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On the propagating waves in the hot-cathode mercury discharge
Volume 20, number 2
P HYSI CS L E T T E RS
IN
THE
1 February 1966
ON THE PROPAGATING WAVES HOT-CATHODE MERCURY DISCHARGE
N. SATO and Y. H A T T A Department of Electronic Engineering, Tohoku University, Sendal, Japan Received 3 January 1966
Experimental results on propagating waves excited by small coil surrounding a discharge tube under a weak axial magnetic field show that the waves excited have the properties of the backward waves on the both sides of the exciting coil. m o v i n g s t r i a t i o n s . E x p e r i m e n t a l r e s u l t s obtained by Craw-ford and Self [6] w e r e c o n c e r n e d with the b a c k w a r d w a v e s e x c i t e d by the s m a l l c o i l in the hotcathode m e r c u r y d i s c h a r g e . But t h e i r a t t e m p t s to d e t e c t the w a v e s on the cathode s i d e of the e x citing c o i l w e r e u n s u c c e s s f u l . In our c a s e , the w a v e s can be d e t e c t e d on the both s i d e s of the e xciting coil. E x p e r i m e n t s a r e m a d e m a i n l y on the w a v e s on the cathode side. The e x p e r i m e n t a l a p p a r a t u s and conditions a r e shown in fig. 1. The w a v e s a r e d e t e c t e d by two p h o t o m u l t i p l i e r s t h r o u g h o p t i cal f i b e r s the d i a m e t e r of which i s 3 m m and the s i g n a l s f r o m two
This l e t t e r r e p o r t s s o m e new e x p e r i m e n t a l r e s u l t s on p r o p a g a t i n g w a v e s e x c i t e d by s m a l l c o i l in the p o s i t i v e c o l u m n of h o t - c a t h o d e m e r c u r y d i s c h a r g e u n d e r a weak m a g n e t i c field. E x p e r i m e n t a l w o r k s on the p r o p a g a t i n g ion w a v e s w e r e done by s e v e r a l a u t h o r s [1-4]. R e c e n t l y , B a r r e t t and L i t t l e [5] showed that t h e r e w e r e two types of the p r o p a g a t i n g w a v e s in the p o s i t i v e column of the m e r c u r y - p o o l a r c d i s c h a r g e . While one of t h e m gave the d i s p e r s i o n r e l a t i o n f o r ion w a v e s , the o t h e r gave that f o r b a c k w a r d w a v e s and was not s u f f i c i e n t l y u n d e r s t o o d though it was thought to be i n t e r p r e t e d as the d i s p e r s i o n r e l a t i o n f o r
DISCHARGE°CURRENT 5mA-- I A HELMHOLI~ COIL"" [ ~ (0-250
GAUSS)
HOT CATHODE
GAS PRESSURE
~]
t
t •~
•
D'~
I\/11% L"~'A
I - 2 x 10"SmmHg
ELECTRON TEMPERATURE2 - 4 x I 0 a °K PLASMA DENSITY I0 s - I0 V / c m s ii
,b__
d~-
" EXC,T,NG CO,"
', £ODE
FEW TENS OF GAUSS~ \ OPTICAl.. FIBER
FILTER
\
DUAL BEAM SYNCHROSCOPE
~g
"1
TuBEA 60
50
L 1027
"rUBEB 35
30
1235
Fig. 1. Experimental apparatus. All dimensions are in millimeters. 161
Volume 20, n u m b e r 2
P HYSI CS L E T T E R S 540"
I0°
360"
1 F e b r u a r y 1966
,7, t.g rr (..9 UJ E:3
-
i
x~ ~.~
~
"x
==
360°
.
=
-a.
o
=
30
/
-
o
,o-,
/~//---x.".Y-.
/.,y ,
,4o0
II
=70cm
I00
= 50
180 %
LU O3
/
/ / /
//yj
.
zlf~"
"1" O...
/~" 0° 0
I 5
I I0 2.
12 = 15.3 cm I I 15 20 (CM)
1~ / / / /
25
Fig. 2. The phase and the amplitude of the waves e x cited on the cathode side for tube A. l is the distance f r o m the n o r m a l i z a t i o n point (7 cm f r o m the exciting coil). d i f f e r e n t p o s i t i o n s a~'e c o m p a r e d on d u a l b e a m synchroscope. On t h e b o t h s i d e s , t h e w a v e s a r e d a m p e d s o heavily that the signals are not detectable near the electrodes unless the exciting coil is very c l o s e to t h e m . T h e p h a s e a n d t h e a m p l i t u d e of t h e w a v e s e x c i t e d on t h e c a t h o d e s i d e c h a n g e c o n tinuously, but those on the anode side not always c o n t i n u o u s l y . It m a y b e s a i d t h a t t h e v a l u e s of the phase velocity and the damping distance on the anode side are rather larger than those on the cathode side. The signals from the center and t h o s e n e a r t h e w a l l a r e 180 ° o u t of p h a s e o n t h e c r o s s s e c t i o n of t h e t u b e e x c e p t f o r t h e r e g i o n n e a r t h e e x c i t i n g c o i l . T h u s t h e m o d e of t h e w a v e s i s c o n s i d e r e d a s n = 0, m = 2 (the B e s s e l f u n c t i o n Jn(kmr) i s a s s u m e d i n t h e r a d i a l d i r e c t i o n a n d m d e n o t e s t h e r u t h z e r o of Jn(kmr)) i n c o n t r a s t w i t h t h e c a s e of L i t t l e e t al. [2, 4, 5] a n d C r a w f o r d a n d S e l f [6] in w h i c h n = 0, m = 1. M e a s u r e m e n t s of t h e p h a s e i n t h e a x i a l d i r e c t i o n o n t h e b o t h s i d e s of t h e e x c i t i n g c o i l s h o w t h a t t h e p h a s e v e l o c i t y i s a l w a y s d i r e c t e d to t h e e x c i t i n g coil. The typical data for the waves excited on t h e c a t h o d e s i d e a r e s h o w n i n fig. 2. T h e w a v e s a r e m o s t e a s i l y e x c i t e d f o r t h e f r e q u e n c y of 20 k c / s - 35 k c / s m u c h a b o v e a n d b e l o w w h i c h t h e a m p l i t u d e b e c o m e s t o o s m a l l to b e d e t e c t e d . T h e p r o p e r t y of t h e w a v e s d o e s n o t e s s e n t i a l l y d e p e n d o n t h e d i s c h a r g e c u r r e n t (I) e x c e p t t h a t t h e i n t e n 162
=150gauss
,o
I LU ¢,D Z W n" i.U LL LL
•
0° 0
,,: 12= 2 3 I I0
I 20 £
I 30
cm I 40
(CM)
Fig. 3. The effect of the steady axial magnetic field (Bo) on the propagation of the waves excited on the cathode side for tube B. x denotes the p h a s e of the s e l f excited waves Or = 29 k c / s , Bo = 62.5 gauss). s i t y of t h e s i g n a l s i n c r e a s e s w i t h 1. It i s s h o w n i n fig. 3 h o w t h e s t e a d y m a g n e t i c f i e l d (Bo) a f f e c t s t h e p r o p a g a t i o n of t h e w a v e s . T h e r e a p p e a r o c casionally self-excited waves at a certain value of t h e m a g n e t i c f i e l d w i t h o u t t h e e x t e r n a l e x c i t a t i o n . T h e p h a s e v e l o c i t y of t h e s e w a v e s i s i n t h e d i r e c t i o n of t h e a n o d e a n d i s n e a r l y c o n s i s t e n t w i t h t h a t of t h e w a v e s w h i c h a r e e x c i t e d e x t e r n a l ly o n t h e c a t h o d e s i d e a n d p r o p a g a t e i n t h e d i r e c t i o n of t h e a n o d e . The wave length does not change appreciably w i t h t h e f r e q u e n c y , b u t i t i s i n c l i n e d to b e c o m e long as the frequency increases and so the waves h a v e t h e p r o p e r t y of t h e b e a c k w a r d w a v e s o n t h e b o t h s i d e s of t h e e x c i t i n g c o i l . It i s n o t w e l l u n d e r stood why the waves are excited more easily on the cathode side and the phase and the amplitude of t h e w a v e s on t h e a n o d e s i d e do n o t c h a n g e s o c o n t i n u o u s l y a s t h o s e on t h e c a t h o d e s i d e . T h e waves are not considered as the so-called moving striations which are understood as the popular e x a m p l e of t h e b a c k w a r d w a v e s i n t h e p o s i t i v e c o l u m n , b e c a u s e t h e p r o p e r t y of t h e w a v e s d o e s not depend on the discharge current and the mode of t h e m i s n= 0, m = 2 c o n t r a r y t o t h e m o v i n g striations for which the light signals from the center and near the wall are in phase on the c r o s s s e c t i o n of t h e t u b e .
Volume 20, number 2
P HYSI CS L E T T E RS
1 February 1966
References 1. Y. Hatta and N. Sato, Proc. Vth Int. Conf. on Ionization Phenomena in Gases, Munich, 1961 (North-Holland Publishing Company, Amsterdam) p.478. 2. P . F . Little, ibid,, p. 1440; P. F. Little, Nature 194 (1962) 1137. 3. A.Y.Wong, N.D'Angelo and R.W. Motley, Phys. Rev. Letters 9 (1962) 415; A. Y. Wong, R.W. Motley and N.D'Angelo, Phys. Rev. 133 (1964) A 436.
THE
TEMPERATURE
DEPENDENCE IN LITHIUM
4. P. F. Little and H. G. Jones, Proc. Phys. Soc. 85 (1965) 979. 5. P. J. Barrett and P. F. Little, Phys. Rev. Letters 14 (1965) 356. 6. F.W. Crawford and S. A. Self, Proc. of VIth Int. Conf. on Ionization Phenomena in Gases, Paris, 1963 (SERMA Publishing Company, Paris), Vol. 3, p.51.
OF OPTICAL NIOBATE
BIREFRINGENCE
J. WARNER, D. S. ROBERTSON and K. F. HULME
Royal Radar Establishment, Great Malvern, Worcestershire, England Received 29 December 1965
The optical birefringence of lithium niobate (LiNbO3) has been measured at 632.8 and 1152.3 my from room temperature up to the ferroelectric Curie temperature. The birefringence falls by a factor of about 8 between room temperature and the Curie point, which was determined to be 1205 + 5°C. The results indicate that the phase above the Curie point is not cubic. It has been shown that l i t h i u m niobate has adv a n t a g e o u s p r o p e r t i e s for m i x i n g l a s e r b e a m s [1]. F o r c e r t a i n m i x i n g p r o c e s s e s one can r e a i i s e the d e s i r a b l e s i t u a ti o n in which phase m a t c h i n g o c c u r s p e r p e n d i c u l a r to the optic axis. F l e x i b i l i t y in the choice of o p t i c a l f r e q u e n c i e s i n t e r a c t i n g in this way a r i s e s f r o m the t e m p e r a t u r e s e n s i t i v i t y of the b i r e f r i n g e n c e of l it h i u m niobate. This has been p r e v i o u s l y m e a s u r e d n e a r r o o m t e m p e r a t u r e [2]. We p r o v i d e b i r e f r i n g e n c e data o v e r a wide t e m p e r a t u r e r a n g e . Single c r y s t a l s of li th iu m niobate have been grown f r o m the m e l t using the C z o c h r a l s k i t e c h nique. The s t a r t i n g m a t e r i a l was obtained f r o m B r i t i s h D r u g Houses L t d . , and a m o d i f i e d f o r m of the c r y s t a l pulling e q u i p m e n t p r e v i o u s l y des c r i b e d [3] was used. The m e l t was contained in a t h i c k - w a l l e d platinum c r u c i b l e which was supp o r t e d by a r e c r y s t a l l i z e d a l u m i n a c r u c i b l e . The c r y s t a l s w e r e pulled f r o m the m e l t into a t e m p e r a t u r e c o n t r o l l e d a f t e r h e a t e r m a i n t a i n e d at a t e m p e r a t u r e of 800°C. The a f t e r h e a t e r t e m p e r a t u r e was then r e d u c e d to 20°C in a c o n t r o l l e d way o v e r a p e r i o d of about 16 hours. C r y s t a l growth was c a r r i e d out in an a t m o s p h e r e of oxygen flowing through the a p p a r a t u s at a r a t e of 0.5 l i t r e p e r minute. T y p i c a l c r y s t a l s g r o w n in this way w e r e about 5 c m long, 1.0 cm in d i a m e t e r , and w e r e pale g r e e n in c o l o r .
A single c r y s t a l was o r i e n t e d by back r e f l e c tion Laue X - r a y t e c h n i q u e s , and a plate about a m i l l i m e t r e thick was cut with the optic axis p a r a l l el to the f a c e s of the plate; the l a t t e r w e r e then o p t i cal l y polished. The c r y s t a l o r i e n t a t i o n was ch eck ed with a p o l a r i s i n g m i c r o s c o p e . A n t i - p a r a l l e l domains [4] probably e x i s t e d in this plate, but w e r e u n d e t e c t a b l e under the p o l a r i s i n g m i c r o s c o p e , and would not affect the b i r e f r i n g e n c e m e a s u r e m e n t s we made. The change of b i r e f r i n g e n c e with t e m p e r a t u r e was m e a s u r e d by allowing the plane p o l a r i s e d light f r o m a gas l a s e r , o p e r a t i n g at e i t h e r 632.8 o r 1152.3 m y , to p a s s n o r m a l l y through the lithium niobate. The optic a x i s of the c r y s t a l was s e t at 45 ° to the plane of p o l a r i s a t i o n . The e m e r gent light p a s s e d through a Nicol p r i s m c r o s s e d w . r . t , the l a s e r p o l a r i s a t i o n , on to a p h o t o - c e l l . The plate was in a f u r n a c e in which an oxygen p r e s s u r e of 1½ a t m o s p h e r e was m a i n t a i n e d to p r e v e n t any l o s s of oxygen f r o m the plate at high t e m p e r a t u r e s . The plate t e m p e r a t u r e was m e a s u r e d by a c a l i b r a t e d P t / 1 3 % R h - P t t h e r m o c o u p l e junction p l a c e d c l o s e to the plate, and r e c o r d e d on one channel of a two channel cha~rt r e c o r d e r . The r e l a t i v e light i n t e n s i t y t r a n s m i t t e d through the s y s t e m at a t e m p e r a t u r e T is given by
[//o = sin2g° = sin2( 2~ lB/~) ;
(1) 163
P HYSI CS L E T T E RS
IN
THE
1 February 1966
ON THE PROPAGATING WAVES HOT-CATHODE MERCURY DISCHARGE
N. SATO and Y. H A T T A Department of Electronic Engineering, Tohoku University, Sendal, Japan Received 3 January 1966
Experimental results on propagating waves excited by small coil surrounding a discharge tube under a weak axial magnetic field show that the waves excited have the properties of the backward waves on the both sides of the exciting coil. m o v i n g s t r i a t i o n s . E x p e r i m e n t a l r e s u l t s obtained by Craw-ford and Self [6] w e r e c o n c e r n e d with the b a c k w a r d w a v e s e x c i t e d by the s m a l l c o i l in the hotcathode m e r c u r y d i s c h a r g e . But t h e i r a t t e m p t s to d e t e c t the w a v e s on the cathode s i d e of the e x citing c o i l w e r e u n s u c c e s s f u l . In our c a s e , the w a v e s can be d e t e c t e d on the both s i d e s of the e xciting coil. E x p e r i m e n t s a r e m a d e m a i n l y on the w a v e s on the cathode side. The e x p e r i m e n t a l a p p a r a t u s and conditions a r e shown in fig. 1. The w a v e s a r e d e t e c t e d by two p h o t o m u l t i p l i e r s t h r o u g h o p t i cal f i b e r s the d i a m e t e r of which i s 3 m m and the s i g n a l s f r o m two
This l e t t e r r e p o r t s s o m e new e x p e r i m e n t a l r e s u l t s on p r o p a g a t i n g w a v e s e x c i t e d by s m a l l c o i l in the p o s i t i v e c o l u m n of h o t - c a t h o d e m e r c u r y d i s c h a r g e u n d e r a weak m a g n e t i c field. E x p e r i m e n t a l w o r k s on the p r o p a g a t i n g ion w a v e s w e r e done by s e v e r a l a u t h o r s [1-4]. R e c e n t l y , B a r r e t t and L i t t l e [5] showed that t h e r e w e r e two types of the p r o p a g a t i n g w a v e s in the p o s i t i v e column of the m e r c u r y - p o o l a r c d i s c h a r g e . While one of t h e m gave the d i s p e r s i o n r e l a t i o n f o r ion w a v e s , the o t h e r gave that f o r b a c k w a r d w a v e s and was not s u f f i c i e n t l y u n d e r s t o o d though it was thought to be i n t e r p r e t e d as the d i s p e r s i o n r e l a t i o n f o r
DISCHARGE°CURRENT 5mA-- I A HELMHOLI~ COIL"" [ ~ (0-250
GAUSS)
HOT CATHODE
GAS PRESSURE
~]
t
t •~
•
D'~
I\/11% L"~'A
I - 2 x 10"SmmHg
ELECTRON TEMPERATURE2 - 4 x I 0 a °K PLASMA DENSITY I0 s - I0 V / c m s ii
,b__
d~-
" EXC,T,NG CO,"
', £ODE
FEW TENS OF GAUSS~ \ OPTICAl.. FIBER
FILTER
\
DUAL BEAM SYNCHROSCOPE
~g
"1
TuBEA 60
50
L 1027
"rUBEB 35
30
1235
Fig. 1. Experimental apparatus. All dimensions are in millimeters. 161
Volume 20, n u m b e r 2
P HYSI CS L E T T E R S 540"
I0°
360"
1 F e b r u a r y 1966
,7, t.g rr (..9 UJ E:3
-
i
x~ ~.~
~
"x
==
360°
.
=
-a.
o
=
30
/
-
o
,o-,
/~//---x.".Y-.
/.,y ,
,4o0
II
=70cm
I00
= 50
180 %
LU O3
/
/ / /
//yj
.
zlf~"
"1" O...
/~" 0° 0
I 5
I I0 2.
12 = 15.3 cm I I 15 20 (CM)
1~ / / / /
25
Fig. 2. The phase and the amplitude of the waves e x cited on the cathode side for tube A. l is the distance f r o m the n o r m a l i z a t i o n point (7 cm f r o m the exciting coil). d i f f e r e n t p o s i t i o n s a~'e c o m p a r e d on d u a l b e a m synchroscope. On t h e b o t h s i d e s , t h e w a v e s a r e d a m p e d s o heavily that the signals are not detectable near the electrodes unless the exciting coil is very c l o s e to t h e m . T h e p h a s e a n d t h e a m p l i t u d e of t h e w a v e s e x c i t e d on t h e c a t h o d e s i d e c h a n g e c o n tinuously, but those on the anode side not always c o n t i n u o u s l y . It m a y b e s a i d t h a t t h e v a l u e s of the phase velocity and the damping distance on the anode side are rather larger than those on the cathode side. The signals from the center and t h o s e n e a r t h e w a l l a r e 180 ° o u t of p h a s e o n t h e c r o s s s e c t i o n of t h e t u b e e x c e p t f o r t h e r e g i o n n e a r t h e e x c i t i n g c o i l . T h u s t h e m o d e of t h e w a v e s i s c o n s i d e r e d a s n = 0, m = 2 (the B e s s e l f u n c t i o n Jn(kmr) i s a s s u m e d i n t h e r a d i a l d i r e c t i o n a n d m d e n o t e s t h e r u t h z e r o of Jn(kmr)) i n c o n t r a s t w i t h t h e c a s e of L i t t l e e t al. [2, 4, 5] a n d C r a w f o r d a n d S e l f [6] in w h i c h n = 0, m = 1. M e a s u r e m e n t s of t h e p h a s e i n t h e a x i a l d i r e c t i o n o n t h e b o t h s i d e s of t h e e x c i t i n g c o i l s h o w t h a t t h e p h a s e v e l o c i t y i s a l w a y s d i r e c t e d to t h e e x c i t i n g coil. The typical data for the waves excited on t h e c a t h o d e s i d e a r e s h o w n i n fig. 2. T h e w a v e s a r e m o s t e a s i l y e x c i t e d f o r t h e f r e q u e n c y of 20 k c / s - 35 k c / s m u c h a b o v e a n d b e l o w w h i c h t h e a m p l i t u d e b e c o m e s t o o s m a l l to b e d e t e c t e d . T h e p r o p e r t y of t h e w a v e s d o e s n o t e s s e n t i a l l y d e p e n d o n t h e d i s c h a r g e c u r r e n t (I) e x c e p t t h a t t h e i n t e n 162
=150gauss
,o
I LU ¢,D Z W n" i.U LL LL
•
0° 0
,,: 12= 2 3 I I0
I 20 £
I 30
cm I 40
(CM)
Fig. 3. The effect of the steady axial magnetic field (Bo) on the propagation of the waves excited on the cathode side for tube B. x denotes the p h a s e of the s e l f excited waves Or = 29 k c / s , Bo = 62.5 gauss). s i t y of t h e s i g n a l s i n c r e a s e s w i t h 1. It i s s h o w n i n fig. 3 h o w t h e s t e a d y m a g n e t i c f i e l d (Bo) a f f e c t s t h e p r o p a g a t i o n of t h e w a v e s . T h e r e a p p e a r o c casionally self-excited waves at a certain value of t h e m a g n e t i c f i e l d w i t h o u t t h e e x t e r n a l e x c i t a t i o n . T h e p h a s e v e l o c i t y of t h e s e w a v e s i s i n t h e d i r e c t i o n of t h e a n o d e a n d i s n e a r l y c o n s i s t e n t w i t h t h a t of t h e w a v e s w h i c h a r e e x c i t e d e x t e r n a l ly o n t h e c a t h o d e s i d e a n d p r o p a g a t e i n t h e d i r e c t i o n of t h e a n o d e . The wave length does not change appreciably w i t h t h e f r e q u e n c y , b u t i t i s i n c l i n e d to b e c o m e long as the frequency increases and so the waves h a v e t h e p r o p e r t y of t h e b e a c k w a r d w a v e s o n t h e b o t h s i d e s of t h e e x c i t i n g c o i l . It i s n o t w e l l u n d e r stood why the waves are excited more easily on the cathode side and the phase and the amplitude of t h e w a v e s on t h e a n o d e s i d e do n o t c h a n g e s o c o n t i n u o u s l y a s t h o s e on t h e c a t h o d e s i d e . T h e waves are not considered as the so-called moving striations which are understood as the popular e x a m p l e of t h e b a c k w a r d w a v e s i n t h e p o s i t i v e c o l u m n , b e c a u s e t h e p r o p e r t y of t h e w a v e s d o e s not depend on the discharge current and the mode of t h e m i s n= 0, m = 2 c o n t r a r y t o t h e m o v i n g striations for which the light signals from the center and near the wall are in phase on the c r o s s s e c t i o n of t h e t u b e .
Volume 20, number 2
P HYSI CS L E T T E RS
1 February 1966
References 1. Y. Hatta and N. Sato, Proc. Vth Int. Conf. on Ionization Phenomena in Gases, Munich, 1961 (North-Holland Publishing Company, Amsterdam) p.478. 2. P . F . Little, ibid,, p. 1440; P. F. Little, Nature 194 (1962) 1137. 3. A.Y.Wong, N.D'Angelo and R.W. Motley, Phys. Rev. Letters 9 (1962) 415; A. Y. Wong, R.W. Motley and N.D'Angelo, Phys. Rev. 133 (1964) A 436.
THE
TEMPERATURE
DEPENDENCE IN LITHIUM
4. P. F. Little and H. G. Jones, Proc. Phys. Soc. 85 (1965) 979. 5. P. J. Barrett and P. F. Little, Phys. Rev. Letters 14 (1965) 356. 6. F.W. Crawford and S. A. Self, Proc. of VIth Int. Conf. on Ionization Phenomena in Gases, Paris, 1963 (SERMA Publishing Company, Paris), Vol. 3, p.51.
OF OPTICAL NIOBATE
BIREFRINGENCE
J. WARNER, D. S. ROBERTSON and K. F. HULME
Royal Radar Establishment, Great Malvern, Worcestershire, England Received 29 December 1965
The optical birefringence of lithium niobate (LiNbO3) has been measured at 632.8 and 1152.3 my from room temperature up to the ferroelectric Curie temperature. The birefringence falls by a factor of about 8 between room temperature and the Curie point, which was determined to be 1205 + 5°C. The results indicate that the phase above the Curie point is not cubic. It has been shown that l i t h i u m niobate has adv a n t a g e o u s p r o p e r t i e s for m i x i n g l a s e r b e a m s [1]. F o r c e r t a i n m i x i n g p r o c e s s e s one can r e a i i s e the d e s i r a b l e s i t u a ti o n in which phase m a t c h i n g o c c u r s p e r p e n d i c u l a r to the optic axis. F l e x i b i l i t y in the choice of o p t i c a l f r e q u e n c i e s i n t e r a c t i n g in this way a r i s e s f r o m the t e m p e r a t u r e s e n s i t i v i t y of the b i r e f r i n g e n c e of l it h i u m niobate. This has been p r e v i o u s l y m e a s u r e d n e a r r o o m t e m p e r a t u r e [2]. We p r o v i d e b i r e f r i n g e n c e data o v e r a wide t e m p e r a t u r e r a n g e . Single c r y s t a l s of li th iu m niobate have been grown f r o m the m e l t using the C z o c h r a l s k i t e c h nique. The s t a r t i n g m a t e r i a l was obtained f r o m B r i t i s h D r u g Houses L t d . , and a m o d i f i e d f o r m of the c r y s t a l pulling e q u i p m e n t p r e v i o u s l y des c r i b e d [3] was used. The m e l t was contained in a t h i c k - w a l l e d platinum c r u c i b l e which was supp o r t e d by a r e c r y s t a l l i z e d a l u m i n a c r u c i b l e . The c r y s t a l s w e r e pulled f r o m the m e l t into a t e m p e r a t u r e c o n t r o l l e d a f t e r h e a t e r m a i n t a i n e d at a t e m p e r a t u r e of 800°C. The a f t e r h e a t e r t e m p e r a t u r e was then r e d u c e d to 20°C in a c o n t r o l l e d way o v e r a p e r i o d of about 16 hours. C r y s t a l growth was c a r r i e d out in an a t m o s p h e r e of oxygen flowing through the a p p a r a t u s at a r a t e of 0.5 l i t r e p e r minute. T y p i c a l c r y s t a l s g r o w n in this way w e r e about 5 c m long, 1.0 cm in d i a m e t e r , and w e r e pale g r e e n in c o l o r .
A single c r y s t a l was o r i e n t e d by back r e f l e c tion Laue X - r a y t e c h n i q u e s , and a plate about a m i l l i m e t r e thick was cut with the optic axis p a r a l l el to the f a c e s of the plate; the l a t t e r w e r e then o p t i cal l y polished. The c r y s t a l o r i e n t a t i o n was ch eck ed with a p o l a r i s i n g m i c r o s c o p e . A n t i - p a r a l l e l domains [4] probably e x i s t e d in this plate, but w e r e u n d e t e c t a b l e under the p o l a r i s i n g m i c r o s c o p e , and would not affect the b i r e f r i n g e n c e m e a s u r e m e n t s we made. The change of b i r e f r i n g e n c e with t e m p e r a t u r e was m e a s u r e d by allowing the plane p o l a r i s e d light f r o m a gas l a s e r , o p e r a t i n g at e i t h e r 632.8 o r 1152.3 m y , to p a s s n o r m a l l y through the lithium niobate. The optic a x i s of the c r y s t a l was s e t at 45 ° to the plane of p o l a r i s a t i o n . The e m e r gent light p a s s e d through a Nicol p r i s m c r o s s e d w . r . t , the l a s e r p o l a r i s a t i o n , on to a p h o t o - c e l l . The plate was in a f u r n a c e in which an oxygen p r e s s u r e of 1½ a t m o s p h e r e was m a i n t a i n e d to p r e v e n t any l o s s of oxygen f r o m the plate at high t e m p e r a t u r e s . The plate t e m p e r a t u r e was m e a s u r e d by a c a l i b r a t e d P t / 1 3 % R h - P t t h e r m o c o u p l e junction p l a c e d c l o s e to the plate, and r e c o r d e d on one channel of a two channel cha~rt r e c o r d e r . The r e l a t i v e light i n t e n s i t y t r a n s m i t t e d through the s y s t e m at a t e m p e r a t u r e T is given by
[//o = sin2g° = sin2( 2~ lB/~) ;
(1) 163