SEMIQUANTITATIVE X-RAY MICROANALYSIS ON PREFORMS FOR OPTICAL WAVEGUIDE FIBRES

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SEMIQUANTITATIVE X-RAY MICROANALYSIS ON PREFORMS FOR OPTICAL WAVEGUIDE FIBRES

J. Hersener, H.-P. Huber, J. von Wienskowski

To cite this version:

J. Hersener, H.-P. Huber, J. von Wienskowski. SEMIQUANTITATIVE X-RAY MICROANALYSIS

ON PREFORMS FOR OPTICAL WAVEGUIDE FIBRES. Journal de Physique Colloques, 1984, 45

(C2), pp.C2-761-C2-764. �10.1051/jphyscol:19842175�. �jpa-00223849�

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Colloque C2, supplkment au n02, T o m e

fCvrier

page C2-761

SEMIQUANTITATIVE X-RAY MICROANALYSIS ON P R E F O R M S FOR OPTICAL WAVEGUIDE FIBRES

J. Hersener, H.-P. Huber and J. Von wienskowskir

BEG-TeZefunken, Forschungsinstitut, Sedanstrasse 10, 0-7900

am,

AEG-TeZefunken, KabeZwerke AG Rheydt, Bonnenbroicher Strasse 2-14, D-4050 MDnchengZadbach 2, F.R.G.

Resume - La distribution du dopant Ge(02) dans les fibres optiques, qui sont fabriquees par VCVD, est determinee par analyse par spectrographie

X

en energie. On obtient une r6solution de

pm avec la raie Ge(L,) et une Gnergie d'excitation de 10 keV. On a observe dans la fabrication des preformes une fluctuation caracteristique de la densite de Ge(02) pour chaque couche dsposee. Cette distribution initiale du dopant Ge(02) est modifiee par la diffusion et l'evaporation lors des phases suivantes de la preparation des preformes et de l'6tirage des fibres.

Abstract - The distribution of the Ge(02)-dopant in optical waveguides

preparedbytheVCVD-process

is determined with an SEM using energy disper- sive microanalysis. Operating at

keV and analysing the Ge(L,)-line, a lateral resolution of approximately

Dm is achieved. A strong charac- teristic fluctuation of the Ge(02)-density within each freshly deposited

layer is observed. This original Ge(02)-distribution in each layer is changed by interdiffusion and evaporation processes during following steps of preform preparation and fibre drawing.

Introduction

The transmission characteristics of an optical fibre strongly depend on its re- fractive index profile. Fig.

shows two typical profiles of multimode fibres:

i )

a step index

i i )

a graded index profile.

Graded inde

I ^I I

' 1

! &

3 ,

Fig. 1 - Typical index profiles and doping materials for Si02 in multimode fibres The graded index profile is optimized for high transmission bandwidth and there- fore of most interest. In actual fibres, however, the index profile is not smooth but has typical defects caused by the manufacturing processes. Most common is an

index ripple along the profile and an index dip in the core centre. Such index de- teriorations can reduce the fibre bandwidth drastically. Therefore it is of ut- most interest to understand the causes for the index deviations to optimize the fabrication process. This paper ~nvestigates details of the Ge02-doping distribu-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842175

C2-762 JOURNAL DE PHYSIQUE

tion in silica fibres in the various processing steps from the preform prepara- tion to the drawn fibre.

Fibre preparation and measuring method

The fibres were prepared by the VCVD-process

which is similar to the well- known MCVD-process. The process is based on a homogeneous vapour phase reaction of SiC14and

to produce soot particles of extremely pure silica glass. To change the reFractive index of the glass, doping materials Ge02, F, P2O5 or others are used. For graded index fibres the dominant doping material is GeO,.

Fig. 2 - Schematic view of the

(a), the drawing process (b) and the dependence of oxide yield on temperature (c)

The starting vapours and gases are fed through a vertically positioned silica glass tube from the top end. In a short high temperature

OC) zone pro- duced by a graphite resistance furnace surrounding the tube the soot particles are created (Fig. 2a). The particles are driven by thermophoretic forces to the inner tube wall below the reaction zone (left half Fig. 2a) where they are de- posited. The slowly downwards moving heat-zone sinters the soot deposit to a glassy layer. The preform preparation includes the deposition of many layers one by the other each layer doped individually to produce the desired index profile.

After the deposition process the preform tube is collapsed to a rod by a high temperature process. The optical fibre is drawn from the preform rod.

Alternatively the fibre canalsobe drawn from the preform tube, as shown in Fig.

2b. In this case the collapsing and drawing steps are carried out simultaneously at the lower end of the preform tube.

An important feature of the vapour phase reactions in the deposition process is that Sic14 reacts completely to Si02 for temperatures above approximately 1500 OC whereas the Ge02-yield increases with temperature to a maximum value at about 1500 OC and decreases for hiqher temoeratures 121, Fig. 2c. In a nonuni- form temperature field which obviously exists along the tube radius one expects therefore the creation of soot particles with different Ge02-concentration.

For the quantitative analysis of the GeOp-distribution a Leitz AMR

SEM with an energy dispersive X-ray analyser EDAX mod 711 is used. Operating with the Ge(L,)- line and primary electrons of

keV a lateral resolution of

approximately 1.5 ym is achieved. The measurements are carried out in the spot mode of the SEM. For the measurements the count rates of five channels are

registrated for about200 sat eight energy values between 900 and 2500 eV

including the peaks of Ge(L,), Si(K,) and P(K4),). With the aid of a special

BASIC-program for anHP85 desk computer first the netto intensities and

normalized relative intensities are determined with the aid of calculated

relative pure element intensities

The ZAF correction program is based on

the NBS-program given by Heinrich 141 with some modifications for the low

energy radiation. The background intensities at the eight measured energy values

are determined for a pure SiOp standard and for a sample with high GeO? dopant

(ca. 50 wt.-%). These values are constants in the background substractlon

procedure and are linearly interpolated iteratively with respect to the

Ge(0 2) content.

Results

-shows SEM-micrographs of anetched cross section of a fibre and of a po- lished preform disk prepared from the neck-down zone of a preform end, respec- tively.

characteristic ring structure corresponding to the individual layers from the preform preparation is revealed. The anticipated inhomogeneous distri- bution of the Ge(q) doping within the individual layers results in unequal et- ching rates. We found an increasing etching rate with increasing Ge-content.

Fig. 3 - SEM micrographs of different process steps of fibre fabrication On the other hand the micrograph of the polished preform disk shows (Fig. 3b) bright zones for Ge-rich and dark zones for Ge-poor regions on the specimen.

This contrast behaviour is caused by the different reflection coefficients for high and low Ge-contents.

Fig. 4 and Fig. 5 show the measured distribution of Ge(4) and P2(%) concen- tration (a small amount of P205 was added to the deposition of silica to reduce the sintering temperature) in the layers at the preform tube and in a disk pre- pared from the collapsed neck-down region when drawing the fibre from the pre- form tube, respectively.

Preform t u b e

uo 0,20-

U C 0 U

2 Oslo-

3

n

Fig. 4 - Measured Ge(02) concentration profile inside the three innermost layers of the preform tube (a) and SEM micrograph of these layers (b)

Fig. 4a reveals a strong characteristic oscillation of the Ge(02) concentration across each layer. In each layer the Ge(0

concentration increases towards the tube centre but is zero at both layer bor2ers. In contrast the P2(05) concentra- tion level is found to be constant over all layers.

Fig. 4b shows the corresponding SEM micrograph. The dark regions correspond to a low GeOpconcentration and the brighter regions indicate higher GeO oncentra- tions. The Ge(0

concentration profile can be explained by the inh&itgeneous re- action of the

due to the strong temperature dependence and by the high eva- poration rate of 6eoZ at the innermost free layer surface during the sintering of the layer, resulting in a depletion of Ge02 at the surface region.

Fig. 5 (thick curve) gives the measured Ge(0

concentration profile in the neck:

down region (sample

whereas the thinner c6rve corresponds to the Ge(02) distri-

bution found in the preform tube but transformed into the collapsed form.

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Fig. 5 - Ge(02)- and P2(05) c o n c e n t r a t i o n p r o f i l e s n e a r t h e c e n t r e o f a d i s k taken from t h e c o l l a p s e d preform t u b e ( t h i c k c u r v e ) and t h e transformed Ge(02) c o n c e n t r a t i o n p r o f i l e of Fig. 3a ( t h i n c u r v e ) ; P2(05) dashed curve A comparison of t h e two p r o f i l e s c l e a r l y demonstrates a change i n t h e Ge(0 ) d i s - t r i b u t i o n which obviously occured during t h e drawing p r o c e s s when t h e

t u b e was heated up and c o l l a p s e d t o t h e neck-down region. The change can be ex- p l a i n e d by d o p a n t - d i f f u s i o n w i t h i n t h e l a y e r s , between a d j a c e n t l a y e r s and by e v a p o r a t i o n on t h e i n n e r f r e e s u r f a c e . The l a t t e r l e a d s t o t h e observed dopant d e p l e t i o n corresponding an index d e p r e s s i o n a t t h e c o r e c e n t r e , t h e so c a l l e d index d i p .

Conclusion

The Ge02 d i s t r i b u t i o n i n graded index preformsand f i b r e s i s n e i t h e r s m o o t h n o r s t e p - l i k e a s should be expected t h e o r e t i c a l l y from t h e d e p o s i t i o n o f doped l a y e r s . S t r o n g o s c i l l a t i o n s a r e found w i t h i n t h e l a y e r s a t t h e preform t u b e . D e t a i l s o f t h e d i s t r i b u t i o n in t h e l a y e r # a s d e p o s i t e d a r e e s s e n t i a l l y a f u n c t i o n of t h e t e m p e r a t u r e p r o f i l e i n t h e r e a c t i o n zone t h a t depends on t h e h e a t source and i s a f u n c t i o n of t h e t e m p e r a t u r e dependence of vapour phase r e a c t i o n f o r Ge$.

The o s c i l l a t i n g d i s t r i b u t i o n i s c o n s i d e r a b l y smoothed o u t d u r i n g preform c o l l a p s - ing and f i b r e drawing. A d d i t i o n a l e v a p o r a t i o n of dopant m a t e r i a l from t h e i n n e r s u r f a c e of t h e preform t u b e d u r i n g t h e c o l l a p s i n g s t e p r e s u l t s i n dopant d e p l e - t i o n in t h e c o r e c e n t r e and i s t h e reason f o r t h e index d i p .

Acknowledgements

The a u t h o r s a r e g r a t e f u l t o P r o f . Dr. S. Maslowski - d i r e c t o r of o u r r e s e a r c h i n s t i t u t e - f o r u s e f u l d i s c u s s i o n s .

L i t e r a t u r e

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/2/ S.R. NAGEL, H.B. McCHESNEY, K.L. WALKER: IEEE T r a n s a c t i o n s on Microwave Theory and Techniques, Vol. MTT-30, 305-322, 1982

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