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Manufacture of fluoride glass preforms
NON-CR'NLiNiSOUDS
Journal of Non-Crystalline Solids 140 (1992) 265-268 North-Holland
Manufacture of fluoride glass preforms K. C l a r k e a n d Y. I t o Telecom Australia Research Laboratories, Clayton, Victoria 3168, Australia
Since vapour deposition methods for heavy metal fluoride glasses are still in the early stages of development, preforms are prepared by pouring two slightly different glass melts sequentially into a heated mould. In this paper, a shutter technique is described, which has been used to make both doped and undoped preforms, and which facilitates the production of more complex fibre structures.
1. Introduction Fluoride glasses are under investigation in many laboratories for their unique potential advantages over silica as a telecommunications fibre material, both for low transmission loss and for their use as hosts in rare-earth doped fibre amplifiers. This work is still at the process development stage and while progress has been substantial, the tendency of the glasses to form microcrystals is the main b a r r i e r which must be overcome. Glass and preform processing techniques must be found which either eliminate crystals entirely or limit their size so that they do not contribute significantly to the optical loss by scattering. At the same time, a process must address issues such as bubble formation, c o r e / c l a d d i n g - i n t e r f a c e quality and uniformity. In this p a p e r a modified preform preparation technique is described which leads to the fabrication of improved quality fluoride glass preforms, and which enables the manufacture of complex fibre structures. The simplest approach in the production of fluoride fibres is to use a Teflon (FEP) cladding on a fluoride glass rod to form the preform for drawing [1]. Subsequently, two-glass fibres were m a d e by the so-called build-in-casting method [2].
Cladding glass was first poured into a heated mould, then after a certain period of time the mould was upturned so that the still molten glass drained from the centre. The high index core glass could then be introduced to produce the two glass preform. The optical quality of c o r e / cladding interface was adversely affected even by exposure to the inert atmosphere of the dry box. In addition, it was difficult to obtain long, uniform preforms. In an effort to avoid the short lengths of highly tapered preform that the previous approach provided, a commonly used alternative technique is to rotate a heated mould at a few thousand rpm to form a tube from a molten cladding glass [3]. The core glass could then be poured into the tube to form a complete preform. The main disadvantage of this method is the temporary formation of a g l a s s / a t m o s p h e r e interface, where conditions for crystal nucleation are favourable due to the cooling and subsequent re-heating of the glass as the core melt is introduced. O t h e r drawbacks include an increased likelihood of contamination and the mechanical complexity of a high-speed, rotating mould. To overcome the limitations of the processes described above, T R L have developed a mould with a manually operated shutter. This is a technique which can routinely provide practical lengths of relatively uniform preform.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
K. Clarke, Y. Ito / Manufacture of fluoride glass preforms
266 Table 1 Z B L A N glass c o m p o s i t i o n s
C o r e / c l a d c o m p o s i t i o n s for the fibre
Core Clad
Index
Z r F4
BaF 2
LaF 3
YF 3
A1F 3
NaF
PbF 2
XF 3
51.5 51.5
19.5 20.5
3.3- x 3.3
2.0 2.0
3.2 3.2
18.0 19.5
2.5 -
x -
1.5125 1.5038 N A = 0.1620
X F 3 is rare earth fluorides: X = Nd, Er, Ho, Tin, Yb. InF 3 (0.24 wt%) was a d d e d as an internal oxidant• Y F 3 is a glass stabiliser.
2. Methodology Typical compositions of the core and cladding ZBLAN glasses are displayed in table 1. The powders are placed into two gold or glassy-carbon crucibles. These are removed from the furnace at 850 °C and are allowed to cool to the appropriate pouring temperature. The cladding glass is first poured smoothly into a brass mould which is preheated to just below the glass-transition temperature. While solidification
\
sets in from the inner mould wall, the core glass is poured on top of the cladding melt. Within a few seconds, when the solidification front has reached the desired diameter, the shutter is quickly opened. As the still molten cladding drains from the centre, the core glass descends, forming the core. The steps described are shown in sequence in fig. 1. Note the taper immediately above the shutter. This is to ensure that the glass melt in this region is the last to solidify, so that it falls
/ \ \ \ \ \ \ \ \ \ \ \ \ \\\N x\x\ \\\N \ \ \ \xxx x \ \ \ \ \ \ x \ \ \ \ x\x~ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \
\ \ \ \
\\\. ,.\\~
ii
.',7.\: ,\\\
I
x
iI[1
\ \ \ \
~iit[
\ \ \ \ \ \ \ \ \ \ x \ \ \ \ \ x \
,-\\'~
I
i
i
\ \ \ ...... \xx
??'.?'.? """2
,¢~ \\\" "~I",-SHUTTE R CLOSED
]
"~""SIIUTTEa CLOSED
SHUTTER CLADDING DRAINS
Fig. 1. Making a preform with the shutter mould.
K. Clarke, Y lto / Manufacture of fluoride glass preforms
away easily when the shutter is opened. Another important aspect of the mould design is the shutter thickness, which is only 1 mm. This prevents excess solidification of the glass caused by thermal conduction, which would prevent the shutter from opening. The thin layer of glass solidified on the top surface of the shutter is ruptured by the pressure of the molten glass above. The important features are as follows. (1) The c o r e / c l a d d i n g interface is not exposed to the atmosphere. This reduces the likelihood of contamination by dust particles, etc. (2) Cladding side of the interface cools with minimum reheating, since the two glass melts can be arranged to have similar temperatures. This prevents excess crystallisation. (3) Mechanical simplicity allows pouring at temperatures as low as 400 ° C, because the whole process only takes a matter of seconds.
3. Results
This technique can be used to produce preforms up to 15 cm in length. The smallest core size achievable over such lengths is generally one-third of the cladding diameter. Therefore, single-mode preforms cannot be obtained in one
267
Fig. 3. 'Pump-core' ZBLAN fibre.
step by this method. However, it is still possible to manufacture such a preform by first forming a tube of cladding glass in a shutter-mould then stretching an existing preform to fit inside [4,5], so that the appropriate core to cladding ratio is achieved. The tubes required are made simply by opening the shutter when the desired wall-thickness is attained. Figure 2 is a photograph of a typical multimode preform cross-section obtained by the shutter method. Such preforms can be drawn into lengths of fibre well in excess of 300 m with the standard 125 Ixm diameter. Note the large core size of the preform, which limits the fibre core dimensions to 40 Ixm or greater. Although longer and more uniform preforms are obtainable by other methods, this procedure has the advantage of simplicity without sacrificing optical quality.
4. Discussion
Fig. 2. Multimode preform cross-section.
Fibres drawn from these preforms have been used successfully for fibre lasers and amplifiers. In these cases, the preform cores were doped with the appropriate levels of rare-earth fluorides such as neodymium and erbium, which produce outputs at wavelengths (1.3 and 1.55 Ixm) useful in telecommunications applications.
268
K. Clarke, Y. Ito /Manufacture of fluoride glass preforms
Importantly, this process also lends itself to the production of other customised fibres such as depressed clad fibres and ' p u m p - c o r e ' fibres (fig. 3). The latter are doped singlemode fibres which have a secondary core region, typically a few tens of micrometres across, surrounding the doped core. This improves the coupling efficiency when using multimode p u m p sources that cannot be focussed to a small spot size. It also helps to maximise the amount of p u m p light in the doped region of the fibre.
The authors thank J. Lowing and his group for precise construction of the moulds, D. Coulson for his fibre-drawing skills, G. R o s m a n for his enthusiastic support, and the joint efforts of the Chemistry D e p a r t m e n t at Monash University for the success of this work. The permission of the Executive General Manager, Telecom Australia Research Laboratories to publish this p a p e r is hereby acknowledged.
References 5. Conclusions A shutter-mould technique has been described which readily produces preforms having a high quality c o r e / c l a d d i n g interface, without the complexities required in other manufacturing methods such as rotational casting. The shutter mould has also been used to produce the preforms, rods, and tubes which form composite preforms for the production of singlemode Z B L A N fibres.
[1] S. Mitachi, Electron. Lett. 17 (1981) 128. [2] S. Mitachi, T. Miyashita and T. Kanamori, Electron. Lett. 17 (1981) 591. [3] D.C. Tran, C.F. Fisher and G.H. Sigel, Electron. Lett. 22 (1982) 657. [4] M. Braglia, M. Ferraris, G. Parisi, R. de Franceschi and L. Salasco, in: Proc. 6th Int. Syrup. on Halide Glasses, 1989, p. 527. [5] H. Poignant, C. Falcou and J. le Mellot, Glass Technol. 28 (1987) 38.
Journal of Non-Crystalline Solids 140 (1992) 265-268 North-Holland
Manufacture of fluoride glass preforms K. C l a r k e a n d Y. I t o Telecom Australia Research Laboratories, Clayton, Victoria 3168, Australia
Since vapour deposition methods for heavy metal fluoride glasses are still in the early stages of development, preforms are prepared by pouring two slightly different glass melts sequentially into a heated mould. In this paper, a shutter technique is described, which has been used to make both doped and undoped preforms, and which facilitates the production of more complex fibre structures.
1. Introduction Fluoride glasses are under investigation in many laboratories for their unique potential advantages over silica as a telecommunications fibre material, both for low transmission loss and for their use as hosts in rare-earth doped fibre amplifiers. This work is still at the process development stage and while progress has been substantial, the tendency of the glasses to form microcrystals is the main b a r r i e r which must be overcome. Glass and preform processing techniques must be found which either eliminate crystals entirely or limit their size so that they do not contribute significantly to the optical loss by scattering. At the same time, a process must address issues such as bubble formation, c o r e / c l a d d i n g - i n t e r f a c e quality and uniformity. In this p a p e r a modified preform preparation technique is described which leads to the fabrication of improved quality fluoride glass preforms, and which enables the manufacture of complex fibre structures. The simplest approach in the production of fluoride fibres is to use a Teflon (FEP) cladding on a fluoride glass rod to form the preform for drawing [1]. Subsequently, two-glass fibres were m a d e by the so-called build-in-casting method [2].
Cladding glass was first poured into a heated mould, then after a certain period of time the mould was upturned so that the still molten glass drained from the centre. The high index core glass could then be introduced to produce the two glass preform. The optical quality of c o r e / cladding interface was adversely affected even by exposure to the inert atmosphere of the dry box. In addition, it was difficult to obtain long, uniform preforms. In an effort to avoid the short lengths of highly tapered preform that the previous approach provided, a commonly used alternative technique is to rotate a heated mould at a few thousand rpm to form a tube from a molten cladding glass [3]. The core glass could then be poured into the tube to form a complete preform. The main disadvantage of this method is the temporary formation of a g l a s s / a t m o s p h e r e interface, where conditions for crystal nucleation are favourable due to the cooling and subsequent re-heating of the glass as the core melt is introduced. O t h e r drawbacks include an increased likelihood of contamination and the mechanical complexity of a high-speed, rotating mould. To overcome the limitations of the processes described above, T R L have developed a mould with a manually operated shutter. This is a technique which can routinely provide practical lengths of relatively uniform preform.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
K. Clarke, Y. Ito / Manufacture of fluoride glass preforms
266 Table 1 Z B L A N glass c o m p o s i t i o n s
C o r e / c l a d c o m p o s i t i o n s for the fibre
Core Clad
Index
Z r F4
BaF 2
LaF 3
YF 3
A1F 3
NaF
PbF 2
XF 3
51.5 51.5
19.5 20.5
3.3- x 3.3
2.0 2.0
3.2 3.2
18.0 19.5
2.5 -
x -
1.5125 1.5038 N A = 0.1620
X F 3 is rare earth fluorides: X = Nd, Er, Ho, Tin, Yb. InF 3 (0.24 wt%) was a d d e d as an internal oxidant• Y F 3 is a glass stabiliser.
2. Methodology Typical compositions of the core and cladding ZBLAN glasses are displayed in table 1. The powders are placed into two gold or glassy-carbon crucibles. These are removed from the furnace at 850 °C and are allowed to cool to the appropriate pouring temperature. The cladding glass is first poured smoothly into a brass mould which is preheated to just below the glass-transition temperature. While solidification
\
sets in from the inner mould wall, the core glass is poured on top of the cladding melt. Within a few seconds, when the solidification front has reached the desired diameter, the shutter is quickly opened. As the still molten cladding drains from the centre, the core glass descends, forming the core. The steps described are shown in sequence in fig. 1. Note the taper immediately above the shutter. This is to ensure that the glass melt in this region is the last to solidify, so that it falls
/ \ \ \ \ \ \ \ \ \ \ \ \ \\\N x\x\ \\\N \ \ \ \xxx x \ \ \ \ \ \ x \ \ \ \ x\x~ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \
\ \ \ \
\\\. ,.\\~
ii
.',7.\: ,\\\
I
x
iI[1
\ \ \ \
~iit[
\ \ \ \ \ \ \ \ \ \ x \ \ \ \ \ x \
,-\\'~
I
i
i
\ \ \ ...... \xx
??'.?'.? """2
,¢~ \\\" "~I",-SHUTTE R CLOSED
]
"~""SIIUTTEa CLOSED
SHUTTER CLADDING DRAINS
Fig. 1. Making a preform with the shutter mould.
K. Clarke, Y lto / Manufacture of fluoride glass preforms
away easily when the shutter is opened. Another important aspect of the mould design is the shutter thickness, which is only 1 mm. This prevents excess solidification of the glass caused by thermal conduction, which would prevent the shutter from opening. The thin layer of glass solidified on the top surface of the shutter is ruptured by the pressure of the molten glass above. The important features are as follows. (1) The c o r e / c l a d d i n g interface is not exposed to the atmosphere. This reduces the likelihood of contamination by dust particles, etc. (2) Cladding side of the interface cools with minimum reheating, since the two glass melts can be arranged to have similar temperatures. This prevents excess crystallisation. (3) Mechanical simplicity allows pouring at temperatures as low as 400 ° C, because the whole process only takes a matter of seconds.
3. Results
This technique can be used to produce preforms up to 15 cm in length. The smallest core size achievable over such lengths is generally one-third of the cladding diameter. Therefore, single-mode preforms cannot be obtained in one
267
Fig. 3. 'Pump-core' ZBLAN fibre.
step by this method. However, it is still possible to manufacture such a preform by first forming a tube of cladding glass in a shutter-mould then stretching an existing preform to fit inside [4,5], so that the appropriate core to cladding ratio is achieved. The tubes required are made simply by opening the shutter when the desired wall-thickness is attained. Figure 2 is a photograph of a typical multimode preform cross-section obtained by the shutter method. Such preforms can be drawn into lengths of fibre well in excess of 300 m with the standard 125 Ixm diameter. Note the large core size of the preform, which limits the fibre core dimensions to 40 Ixm or greater. Although longer and more uniform preforms are obtainable by other methods, this procedure has the advantage of simplicity without sacrificing optical quality.
4. Discussion
Fig. 2. Multimode preform cross-section.
Fibres drawn from these preforms have been used successfully for fibre lasers and amplifiers. In these cases, the preform cores were doped with the appropriate levels of rare-earth fluorides such as neodymium and erbium, which produce outputs at wavelengths (1.3 and 1.55 Ixm) useful in telecommunications applications.
268
K. Clarke, Y. Ito /Manufacture of fluoride glass preforms
Importantly, this process also lends itself to the production of other customised fibres such as depressed clad fibres and ' p u m p - c o r e ' fibres (fig. 3). The latter are doped singlemode fibres which have a secondary core region, typically a few tens of micrometres across, surrounding the doped core. This improves the coupling efficiency when using multimode p u m p sources that cannot be focussed to a small spot size. It also helps to maximise the amount of p u m p light in the doped region of the fibre.
The authors thank J. Lowing and his group for precise construction of the moulds, D. Coulson for his fibre-drawing skills, G. R o s m a n for his enthusiastic support, and the joint efforts of the Chemistry D e p a r t m e n t at Monash University for the success of this work. The permission of the Executive General Manager, Telecom Australia Research Laboratories to publish this p a p e r is hereby acknowledged.
References 5. Conclusions A shutter-mould technique has been described which readily produces preforms having a high quality c o r e / c l a d d i n g interface, without the complexities required in other manufacturing methods such as rotational casting. The shutter mould has also been used to produce the preforms, rods, and tubes which form composite preforms for the production of singlemode Z B L A N fibres.
[1] S. Mitachi, Electron. Lett. 17 (1981) 128. [2] S. Mitachi, T. Miyashita and T. Kanamori, Electron. Lett. 17 (1981) 591. [3] D.C. Tran, C.F. Fisher and G.H. Sigel, Electron. Lett. 22 (1982) 657. [4] M. Braglia, M. Ferraris, G. Parisi, R. de Franceschi and L. Salasco, in: Proc. 6th Int. Syrup. on Halide Glasses, 1989, p. 527. [5] H. Poignant, C. Falcou and J. le Mellot, Glass Technol. 28 (1987) 38.