- Email: [email protected]
The role of GaF3 in high numerical aperture heavy metal fluoride fibres
J O U R N A l , OF
ELSEVIER
Journal of Non-Crystalline Solids 184 (1995) 244-248
The role of GaF 3 in high numerical aperture heavy metal fluoride fibres R.S. Rowe "'*, G. Rosman
a C.G.
Byrne a, j. Javorniczky D.R. MacFarlane b
b p.j.
Newman
b
a Telstra Research Laboratories, 770 Blackburn Road, Clayton, Victoria 3168, Australia b Department of Chemistry, Monash UniL,ersity, Clayton, Victoria 3168, Australia
Abstract There is a role for GaF3 in both the core and cladding of heavy metal fluoride glass fibres. Substitution for A1F 3 in the core can prevent tensile stresses occurring in the cladding during cooling, by increasing the thermal expansion coefficient in the core, with no measurable effect on the refractive index. When substituted for BaFz in the cladding, the refractive index is decreased and the thermal expansion coefficient is increased. Use of GaF3 in conjunction with HfF4 gives a useful additional degree of freedom in the design of the cladding glass. In combination with GaF3 for A1F3 substitution in the core glass, this substitution allows a higherindex difference to be achieved in a compatible pair of compositions for a given concentration of PbF2 in the core.
1. Introduction It can be shown that the gain obtainable from a pr3+-doped heavy metal fluoride fibre amplifier, for a given pump power, is directly proportional to the difference in refractive index between the core and cladding [1]. This index difference must be achieved with appropriate matching of the thermomechanical properties of the core and cladding glasses [2,3]. A n y component which can be used to manipulate the refractive index, N o , and thermal expansion coefficient, a , is therefore welcome, providing an additional degree of freedom in the selection of compatible pairs of glasses with a large difference in index.
* Corresponding author. Tel. + 61-3 253 6698. Telefax + 61-3 253 6664. E-mail: [email protected].
We show that G a F 3 is useful in both the core and cladding of fibres designed for high numerical aperture (NA), enabling a higher difference in index to be achieved, in a thermomechanically matched design, for a given concentration of PbF e in the core glass.
2. Experimental Refractive index measurements were made using an A b b e refractometer with white light. Thermal expansion coefficients were measured on 8 mm diameter rods of length 35__+0.1 mm using an A d a m e l - L h o m a r g y DI-24 dilatometer. The mean value of a for Z B L A N - 2 0 was calculated, with confidence limits of 99%, to be 192 + 0.9 × 10 - 7 K -1 '
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 6 1 9 - 9
245
R.S. Rowe et al. / J o u r n a l of Non-Crystalline Solids 184 (1995) 244-248
Glasses were made from high-purity anhydrous m e t a l fluorides, w h i c h w e r e b a t c h e d u n d e r dry nitrog e n in v i t r e o u s c a r b o n c r u c i b l e s w i t h closely fitting p l a t i n u m lids. M e l t i n g w a s c a r r i e d out at ~ 8 5 0 ° C for 3 - 4 h in an rf f u r n a c e , w i t h a silica l i n e r t u b e a t t a c h e d to the f l o o r o f a n i t r o g e n p u r g e d d r y - b o x . T h e g l a s s m e l t s w e r e t h e n p o u r e d into b r a s s m o u l d s h e l d j u s t b e l o w the g l a s s t r a n s i t i o n t e m p e r a t u r e (typically 2 6 0 ° C ) a n d a n n e a l e d for 4 h. G l a s s t r a n s i t i o n t e m p e r a t u r e s , Tg, w e r e m e a s u r e d u s i n g a T A instruments 910 DSC differential scanning calorimeter.
SD
. . . . . . . . . . . . .
' ....
I ....
' ....
I ....
' ....
ZBLAN-20 ZBLGN-20 • Ga for AI _n
1.50(
~ ~ G Ga for Ba~
1.49(
i
for Zr ~ Z~-8,BLAN-2O,G-5
1.48(
ZBLAN-20,G-10
, , , j , , ,
I ....
, ....
190
I ....
, ....
200
I ....
,
....
210
Of,(10"~K4) 3.
Results
Fig. 1. N o - a diagram showing the effect of GaF3 substitution for AIF3 or BaF2 in ZBLAN-20 glass (the error in measurement of a is represented by the bar on the ZBLAN-20, G-10 marker, and that of the index is smaller than the marker; the solid lines illustrate the assumption of linearity of both N o and ot as functions of change in concentration of components as labelled).
E x p e r i m e n t s h a v e s h o w n that the A1F 3 in a standard ZBLAN-20 glass can be entirely replaced by GaF3, p r o d u c i n g a s i g n i f i c a n t i n c r e a s e in e x p a n s i o n c o e f f i c i e n t . S u b s t i t u t i o n o f G a F 3 for the B a F 2 c a n also result in s t a b l e glasses. In Fig. 1, w e see h o w e a c h o f t h e s e s u b s t i t u t i o n s traces a d i f f e r e n t path o n the d i a g r a m o f r e f r a c t i v e i n d e x p l o t t e d as a f u n c t i o n o f o~. T h e A I F 3 r e p l a c e m e n t r e s u l t s in a large inc r e a s e in c~ w i t h n o s i g n i f i c a n t effect o n r e f r a c t i v e index. O n the o t h e r h a n d , the r e p l a c e m e n t o f B a F 2 c a u s e s a large r e d u c t i o n in i n d e x t o g e t h e r w i t h a n i n c r e a s e in c~. T h e s u b s t i t u t i o n o f 3 % G a F 3 for A I F 3 p r o d u c e s a g r e a t e r i n c r e a s e in o~ t h a n a 1 0 % substit u t i o n for B a F 2. T h e s u b s t i t u t i o n o f up to 5 m o l % G a F 3 for Z r F 4 is
also s h o w n in Fig. 1. It a p p e a r s to b e o f little p r a c t i c a l s i g n i f i c a n c e , g i v i n g the s a m e o r d e r o f inc r e a s e in a p e r m o l % , as G a F 3 for A I F 3, c o u p l e d w i t h a d e c r e a s e in i n d e x e q u i v a l e n t to that o n substit u t i o n o f G a F 3 for BaF2, p e r m o l % . E x p e r i m e n t s h a v e b e e n c a r r i e d o u t to d e t e r m i n e w h e t h e r the e f f e c t s o f G a F 3 for A I F 3 a n d G a F 3 for B a F 2 s u b s t i t u t i o n o c c u r for o t h e r b a s e c o m p o s i t i o n s , e s p e c i a l l y t h o s e c o n t a i n i n g L i F or H f F 4 w h i c h find
Table 1 Thermal expansion coefficient, o~, and refractive index, No, for a range of fluoride glass compositions ZrF4 (mol%)
HfF4 (mol%)
53 53 53 53 53 53 53 53 52 53 53 53 53
BaF2 (mol%)
LaF3
AIF3
(mol%)
(mol%)
20 30 20 27 15 20 17 17 15
4 4 4 4 4 4 4 4 5
3 3
3
20 20 10 10
4 4 4 4
3 3 3 3
NaF (mol%)
LiF (mol%) 20 10 20 13 20 10 13 13 10
3 3 3
20 20 20 20
PbF2 (mol%)
a (50-200°C) (10-7 K a)
100 100 100 100 100
1.5095 1.5150 1.5090 1.5137 1.5225 1.5415 1.5400 1.5400 1.5535
174.0 181.4 190.5 177.6 189.0 181.5 177.7 194.1 183.3
100 100 100 100
1.5000 1.4870 1.4810 1.4680
192.5 182.5 199.8 187.5
Total
(mol%)
3 5 10 10 10 15
ND
GaF3
(mol%)
3
3
10 10
100 100 100
246
R.S. Rowe et al. /Journal of Non-Crystalline Solids 184 (1995) 244-248
ND ~ ' . . . . . . . ~. . . . . . . . . ~. . . . . . . . . ~. . . . . . . . . I . . . . . . . . ~-
Ga 0
I
~.52, ta-lo
o , ; ::: ::Z~ ......................... ta-13 u~0
1.55( Pb for Ba Ba for Li _-~
Ga for AI
,.5i~
Ga 3
:-
Pb 10
--
c~ ................................. I~ O': ::: : ::i ii ............ t~-~'me~,4~............ Ia-13 Li-10............................... .:,.~1
Pb5
~-
H f ~ r Z~
~"~'~
~ , .........
, .........
t70
i
~ " ~ ~
,.45( ~, ........
Pbl5
m ......... 15-13,~-3 D
O': ::: :-7-::: ...... "~..~ ~.."....~. ~.?~ ;:':-'.~-.::" ....
Ga-20
, .........
,80
190
I ........
-
200
0 [ (10-7 K q )
Fig. 2. N o - a diagram illustrating thermomechanical matching of core and cladding glass pairs for high NA preform fabrication (the error in measurement of ot is represented by the bar on the HfBLAN-20 marker, and that of the index is smaller than the marker; the filled rectangles and solid lines fulfil the same role as in Fig. 1, whereas the hollow rectangles and dotted lines represent projections).
application in fibres with high numerical aperture. The general formula for core glass composition was, in mol%, 53ZrF 4 - (40 - x - y)BaF2 • 4LaF 3 • (3 - z ) A I F 3 •xLiF- yPbF 2 •zGaF3 (10 < x < 20, 0 < y <
15, 0 < z < 3 ) .
An exception to this formula was the highest-index glass, which contained 52% ZrF4 and 5% LaF 3 contents for reasons of glass stability. The general formula for cladding glass composition was, in mol%, (53 - x ) Z r F 4 . x H f F 4 • (20 - y ) B a F 2 - 4 L a F 3 • 3A1F3 • 20NaF • y G a F 3 (0 < x < 53, 0 < y < 10).
N D and a results are presented in Table 1, and plotted in Fig. 2 as an ND-a diagram to facilitate visualisation. The parallelogram in the lower part of the diagram encloses all points which can be reached by the combined use of HfF4 (up to 53 mol%) and GaF 3 (up to 10 mol%) in the cladding. Experimentally determined points, from Table 1, are marked with solid rectangles and the projections by hollow rectangles• It should be noted that glasses can be prepared with > 10 mol% G a F 3 but for the purposes of this study we have confined our experimental measurements to an upper limit of 10%. It is also important to match the glass transition temperatures of core and cladding glasses for annealing and fibre drawing purposes. Experimental Tg values are reported in Table 2 for some of the glasses studied, illustrating the effect on Tg of the following substitutions in a Z B L A N - 2 0 glass: G a / B a , L i / N a , P b / B a and H f / Z r .
4. Discussion The selection of thermomechanically compatible mixtures simply involves choosing two points on the diagram, with the required index difference, such that a for the core is equal to or slightly greater than that for the cladding. If the line joining the two points slopes significantly to the left, then there is the danger that, when the preform cools, the higher contraction of the cladding will result in tensile stresses sufficient to cause cracking (the degree of stress depends also on the core cladding diameter ratios [3]). In representing the effect of PbF 2 substitutions we have made the approximation that the substitution of PbF 2 for BaF 2 has a negligible effect on 0~, which is
Table 2 Glass transition temperatures, Tg, measured on selected glasses (-t-2°C) ZrF4 (mol%)
HfF4 (mol%)
BaF 2 (mol%)
LaF 3 (mol%)
AIF3 (mol%)
NaF (tool%)
4 4 4 4 4
3 3 3 3 3
20 20
53
20 10 20 15 20
53 53 53 53
LiF (mol%)
GaF3 (mol%) 10
20 20 20
PbF 2 (mol%)
Total (mol%)
Tg (°C)
100 100 100 100 100
266.2 259.8 252.3 259.2 277.7
R.S. Rowe et al. /Journal of Non-Crystalline Solids 184 (1995) 244-248
supported by our experimental data, and has been reported for glasses containing NaF [4]. T h e range of N o and o~ for each PbF 2 concentration (0, 5, 10, 15 mol%) is represented in Fig. 2 by a series of ascending parallelograms. The vertical separation of these parallelograms is determined by the experimentally measured increase of index with substitution of PbF 2 for BaF 2 (2.6 × 10 3 per mol% PbF2), which is in accordance with other reported data [5]. The increase in c~ which occurs on complete replacement of the 3% AIF 3 by GaF 3 is on average 16 X 10 - 7 K -1 for the ZBLALi glasses. This value has been used to determine the width of the parallelograms at each level of PbF 2 concentration. The height of each parallelogram is found by considering the line joining the experimental Li-20 and Li-10 points (BaF 2 for LiF substitution). The length and orientation of this line at 0% PbF 2 is reproduced in each of the parallelograms representing higher levels of PbF 2. The good agreement between experimental points and the projected points on the parallelograms indicates that the approximation is reasonable. There may be a small dependence on base composition, which can be seen from the distortion of the parallelogram formed by the 53% H f F 4 for Z r F 4 and GaF 3 for BaF 2 substitutions. However, the effect seems to arise only with relatively major differences in base composition, and in any case it is sufficiently small to be ignored for practical purposes. It can be seen from the diagram that the substitution of GaF 3 for BaF 2 in the cladding causes a substantial reduction of refractive index, to the extent that the change in index for 10 mol% GaF 3 with 53 mol% HfF4 is more than double that obtained from the use of 53% HfF4 alone. However, the concomitant increase in c~ would cause difficulty, with standard cores containing LiF and PbF 2, if the point describing the cladding glass were to move to the right of the 10% LiF point on the c~ scale, as would be the case for a cladding incorporating 10% GaF 3. The replacement of all or part of the A1F3 in the core by GaF 3 increases c~ to the extent that a thermomechanically stable design is possible. The effect of selected substitutions on Tg is shown in Table 2. Relative to ZBLAN-20 Tg is decreased by the substitution of GaF 3 for BaF2, PbF e for BaF 2 and LiF for NaF, and increased by the substitution of HfF4 for ZrF4. The complete substitution (3 mol%)
247
of GaF 3 for AIF3 also causes Tg to decrease [6]. Hence care must be taken in selecting pairs of glasses which are otherwise thermomechanically matched that the Tg values are not too different. The GaF 3 used in the experiments described here was ' w e t ' and so it is difficult to assess the true effect of GaF 3 for AIF 3 or GaF 3 for BaF e substitution on glass stability. However, measurements of Tx - Tg made at Monash University, using the same GaF 3, show that in the case of GaF 3 for A1F3 substitution Tx - T g drops by about 30-90°C and > 10 mol% of GaF 3 in the glass causes Tx - T g to fall rapidly [6]. We now report that, using the design technique described above, a preform with NA of 0.462 has been made. Its N A has been confirmed by index measurements on the core and cladding glass, and measurement of the maximum angle at which a H e - N e laser in its core shows total internal reflection.
5. Conclusion The design of high numerical aperture heavy metal fluoride fibres is facilitated by the use of GaF 3 in both core and cladding glasses, allowing a higher NA to be obtained for a given concentration of PbF 2 in the core glass and HfF4 in the cladding. The highest index difference in a preform which is predicted from the experimental points on the No-a diagram (Fig. 2) is 8.6 × 10 2. This is equivalent to an N A of 0.51. An NA of 0.46 has already been achieved in a GaF3-containing preform. The authors wish to thank Mr J. Mitchell of the Department of Materials Engineering, Monash University, for access to the dilatometer and technical assistance with measurement of the thermal expansion coefficients. The Telstra authors acknowledge the permission of the Director, Telstra Research Laboratories, to publish this work.
References [1] M.J.F. Digonnet, IEEE J. Quantum Electron. QE-26 (1990) 1788.
248
R.S. Rowe et al. /Journal of Non-Crystalline Solids 184 (1995) 244-248
[2] J.M. Parker and P.W. France, in: Fluoride Glass Optical Fibres, ed. P.W. France, M.G. Drexhage, J.M. Parker, M.W. Moore, S.F. Carter and J.V. Wright (Blackie, Glasgow and London, 1990) ch. 2, p. 32. [3] J.M. Parker and A.G. Clare, Mater. Sci. Forum 67&68 (1991) 549.
[4] R. Lebullenger, S. Benjaballah, C. Le Deit and M. Poulain, J. Non-Cryst. Solids 161 (1993) 217. [5] T. Kogo, M. Onishi, H. Kanamori and H. Yokota, J. Non-Cryst. Solids 161 (1993) 169. [6] J.S. Javorniczky, P.J. Newman and D.R. MacFarlane, these Proceedings, p. 336.
ELSEVIER
Journal of Non-Crystalline Solids 184 (1995) 244-248
The role of GaF 3 in high numerical aperture heavy metal fluoride fibres R.S. Rowe "'*, G. Rosman
a C.G.
Byrne a, j. Javorniczky D.R. MacFarlane b
b p.j.
Newman
b
a Telstra Research Laboratories, 770 Blackburn Road, Clayton, Victoria 3168, Australia b Department of Chemistry, Monash UniL,ersity, Clayton, Victoria 3168, Australia
Abstract There is a role for GaF3 in both the core and cladding of heavy metal fluoride glass fibres. Substitution for A1F 3 in the core can prevent tensile stresses occurring in the cladding during cooling, by increasing the thermal expansion coefficient in the core, with no measurable effect on the refractive index. When substituted for BaFz in the cladding, the refractive index is decreased and the thermal expansion coefficient is increased. Use of GaF3 in conjunction with HfF4 gives a useful additional degree of freedom in the design of the cladding glass. In combination with GaF3 for A1F3 substitution in the core glass, this substitution allows a higherindex difference to be achieved in a compatible pair of compositions for a given concentration of PbF2 in the core.
1. Introduction It can be shown that the gain obtainable from a pr3+-doped heavy metal fluoride fibre amplifier, for a given pump power, is directly proportional to the difference in refractive index between the core and cladding [1]. This index difference must be achieved with appropriate matching of the thermomechanical properties of the core and cladding glasses [2,3]. A n y component which can be used to manipulate the refractive index, N o , and thermal expansion coefficient, a , is therefore welcome, providing an additional degree of freedom in the selection of compatible pairs of glasses with a large difference in index.
* Corresponding author. Tel. + 61-3 253 6698. Telefax + 61-3 253 6664. E-mail: [email protected].
We show that G a F 3 is useful in both the core and cladding of fibres designed for high numerical aperture (NA), enabling a higher difference in index to be achieved, in a thermomechanically matched design, for a given concentration of PbF e in the core glass.
2. Experimental Refractive index measurements were made using an A b b e refractometer with white light. Thermal expansion coefficients were measured on 8 mm diameter rods of length 35__+0.1 mm using an A d a m e l - L h o m a r g y DI-24 dilatometer. The mean value of a for Z B L A N - 2 0 was calculated, with confidence limits of 99%, to be 192 + 0.9 × 10 - 7 K -1 '
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 6 1 9 - 9
245
R.S. Rowe et al. / J o u r n a l of Non-Crystalline Solids 184 (1995) 244-248
Glasses were made from high-purity anhydrous m e t a l fluorides, w h i c h w e r e b a t c h e d u n d e r dry nitrog e n in v i t r e o u s c a r b o n c r u c i b l e s w i t h closely fitting p l a t i n u m lids. M e l t i n g w a s c a r r i e d out at ~ 8 5 0 ° C for 3 - 4 h in an rf f u r n a c e , w i t h a silica l i n e r t u b e a t t a c h e d to the f l o o r o f a n i t r o g e n p u r g e d d r y - b o x . T h e g l a s s m e l t s w e r e t h e n p o u r e d into b r a s s m o u l d s h e l d j u s t b e l o w the g l a s s t r a n s i t i o n t e m p e r a t u r e (typically 2 6 0 ° C ) a n d a n n e a l e d for 4 h. G l a s s t r a n s i t i o n t e m p e r a t u r e s , Tg, w e r e m e a s u r e d u s i n g a T A instruments 910 DSC differential scanning calorimeter.
SD
. . . . . . . . . . . . .
' ....
I ....
' ....
I ....
' ....
ZBLAN-20 ZBLGN-20 • Ga for AI _n
1.50(
~ ~ G Ga for Ba~
1.49(
i
for Zr ~ Z~-8,BLAN-2O,G-5
1.48(
ZBLAN-20,G-10
, , , j , , ,
I ....
, ....
190
I ....
, ....
200
I ....
,
....
210
Of,(10"~K4) 3.
Results
Fig. 1. N o - a diagram showing the effect of GaF3 substitution for AIF3 or BaF2 in ZBLAN-20 glass (the error in measurement of a is represented by the bar on the ZBLAN-20, G-10 marker, and that of the index is smaller than the marker; the solid lines illustrate the assumption of linearity of both N o and ot as functions of change in concentration of components as labelled).
E x p e r i m e n t s h a v e s h o w n that the A1F 3 in a standard ZBLAN-20 glass can be entirely replaced by GaF3, p r o d u c i n g a s i g n i f i c a n t i n c r e a s e in e x p a n s i o n c o e f f i c i e n t . S u b s t i t u t i o n o f G a F 3 for the B a F 2 c a n also result in s t a b l e glasses. In Fig. 1, w e see h o w e a c h o f t h e s e s u b s t i t u t i o n s traces a d i f f e r e n t path o n the d i a g r a m o f r e f r a c t i v e i n d e x p l o t t e d as a f u n c t i o n o f o~. T h e A I F 3 r e p l a c e m e n t r e s u l t s in a large inc r e a s e in c~ w i t h n o s i g n i f i c a n t effect o n r e f r a c t i v e index. O n the o t h e r h a n d , the r e p l a c e m e n t o f B a F 2 c a u s e s a large r e d u c t i o n in i n d e x t o g e t h e r w i t h a n i n c r e a s e in c~. T h e s u b s t i t u t i o n o f 3 % G a F 3 for A I F 3 p r o d u c e s a g r e a t e r i n c r e a s e in o~ t h a n a 1 0 % substit u t i o n for B a F 2. T h e s u b s t i t u t i o n o f up to 5 m o l % G a F 3 for Z r F 4 is
also s h o w n in Fig. 1. It a p p e a r s to b e o f little p r a c t i c a l s i g n i f i c a n c e , g i v i n g the s a m e o r d e r o f inc r e a s e in a p e r m o l % , as G a F 3 for A I F 3, c o u p l e d w i t h a d e c r e a s e in i n d e x e q u i v a l e n t to that o n substit u t i o n o f G a F 3 for BaF2, p e r m o l % . E x p e r i m e n t s h a v e b e e n c a r r i e d o u t to d e t e r m i n e w h e t h e r the e f f e c t s o f G a F 3 for A I F 3 a n d G a F 3 for B a F 2 s u b s t i t u t i o n o c c u r for o t h e r b a s e c o m p o s i t i o n s , e s p e c i a l l y t h o s e c o n t a i n i n g L i F or H f F 4 w h i c h find
Table 1 Thermal expansion coefficient, o~, and refractive index, No, for a range of fluoride glass compositions ZrF4 (mol%)
HfF4 (mol%)
53 53 53 53 53 53 53 53 52 53 53 53 53
BaF2 (mol%)
LaF3
AIF3
(mol%)
(mol%)
20 30 20 27 15 20 17 17 15
4 4 4 4 4 4 4 4 5
3 3
3
20 20 10 10
4 4 4 4
3 3 3 3
NaF (mol%)
LiF (mol%) 20 10 20 13 20 10 13 13 10
3 3 3
20 20 20 20
PbF2 (mol%)
a (50-200°C) (10-7 K a)
100 100 100 100 100
1.5095 1.5150 1.5090 1.5137 1.5225 1.5415 1.5400 1.5400 1.5535
174.0 181.4 190.5 177.6 189.0 181.5 177.7 194.1 183.3
100 100 100 100
1.5000 1.4870 1.4810 1.4680
192.5 182.5 199.8 187.5
Total
(mol%)
3 5 10 10 10 15
ND
GaF3
(mol%)
3
3
10 10
100 100 100
246
R.S. Rowe et al. /Journal of Non-Crystalline Solids 184 (1995) 244-248
ND ~ ' . . . . . . . ~. . . . . . . . . ~. . . . . . . . . ~. . . . . . . . . I . . . . . . . . ~-
Ga 0
I
~.52, ta-lo
o , ; ::: ::Z~ ......................... ta-13 u~0
1.55( Pb for Ba Ba for Li _-~
Ga for AI
,.5i~
Ga 3
:-
Pb 10
--
c~ ................................. I~ O': ::: : ::i ii ............ t~-~'me~,4~............ Ia-13 Li-10............................... .:,.~1
Pb5
~-
H f ~ r Z~
~"~'~
~ , .........
, .........
t70
i
~ " ~ ~
,.45( ~, ........
Pbl5
m ......... 15-13,~-3 D
O': ::: :-7-::: ...... "~..~ ~.."....~. ~.?~ ;:':-'.~-.::" ....
Ga-20
, .........
,80
190
I ........
-
200
0 [ (10-7 K q )
Fig. 2. N o - a diagram illustrating thermomechanical matching of core and cladding glass pairs for high NA preform fabrication (the error in measurement of ot is represented by the bar on the HfBLAN-20 marker, and that of the index is smaller than the marker; the filled rectangles and solid lines fulfil the same role as in Fig. 1, whereas the hollow rectangles and dotted lines represent projections).
application in fibres with high numerical aperture. The general formula for core glass composition was, in mol%, 53ZrF 4 - (40 - x - y)BaF2 • 4LaF 3 • (3 - z ) A I F 3 •xLiF- yPbF 2 •zGaF3 (10 < x < 20, 0 < y <
15, 0 < z < 3 ) .
An exception to this formula was the highest-index glass, which contained 52% ZrF4 and 5% LaF 3 contents for reasons of glass stability. The general formula for cladding glass composition was, in mol%, (53 - x ) Z r F 4 . x H f F 4 • (20 - y ) B a F 2 - 4 L a F 3 • 3A1F3 • 20NaF • y G a F 3 (0 < x < 53, 0 < y < 10).
N D and a results are presented in Table 1, and plotted in Fig. 2 as an ND-a diagram to facilitate visualisation. The parallelogram in the lower part of the diagram encloses all points which can be reached by the combined use of HfF4 (up to 53 mol%) and GaF 3 (up to 10 mol%) in the cladding. Experimentally determined points, from Table 1, are marked with solid rectangles and the projections by hollow rectangles• It should be noted that glasses can be prepared with > 10 mol% G a F 3 but for the purposes of this study we have confined our experimental measurements to an upper limit of 10%. It is also important to match the glass transition temperatures of core and cladding glasses for annealing and fibre drawing purposes. Experimental Tg values are reported in Table 2 for some of the glasses studied, illustrating the effect on Tg of the following substitutions in a Z B L A N - 2 0 glass: G a / B a , L i / N a , P b / B a and H f / Z r .
4. Discussion The selection of thermomechanically compatible mixtures simply involves choosing two points on the diagram, with the required index difference, such that a for the core is equal to or slightly greater than that for the cladding. If the line joining the two points slopes significantly to the left, then there is the danger that, when the preform cools, the higher contraction of the cladding will result in tensile stresses sufficient to cause cracking (the degree of stress depends also on the core cladding diameter ratios [3]). In representing the effect of PbF 2 substitutions we have made the approximation that the substitution of PbF 2 for BaF 2 has a negligible effect on 0~, which is
Table 2 Glass transition temperatures, Tg, measured on selected glasses (-t-2°C) ZrF4 (mol%)
HfF4 (mol%)
BaF 2 (mol%)
LaF 3 (mol%)
AIF3 (mol%)
NaF (tool%)
4 4 4 4 4
3 3 3 3 3
20 20
53
20 10 20 15 20
53 53 53 53
LiF (mol%)
GaF3 (mol%) 10
20 20 20
PbF 2 (mol%)
Total (mol%)
Tg (°C)
100 100 100 100 100
266.2 259.8 252.3 259.2 277.7
R.S. Rowe et al. /Journal of Non-Crystalline Solids 184 (1995) 244-248
supported by our experimental data, and has been reported for glasses containing NaF [4]. T h e range of N o and o~ for each PbF 2 concentration (0, 5, 10, 15 mol%) is represented in Fig. 2 by a series of ascending parallelograms. The vertical separation of these parallelograms is determined by the experimentally measured increase of index with substitution of PbF 2 for BaF 2 (2.6 × 10 3 per mol% PbF2), which is in accordance with other reported data [5]. The increase in c~ which occurs on complete replacement of the 3% AIF 3 by GaF 3 is on average 16 X 10 - 7 K -1 for the ZBLALi glasses. This value has been used to determine the width of the parallelograms at each level of PbF 2 concentration. The height of each parallelogram is found by considering the line joining the experimental Li-20 and Li-10 points (BaF 2 for LiF substitution). The length and orientation of this line at 0% PbF 2 is reproduced in each of the parallelograms representing higher levels of PbF 2. The good agreement between experimental points and the projected points on the parallelograms indicates that the approximation is reasonable. There may be a small dependence on base composition, which can be seen from the distortion of the parallelogram formed by the 53% H f F 4 for Z r F 4 and GaF 3 for BaF 2 substitutions. However, the effect seems to arise only with relatively major differences in base composition, and in any case it is sufficiently small to be ignored for practical purposes. It can be seen from the diagram that the substitution of GaF 3 for BaF 2 in the cladding causes a substantial reduction of refractive index, to the extent that the change in index for 10 mol% GaF 3 with 53 mol% HfF4 is more than double that obtained from the use of 53% HfF4 alone. However, the concomitant increase in c~ would cause difficulty, with standard cores containing LiF and PbF 2, if the point describing the cladding glass were to move to the right of the 10% LiF point on the c~ scale, as would be the case for a cladding incorporating 10% GaF 3. The replacement of all or part of the A1F3 in the core by GaF 3 increases c~ to the extent that a thermomechanically stable design is possible. The effect of selected substitutions on Tg is shown in Table 2. Relative to ZBLAN-20 Tg is decreased by the substitution of GaF 3 for BaF2, PbF e for BaF 2 and LiF for NaF, and increased by the substitution of HfF4 for ZrF4. The complete substitution (3 mol%)
247
of GaF 3 for AIF3 also causes Tg to decrease [6]. Hence care must be taken in selecting pairs of glasses which are otherwise thermomechanically matched that the Tg values are not too different. The GaF 3 used in the experiments described here was ' w e t ' and so it is difficult to assess the true effect of GaF 3 for AIF 3 or GaF 3 for BaF e substitution on glass stability. However, measurements of Tx - Tg made at Monash University, using the same GaF 3, show that in the case of GaF 3 for A1F3 substitution Tx - T g drops by about 30-90°C and > 10 mol% of GaF 3 in the glass causes Tx - T g to fall rapidly [6]. We now report that, using the design technique described above, a preform with NA of 0.462 has been made. Its N A has been confirmed by index measurements on the core and cladding glass, and measurement of the maximum angle at which a H e - N e laser in its core shows total internal reflection.
5. Conclusion The design of high numerical aperture heavy metal fluoride fibres is facilitated by the use of GaF 3 in both core and cladding glasses, allowing a higher NA to be obtained for a given concentration of PbF 2 in the core glass and HfF4 in the cladding. The highest index difference in a preform which is predicted from the experimental points on the No-a diagram (Fig. 2) is 8.6 × 10 2. This is equivalent to an N A of 0.51. An NA of 0.46 has already been achieved in a GaF3-containing preform. The authors wish to thank Mr J. Mitchell of the Department of Materials Engineering, Monash University, for access to the dilatometer and technical assistance with measurement of the thermal expansion coefficients. The Telstra authors acknowledge the permission of the Director, Telstra Research Laboratories, to publish this work.
References [1] M.J.F. Digonnet, IEEE J. Quantum Electron. QE-26 (1990) 1788.
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[2] J.M. Parker and P.W. France, in: Fluoride Glass Optical Fibres, ed. P.W. France, M.G. Drexhage, J.M. Parker, M.W. Moore, S.F. Carter and J.V. Wright (Blackie, Glasgow and London, 1990) ch. 2, p. 32. [3] J.M. Parker and A.G. Clare, Mater. Sci. Forum 67&68 (1991) 549.
[4] R. Lebullenger, S. Benjaballah, C. Le Deit and M. Poulain, J. Non-Cryst. Solids 161 (1993) 217. [5] T. Kogo, M. Onishi, H. Kanamori and H. Yokota, J. Non-Cryst. Solids 161 (1993) 169. [6] J.S. Javorniczky, P.J. Newman and D.R. MacFarlane, these Proceedings, p. 336.