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ANALYSIS OF CHALCOGENIDE GLASS OPTICAL
FIBRES BY EXAFS
C. Hervo, J.-Y. Barraud, A. Flank, Dominique Bazin, H. Dexpert, P. Lagarde
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, supplément au n°12, Tome 46, décembre 1985 page C8-555
ANALYSIS OF CHALCOGENIDE GLASS OPTICAL FIBRES BY EXAFS
C. Hervo, J . - Y . B a r r a u d , A.M. Flank+ , D. Bazin+ , H. Dexpert+ and P. L a g a r d e+
C.G.E. Research Center, Route de Nozay, 91460 Marcoussis, France
+L.U.R.E., Bailment 209 C, 91405 Orsay, France
Résumé - Les fibres optiques infrarouges de type chalcogénure GeAsSeTe (transmet-tant entre 4 et 10,6 pm) présentent un grand intérêt dans le domaine de la chirurgie (laser C 02 à 10,6 um) des capteurs (capteurs de température), de l'imagerie et dans le domaine militaire (détection). Afin d'étudier les causes d'absorption intrinsèque pouvant être liées à la structure à courte distance, une étude spectroscopique par EXAFS a été menée sur deux verres A et B avant et après fibrage. Les résultats montrent que l'introduction de tellure n'affecte pas l'environnement local du sélénium, contrairement à ceux du Ge et As.
A b s t r a c t Chalcogenide optical fibres (GeAsTeSe type) a r e interesting for t r a n s -mission in t h e 4-11 um range, t h e introduction of tellurium into this type of glass resulting in a shifting of t h e infrared absorption edge toward longer wavelengths. To study t h e intrinsic absorption causes, we performed by EXAFS an analysis of t h e short range order of glasses before and a f t e r fibre drawing. R e s u l t s show t h a t t h e addition of tellurium does not affect t h e local s t r u c t u r e of t h e selenium, while t h a t of Ge or As seems modified.
INTRODUCTION
Beside t h e development of silica glass fibres for t e l e c o m m u n i c a t i o n s many investigations have been made recently on new m a t e r i a l s for infrared t r a n s m i t t i n g optical fibres, operating in t h e 4-11 micron band where silica is not t r a n s p a r e n t .
The availability of such flexible light conductors will play an i m p o r t a n t role in t h e develop-m e n t of develop-many infrared devices in t h e fields of idevelop-mage relay, r e develop-m o t e sensing, and power applications using C 02 laser in surgery as well, welding and h e a t t r e a t m e n t .
The influence of various processing conditions on t h e infrared transmission of glass blocks and fibres has been studied (1). Most of t h e investigations have been made with t h e As1 5Ge3 0 Se 55 composition because of its high transition t e m p e r a t u r e and low c r y s t a l c o n t e n t but a t t e m p t s have also been c a r r i e d out on t h e A s1 3G e2 0S e2 7T e4 0 composition since t h e introduction of tellurium into t h e glass results in a shift of t h e infrared absorption edge toward longer wavelengths.
The partial substitution of tellurium for selenium in t h e original composition reduces t h e a t t e n u a t i o n a t 10,6 um of 3 dB/m and results in promising fibres for C 02 laser applications (figure 1).
However t h e a t t e n u a t i o n a t 10,6 um may be more reduced. The aim of this work is t o understand t h e intrinsic absorption causes by determining t h e local environment through EXAFS m e a s u r e m e n t s .
EXAFS STUDY
EXAFS has been successfully applied for local s t r u c t u r e d e t e r m i n a t i o n of a wide variety of m a t e r i a l s . This absorption spectroscopy t e c h n i q u e allows an a c c u r a t e d e t e r m i n a t i o n of t h e s t r u c t u r a l p a r a m e t e r s ( i n t e r a t o m i c distances R, coordination numbers N) of t h e close shells surrounding t h e absorber a t o m . When large disorder is present, as it is t h e c a s e in amorphous m a t e r i a l s , this technique p r e s e n t s some l i m i t a t i o n s . To break free of t h e s e
C8-556 JOURNAL DE PHYSIQUE
limitations we performed a comparative study of t h e short distance order of glasses before and a f t e r fibering.
I
3
5
7
9
1 1
Wawelenght ( ~ m )
- Fibre As,,Ge2,Se2,Te,,
Experimental procedure
-
P u r e e l e m e n t s (Ge, Se, As, Te) used a s references and t h e samples have been calibrated such a s t h e grain size is always less than 2 5 pm. The experiments have been c a r r i e d out a t t h e synchrotron radiation facility (LURE-DCI) with t h e storage ring operating at two different energies : 1.56 and 1.85 Gev and a n average current of 200 mA. Ge, As and Se K edges (11104, 11863 and 12658 eV) have been measured a t room temperature, using e i t h e r t h e channel c u t or t h e double crystal monochromator s e t up a t DCI (3).Analysis - The EXAFS functions X(k) a r e obtained from t h e X-ray absorption coefficient p (h
u )
by appropriate background removal and normalization : t h e preedge absorption is f i r s t matched t o a Victoreen expression, which is then substracted from t h e signal a f t e r t h e edge, t h e monotonic like absorption being approximated by a second d e g r e e polynomial expression. The remaining long wavelength oscillations still present in t h e EXAFS signal a r e then removed by a multiiteration curve smoothing. The k weighted d a t a a r e then Fourier transformed through a Hanning window spanning from 30 t o 500 ev. In order t o avoid any spurious e f f e c t s arising from this mathematical processing all t h e analysis have been carried o u t in t h e s a m e way for t h e different samples.Results - Fig. 2 compares t h e Fourier Transform a t t h e t h r e e edges respectively f o r t h e standard, t h e block and t h e f i b r e for t w o samples issued from different starting materials. Due t o t h e limited energy range between t h e different L edges of t h e tellurium, we did not measure t h e EXAFS d a t a on this edge, except f o r t h e pure T e element a s a check of t h e electronic parameters.
In Fig. 2, distances a r e uncorrected from phase shifts. I t is obvious t o .remark that, in coming from t h e model compound t o t h e glass or t h e fibre, t h e environment of t h e selenium is only l i t t l e affected, while those of germanium or arsenic seems strongly modified : due t o t h e closeness of t h e t h r e e e l e m e n t s (Ge, As, Se) a s f a r a s EXAFS is concerned, we c a n already conclude t h a t t h e modification is due t o t h e presence of tellurium.
A more detailed analysis is done by Fourier filtering t h e main peak and fitting t h e X (k)
ponding q u a n t i t i e s in T e O a n d L e e ' s c a l c u l a t i o n s (4). T h e a m p l i t u d e s used a r e t h e t h e o r e t i c a l ones, s o m e t i m e s c o r r e c t e d using e l e m e n t a l model compounds, and in t h e analysis, t h e m e a n f r e e p a t h p a r a m e t e r r a n d t h e Debye-Waller f a c t o r 0 h a v e b e e n k e p t f i x e d t o t h e i r values o n p u r e elements. Only Eo, t h e origin of t h e p h o t o e l e c t r o n energy, h a s b e e n allowed t o vary slightly in o r d e r t o a c c o u n t f o r small phase-shift distorsions. Finally, t h e s e analysis h a v e , b e e n done within t h e hypothesis t h a t t h e c o o r d i n a n c e of e a c h e l e m e n t s r e m a i n s equal t o i t s usual value, i.e. 4 f o r Ge, 3 f o r As a n d 2 f o r Se. Nevertheless, a s usual, t h e following EXAFS results r e p r e s e n t only a n a v e r a g e of t h e local e n v i r o n m e n t of e a c h e l e m e n t o n t h e m a t e r i a l .
, .
,.*> A s b l o c k 6-
- R I A 1
O.
t
t
7 .Fig.2
-
C o m p a r e d m a g n i t u d e of t h e F.T. of t h e k X ( k ) o n Ge, As, S e e d g e f o r t h e s t a n d a r d , t h e block a n d t h e f i b r e of t w o d i f f e r e n t edges.Examinating f i r s t t h e results in T a b l e I, t h e a r g u m e n t g o e s a s follow : o n G e a n d As edges, t h e f i t of t h e EXAFS d a t a needs o n e "short" distance, around 2.52
a
f o r A s and 2.41 f o r Ge, and o n e "large" which m u s t b e a t t r i b u t e d t o T e neighbours. T h e s e m e a s u r e d d i s t a n c e s of h e t e r o g e n e o u s pairs m u s t h a v e t h e s a m e v a l u e when m e a s u r e d fr0.n o n e a t o m o r f r o m t h e o t h e r o o n e : s o a result of 2.531
f o r t h e As-Ce pair is incompatible with t h e result of 2.41 A f o r t h e Ge-As one. We c a n t h e r e f o r e e x c l u d e Ge-As pairs.Looking now a t t h e sele%ium environment, t h e EXAFS f i t t i n g n e e d s o n e d i s t a n c e a t 2.29 A a n d a n o t h e r o n e a t 2.37 A, a n y a t t e m p t t o u s e onlyoone shell, a s in t h e stanQard, leading t o u n a c c e p t a b l e fits. While t h e Se-Ge result of 2.37 A is c l o s e f r o m t h e 2.4 A of Ge-Se, w e
see t h e discrepancy b e t w e e n t h e t w o v a l u e s f o r As-Se pairs, this As-Se value of 2.5 being a l s o t o o f a r f r o m t h e c o v a l e n t radius of 2.41. Therefore, w e also e x c l u d e As-Se h e t e r o g e - n e o u s pairs. AH t h e numerical r e s u l t s l i s t e d in T a b l e I r e p r e s e n t t h e s a m e q u a n t i t a t i v e a g r e e m e n t b e t w e e n t h e e x p e r i m e n t a l d a t a a n d t h e c a l c u l a t e d model.
Finally, w e conclude t h a t :
- selenium h a s n o a r s e n i c o r tellurium c l o s e t o i t
- a r s e n i c is only surrounded by a r s e n i c a n d tellurium (Fig. 3)
J O U R N A L DE PHYSIQUE
Table I - Best fitting distances obtained for different two-shell model environment about Se,
As and Ge atoms. Z covalent radii (ev I T h e o r i c a l c o n t r i b u t i o n o f T e ( - ) a n d A s (....)
i .
T o t a l m o d e l l i n g c o n t r b b u t i o n ( A s a T e ) 2.37- Best f i t (-) for t h e EXAFS
(...I
on As edge.A m o r e d e t a i l e d analysis leads t o t h e results of t a b l e 11.
T a b l e I1 - Local e n v i r o n m e n t a b o u t Se, As, G e atoms. N i s t h e coordination number, R t h e d i s t a n c e a n d
a
t h e m e a n s q u a r e displacement. T h e inelastic p a r a m e t e r is a l w a y equal t o .75 A-2.N
G e S e 2 [ o r S e G e G e1::
1 T e 2 As A s I T eFinally, w e looked at t h e evolution of As e n v i r o n m e n t during t h e f i b r e drawing. Q u a n t i t a t i v e s t u d i e s a r e now in progress. But t h e f i r s t r e s u l t s show t h a t t h e main e f f e c t of t h e drawing is a r e l a x a t i o n of t h e As-Te d i s t a n c e of a b o u t .03 A. ACKNOWLEDGMENTS
R
(A) 2.38 2.29 2.43 2.38 2.58 2.52 2.68This work w a s c a r r i e d o u t with t h e support of DRET.
We would like t o thank J-P. PARANT a n d C . LE SERGENT f o r t h e s a m p l e elaboration. We a r e indebted t o t h e L a b o r a t o i r e d e I'Acc6ldrateur L i n d a i r e f o r d e d i c a t e d s h i f t s of t h e DCI s t o r a g e ring.
a
(A) .06 .06 -12 .06 .12 REFERENCES( 1 ) J-P. P A R A N T a n d al., C h a l c o g e n i d e Glass O p t i c a l F i b e r s
-
F i r s t I n t e r n a t i o n a l Symposium o n H a l i d e a n d O t h e r Nonoxide Glasses - C a m b r i d g e , M a r c h 23-26, 1982.(2) G.S. CARGILL, J N C S 6 1 8 6 2 (1984) 261-272. (3) D. RAOUX, R e v . Phys. Appl. 15 (1980).
