u r c e impedance of 50 n s e c e l e c t r i c a l The two d r i v i n g as addition, these s p e c i f i c a t i o n s speed t r a n s i s t o r s the pulses w i l l p u l s e s have an a m p l i t u d e all erator driving in iden- be e q u a l pro- a third output. in width discharge cables. characteristics of the generator follows: Input trigger I eve I : 300 mV V Output pulse amplitudes ± 5 i nto Output pulse widths: 100 Outp ut rise t i mes : I ess Output fall ti mes: 5 nsec 10 10 ohm Ioad ns than Sync pulse amplitude: + Sync pulse width: approx. Sync pulse risetime: 5 V nsec 2 i nto I nsec 50 ysec ohm Ioad are Figure A-2. Avalanche T r a n s i s t o r Pulse Generator. Gate 69 Figure pulse able generator. (A-3) shows t h e The v o l t a g e s avalanche operation power s u p p l y are a d j u s t e d and e q u a l output for to pulse the give gate reli- amplitudes. Figure A-3. Power S u p p l y for Gate P u l s e Generator. o
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Development of a laser oscillator-amplifier combination and a multi-channel spectral detection system… Albach, Gary George 1972
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Title | Development of a laser oscillator-amplifier combination and a multi-channel spectral detection system for light scattering experiments |
Creator |
Albach, Gary George |
Publisher | University of British Columbia |
Date Issued | 1972 |
Description | In preparation for laser light scattering experiments on plasmas in magnetic fields a pulsed ruby laser has been developed in conjunction with a multichannel spectral analyser for detection of the scattered light. The laser, consisting of separate oscillator and amplifier rods has a spectral line width of .08 Å at powers up to 100 Megawatts. The use of a Pockels Cell as the Q-switch permits accurate synchronization with the spectral analyser and all external electronics. For the multichannel detection system five fiber optics slit bundles transmit light from the output of a monochromater to five photomultipiier tubes, which are gated on for 100 nsec during the laser pulse. The pulses are displayed sequentially to give an intensity vs. wavelength profile on an oscilloscope screen. |
Subject |
Light -- Scattering |
Genre |
Thesis/Dissertation |
Type |
Text |
Language | eng |
Date Available | 2011-04-15 |
Provider | Vancouver : University of British Columbia Library |
Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
DOI | 10.14288/1.0084863 |
URI | http://hdl.handle.net/2429/33702 |
Degree |
Master of Science - MSc |
Program |
Physics |
Affiliation |
Science, Faculty of Physics and Astronomy, Department of |
Degree Grantor | University of British Columbia |
Campus |
UBCV |
Scholarly Level | Graduate |
Aggregated Source Repository | DSpace |
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