ELECTRON MICROPROBE STUDIES ON MONOMODE OPTICAL FIBRES AND PREFORMS PREPARED BY THE MCVD TECHNIQUE

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Submitted on 1 Jan 1984

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ELECTRON MICROPROBE STUDIES ON

MONOMODE OPTICAL FIBRES AND PREFORMS PREPARED BY THE MCVD TECHNIQUE

A. Skeats, B. Ainslie

To cite this version:

A. Skeats, B. Ainslie. ELECTRON MICROPROBE STUDIES ON MONOMODE OPTICAL FIBRES AND PREFORMS PREPARED BY THE MCVD TECHNIQUE. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-757-C2-760. �10.1051/jphyscol:19842174�. �jpa-00223848�

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Colloque C2, suppl6ment au n02, Tome 45, fgvrier 1984 page C2-757

ELECTRON MICROPROBE S T U D I E S ON MONOMODE O P T I C A L F I B R E S AND PREFORMS PREPARED BY THE MCVD TECHNIQUE

A . P . Skeats and B . J . Ainslie

British TeZecom Research Laboratories, MartZesham Heath, Ipswich, SuffoZk I P S 7RE, U.K.

RBsumB - A l ' a i d e d ' u n e m i c r o - s o n d e B l e c t r o n i q u e , o n a 6 t u d i 6 l a d i s t r i b u t i o n d e s B l 6 m e n t s d a n s l e s f i b r e s o p t i q u e s monomodes e t d a n s l e s p r B f o r m e s p r 6 p a r G e s s e l o n l a mBthode MCVD ( d 6 p 6 t p a r p h a s e v a p e u r m o d i f i g e ) .

A b s t r a c t - The e l e c t r o n m i c r o p r o b e h a s been u s e d t o s t u d y t h e e l e m e n t a l d i s t r i b u t i o n i n monomode o p t i c a l f i b r e s and p r e f o r m s p r e p a r e d by t h e MCVD ( m o d i f i e d c h e m i c a l v a p o u r d e p o s i t i o n ) t e c h n i q u e .

T h e r e i s c u r r e n t l y w o r l d - w i d e i n t e r e s t i n t h e m a k i n g o f o p t i c a l f i b r e s f o r t e l e c o m m u n i c a t i o n a p p l i c a t i o n s . For t r u n k t r a n s m i s s i o n i n w h i c h h i g h d a t a r a t e s a r e u s e d , v e r y l o w l e v e l s o f a t t e n u a t i o n and d i s p e r s i o n a r e p a r t i c u l a r l y i m p o r t a n t . One of t h e m o s t s u i t a b l e t y p e s o f o p t i c a l f i b r e f o r t h i s a p p l i c a t i o n i s monomode. Our l a b o r a t o r i e s h a v e d e v e l o p e d a monomode f i b r e w h i c h c o n s i s t s o f a s m a l l c e n t r a l c o r e o f h i g h r e f r a c t i v e i n d e x ( R I ) m a t e r i a l ( s i l i c a d o p e d w i t h g e r m a n i a ) s u r r o u n d e d by a c l a d d i n g l a y e r o f l o w e r R I m a t e r i a l ( s i l i c a d o p e d w i t h l o w l e v e l s o f p h o s p h o r u s and f l u o r i n e ) ( 1 ) . A l t h o u g h s u c h f i b r e s h a v e been made w i t h l o s s e s a s l o w a s O.2ldB/km, s t r e n u o u s e f f o r t s a r e b e i n g made t o i m p r o v e t h e f i b r e t r a n s m i s s i o n c h a r a c t e r i s t i c s , n o t o n l y i n t e r m s o f l o s s and d i s p e r s i o n b u t a l s o i n o t h e r i m p o r t a n t f a c t o r s w h i c h a r e r e l a t e d t o f i b r e s t r u c t u r e s u c h a s s e n s i t i v i t y t o b e n d i n g and e a s e o f j o i n t i n g .

The f i b r e s i n t h i s s t u d y h a v e been made u s i n g t h e m o d i f i e d c h e m i c a l v a p o u r d e p o s i t i o n ( M C V D ) p r o c e s s ( 2 ) i n w h i c h l a y e r s o f s u i t a b l y d o p e d s i l i c a a r e d e p o s i t e d on t h e i n n e r w a l l o f a s i l i c a s u b s t r a t e t u b e by t h e d e c o m p o s i t i o n of a g a s e o u s m i x t u r e . An e x t e r n a l o x y g e n / h y d r o g e n b u r n e r i s t r a v e r s e d back and f o r t h a l o n g t h e t u b e , b u i l d i n g l a y e r s o f d e p o s i t e d g l a s s . A f t e r d e p o s i t i o n t h e t u b i n g i s c o l l a p s e d i n t o a s o l i d r o d p r e f o r m and i s s u b s e q u e n t l y drawn i n t o f i b r e . Such a p r o c e s s s h o u l d a l l o w t h e t a i l o r i n g o f R I - I

p r o f i l e s a c r o s s t h e f i b r e by -lmm+ ^II

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s u c c e s s i v e p a s s e s o f t h e b u r n e r . I I

However, a s c a n be s e e n f r o m f i g u r e 1 , w h i c h shows t h e R I p r o f i l e a c r o s s t h e c e n t r e o f a p r e f o r m , t h e p r o f i l e s o b t a i n e d a r e n o t s i m p l e . I n p a r t i c u l a r

" r i p p l i n g T 1 , i . e . v a r i a t i o n s i n R I a c r o s s o n e d e p o s i t e d l a y e r , i s s e e n i n b o t h t h e c o r e and

c l a d d i n g , a s i s a s i g n i f i c a n t d i p F i g u r e 1 : R I p r o f i l e a c r o s s i n R I a t t h e c e n t r e o f t h e c o r e . a p r e f o r m c o r e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842174

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C2-758 JOURNAL DE PHYSIQUE

T h i s p a p e r d e s c r i b e s t h e e l e c t r o n m i c r o p r o b e work c o n d u c t e d a t BTRL t o d e t e r m i n e t h e m a t e r i a l c a u s e o f t h e s e e f f e c t s a n d t h e m o n i t o r i n g o f e x p e r i m e n t s d e s i g n e d t o i m p r o v e t h e f i b r e c h a r a c t e r i s t i c s .

EXPERIMENTAL DETAILS:

The m i c r o p r o b e work t o be d e s c r i b e d c o n s i s t e d o f e l e m e n t a l l i n e s c a n s a c r o s s p o l i s h e d c r o s s - s e c t i o n s o f p r e f o r m s and drawn f i b r e . The work was c o n d u c t e d i n a Cambridge I n s t r u m e n t s S180 s c a n n i n g e l e c t r o n m i c r o s c o p e f i t t e d w i t h a M i c r o s p e c WDX-2A f u l l y f o c u s s i n g s p e c t r o m e t e r . L i n e s c a n s on t h e l a r g e r s p e c i m e n s w e r e d o n e by d r i v i n g t h e s p e c i m e n a t a c o n s t a n t r a t e u n d e r a s t a t i o n a r y e l e c t r o n beam. The s m a l l e r s p e c i m e n s w e r e h e l d s t a t i o n a r y and t h e e l e c t r o n beam r e p e a t e d l y s c a n n e d a c r o s s t h e s u r f a c e , t h e r e s u l t a n t c o u n t s b e i n g f e d t o t h e d i g i t a l l i n e - s c a n u n i t o f a n EDAX 9160 e n e r g y d i s p e r s i v e s y s t e m and s t o r e d i n i t s m u l t i c h a n n e l a n a l y s e r . T h i s d i g i t a l t e c h n i q u e e n s u r e s t h e a b s e n c e o f t i m e c o n s t a n t e f f e c t s a s w e l l a s m i n i m i s i n g t h e e f f e c t o f a n y d r i f t i n beam c u r r e n t .

R e f r a c t i v e i n d e x p r o f i l e s w e r e c a r r i e d o u t u s i n g a P I 0 1 York T e c h n o l o g y P r e f o r m P r o f i l e r .

GENERAL RESULTS:

F i g u r e s 2 , 3 and 4 show t h e e l e m e n t a l s c a n s f o r Ge, P and F a c r o s s t h e same p r e f o r m a s t h e R I p r o f i l e shown i n f i g u r e 1 . No

a t t e m p t h a s been made t o F i g u r e 2: Ge s c a n a c r o s s a q u a n t i f y a c c u r a t e l y t h e a n a l y s e s p r e f o r m c o r e .

b u t t h e a p p r o x i m a t e peak e l e m e n t a l c o n c e n t r a t i o n s a r e t y p i c a l l y 7%Ge, l%P and 0.2%F by w e i g h t . I t

c a n be c l e a r l y s e e n t h a t t h e R I -1mm- f

r i p p l i n g i n t h e c o r e i s c a u s e d by ^I ^I

a v a r i a t i o n i n t h e Ge l e v e l i n ^I₁ ^I

e a c h d e p o s i t e d l a y e r , w h i l e t h e ¹

r i p p l i n g i n t h e c l a d d i n g i s c a u s e d s o l e l y by v a r i a t i o n i n t h e P c o n c e n t r a t i o n . It i s somewhat s u r p r i s i n g t h a t t h e r e i s n o o b v i o u s r i p p l i n g i n t h e F l e v e l i n t h e c l a d d i n g . However,

c o n s i d e r a b l e d i f f u s i o n o f F i n t o F i g u r e 3: P s c a n a c r o s s a t h e c o r e h a s r e s u l t e d , a s c a n be p r e f o r m c o r e .

s e e n i n f i g u r e 4 , and i t i s l i k e l y

= h a t s u c h d i f f u s i o n h a s s m o o t h e d

o u t t h e F l e v e l i n t h e c l a d d i n g . I I

It m i g h t be t h o u g h t t h a t t h e I l m m I

d i f f u s i o n o f F i n t o t h e c o r e e d g e s would be a c c o m p a n i e d by a r e d u c t i o n i n R I ; i n t e r e s t i n g l y t h i s i s n o t o b s e r v e d f r o m t h e R I p r o f i l e s .

The r i p p l i n g o f t h e Ge i n t h e c o r e i s a n u n d e s i r a b l e f e a t u r e a s s h a r p c o m p o s i t i o n a l f l u c t u a t i o n s l e a d t o

s t r e s s g r a d i e n t s and t h e s e i n t u r n F i g i r e 4 : F s c a n a c r o s s a i n c r e a s e t h e f i b r e t r a n s m i s s i o n p r e f o r m c o r e .

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p a r t i c u l a r , a v e r y s h a r p Ge b o u n d a r y e x i s t s and s u g g e s t s t h a t r e d u c e d r i p p l i n g i n t h i s r e g i o n would y i e l d i m p r o v e d f i b r e l o s s e s .

A f u r t h e r f e a t u r e o f t h e c o r e s t r u c t u r e i s t h e r e d u c t i o n i n i n d e x i n t h e c e n t r a l r e g i o n . T h e r e i s good a g r e e m e n t b e t w e e n t h e Ge s c a n and t h e R I p r o f i l e i n t h i s r e g i o n w h i c h s u g g e s t s t h a t t h e r e d u c t i o n i n i n d e x i s d u e t o t h e e v a p o r a t i o n o f g e r m a n i a d u r i n g t h e c o l l a p s i n g s t a g e o f t h e p r o c e s s . T h i s i n d e x d i p n e c e s s i t a t e s a n o v e r a l l i n c r e a s e i n t h e Ge l e v e l i n t h e r e s t o f t h e c o r e ( w h i c h w i l l i n c r e a s e f i b r e l o s s ) ; o t h e r w i s e l e s s t h a n optimum g u i d i n g o f o p t i c a l power and a n i n c r e a s e d s e n s i t i v i t y t o b e n d i n g o c c u r s .

ETCHING EXPERIMENTS:

A method o f o v e r c o m i n g t h e R I d i p p r o b l e m s i s t o e t c h away t h e d e p l e t e d l a y e r f r o m t h e i n n e r s u r f a c e of a p a r t i a l l y c o l l a p s e d t u b e b e f o r e f i n a l c o l l a p s e i n t o a r o d ( 4 ) . The g a s p h a s e e t c h i n g p r o c e s s i s c a r r i e d o u t by p a s s i n g CC1 F2 i n O2 t h r o u g h t h e t u b i n g and f o r m i n g t h e v o l a t i l e S i F 4 a c c o r i i n g t o :

F i g u r e 5 shows a Ge s c a n a c r o s s s u c h a p r e f o r m s h o w i n g j u s t how s u c c e s s f u l s u c h a t e c h n i q u e c a n be ( c f f i g u r e 2 ) . F i g u r e 6 s h o w s a F s c a n a c r o s s t h e same p r e f o r m . A s w e l l a s t h e p r e v i o u s l y o b s e r v e d

I--- I mm ---I

I--- lmm

F i g u r e 5: Ge s c a n a c r o s s a F i g u r e 6 : F s c a n a c r o s s a p r e f o r m c o r e ( e t c h i n g p r e f o r m c o r e ( e t c h i n g

e x p e r i m e n t ) . e x p e r i m e n t 1.

d i f f u s i o n o f F i n t o t h e c o r e e d g e s f r o m t h e c l a d d i n g i t i s now f o u n d t h a t F h a s b e e n p i c k e d u p a t t h e c e n t r e o f t h e c o r e d u r i n g t h e e t c h i n g p r o c e s s . T h i s i s a s u r p r i s i n g o b s e r v a t i o n a s S i F 4 i s a v e r y v o l a t i l e s p e c i e s .

STRESS REDUCTION:

I

^Imm

S t r e s s a t t h e c o r e - c l a d d i n g I I

i n t e r f a c e i s l a r g e l y a s s o c i a t e d w i t h t h e r a p i d c h a n g e i n t h e g e r m a n i a l e v e l i n t h i s r e g i o n ( 3 ) . One method o f r e d u c i n g t h i s s t r e s s i s t o u s e a c o r e o f p u r e s i l i c a and t o d e p r e s s t h e R I o f t h e c l a d d i n g by u s e o f h i g h l e v e l s o f

F ( a p p r o x i m a t e l y 2.5Wt% 1. F i g u r e 7 F i g u r e 7 : F s c a n a c r o s s a shows a s c a n f o r F a c r o s s s u c h a p r e f o r m c o r e ( p u r e s i l i c a p r e f o r m . It i s i n t e r e s t i n g t o n o t e c o r e 1.

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C2-760 JOURNAL DE PHYSIQUE

A d i f f e r e n t m e t h o d o f s t r e s s r e d u c t i o n a d o p t e d a t BTRL ( 5 ) i s t o p r o d u c e a c o r e w i t h a t r i a n g u l a r R I p r o f i l e , r a t h e r t h a n a s u d d e n s t e p . T h i s m e t h o d c a n b e e m p l o y e d t o p r o d u c e a f i b r e w h i c h h a s a d i s p e r s i o n minimum a t a l o n g e r w a v e l e n g t h . T h i s a p p r o a c h r e q u i r e s v e r y p r e c i s e c o n t r o l o f t h e d e p o s i t i o n p r o c e s s a n d a minimum o f r i p p l e o n t h e c o r e s t r u c t u r e . F i g u r e 9 c l e a r l y i l l u s t r a t e s t h a t t h e c o r e h a s b e e n w e l l g r a d e d t o p r o d u c e a t r i a n g u l a r s h a p e a l t h o u g h some r i p p l e i s s t i l l i n e v i d e n c e .

t h a t t h e r e i s c o m p a r a t i v e l y l i t t l e s i g n o f d i f f u s i o n o f F i n t o t h e c o r e a t t h i s s t a g e . T h i s i s p r o b a b l y a s s o c i a t e d w i t h t h e l o w e r

f a b r i c a t i o n t e m p e r a t u r e e m p l o y e d t o make t h i s p r e f o r m . F i g u r e 8 s h o w s a d i g i t a l s c a n a c r o s s t h e same m a t e r i a l a f t e r p u l l i n g i n t o f i b r e ,

s h o w i n g t h a t d i f f u s i o n o f F h a s t a k e n p l a c e d u r i n g t h e h i g h t e m p e r a t u r e p u l l i n g p r o c e s s . T h i s

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p r e f o r m c o r e ( t r i a n g u l a r p r o f i l e c o r e ) .

r e s u l t m a k e s t h e i m p o r t a n t F i g u r e 8: D i g i t a l F s c a n o b s e r v a t i o n t h a t p r e f o r m p r o f i l e s a c r o s s a f i b r e c o r e ( p u r e w i l l n o t e x a c t l y r e p r e s e n t f i b r e s i l i c a c o r e ) .

p r o f i l e s , w h i c h h a v e l e s s s h a r p

b o u n d a r i e s . C a r e h a s t o b e t a k e n when i n t e r p r e t i n g s c a n s a c r o s s s u c h s m a l l s a m p l e s a s t h e s i z e o f t h e x - r a y e x c i t a t i o n v o l u m e i s now s i g n i f i c a n t .

ACKNOWLEDGEMENTS:

A c k n o w l e d g e m e n t i s made t o t h e D i r e c t o r of R e s e a r c h o f B r i t i s h T e l e c o m f o r p e r m i s s i o n t o p u b l i s h t h i s w o r k . T h a n k s a r e a l s o g i v e n t o M r . J . A n s e l l f o r s a m p l e p r e p a r a t i o n .

REFERENCES:

( 1 ) AINSLIE-B. J , BEALES. K. J , DAY. C. R a n d RUSH. J . D . , IEEE.

J . Q a n t . E l e c . 1 8 , No.4 ( 1 9 8 2 ) 5 1 4

( 2 ) Mac CHESNEY. J . B , 0 ' CONNOR. P. B. a n d PRESBY. H. M, P r o c . IEEE, 6 2 , ( 1 9 7 4 ) 1 2 7 8

( 3 ) AINSLIE-B. J , BEALES. K. J , DAY. C. R a n d RUSH. J . D , J. Q u a n t . E l e c . , QE17, 6 , ( 1 9 8 0 ) 8 5 4

( 4 ) AINSLIE. B. J , BEALES. K. J , COOPER. D. M , DAY. C. R a n d RUSH. J . D , E l e c . L e t t . , 1 8 , No.19, ( 1 9 8 2 ) 8 4 2

( 5 ) AINSLIE. B. J , BEALES. K. J , COOPER. D. M a n d D A Y . C. R , E l e c . L e t t . , 1 8 , No.19, ( 1 9 8 2 ) 8 0 9