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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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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.  REFERENCES  S t e e l e , E . 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L a s e r s , L i g h t A m p l i f i e r s and Oscillators. A c a d e m i c 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. L a s e r Mode S e l e c t o r . " A p p l . O p t . , 2:  [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 Solid-State Lasers." Optics & Laser Technology, 1(5): 231-39.  [15]  S o o y , W. 1965. "The N a t u r a l a Passive Q-Switched L a s e r . " 36.  [16]  C o o p e r , J . and J . R . G r e i g . A Rapid Scanning FabryPerot Spectrometer. 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. 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 . " E l e c t r o n i c s , 1: 94-99.  [18]  M a g y a r , G. 1967. "Simple High S p e c t r a l B r i g h t n e s s . "  [19]  S t e i n , A. 1967. "Mode S e l e c t i o n Ruby L a s e r s . " Appl. Opt., 6(12):  [20]  Corning Glass Works. ati ons.  [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. " 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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