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Challenges in obtaining low loss fluoride glass fibers
]OURNA
Journal of Non-Crystalline Solids 140 (1992) 146-149 North-Holland
L OF
NON-CRYSTALLISOLIDS NE
Challenges in obtaining low loss fluoride glass fibers J.S. S a n g h e r a b, B.B. H a r b i s o n a a n d I.D. A g g a r w a l a a Naval Research Laboratory, Washington, DC 20375, USA b Geo-centers Inc., Ft. Washington, MD 20744, USA
The contribution from the different mechanisms to the total loss at 2.55 ~xm is reviewed. The observations suggest that extrinsic scattering induced by the different fiberizing techniques is the biggest limitation to achieving low loss in fluoride fibers. Extrinsic scattering is mainly due to microcrystals which grow on heterogeneities which can be subsequently eliminated and bubbles which are frequently present in all fiberizing techniques. A new fiberizing technique called the core injection technique (CIT) is discussed which potentially eliminates the presence of bubbles.
T h e optical attenuation of fluoride glass fibers is still over two orders of m a g n i t u d e higher than the theoretically predicted value of approximately 0.01 d B / k m at 2.55 p.m [1]. Despite this, advances have b e e n m a d e on short lengths of fibers where scattering losses of 0.025 d B / k m have b e e n r e p o r t e d at 2.55 fxm [2]. These results indicate that low loss is feasible. However, the question arises as to what is the factor limiting ultralow loss on longer lengths. T h e optical attenuation of a fiber is determ i n e d by the s u m m a t i o n of three terms, namely the waveguide, absorption and scattering losses. By a p p r o p r i a t e control of fiber processing techniques and drawing, the waveguide losses can be minimized. O n the o t h e r hand, the absorption losses arise from transition metal and rare earth ion impurities in the precursors a n d / o r during their processing into opical fibers. Table 1 shows
the transition metal ion c o n t e n t of a fluoride glass by G F A A direct-solid analysis in ppb-wt. T h e N d content was d e t e r m i n e d by photoluminescence spectroscopy. Also shown is the contribution from each ion to the loss at 2.55 Ixm. T h e contribution of extrinsic absorption to the total loss at 2.55 txm is therefore calculated to be < 0.280 d B / k m . Therefore, extrinsic absorption is not the major contributor to the total loss observed in current fibers. T h e major contributing m e c h a n i s m to total loss at 2.55 Ixm is extrinsic scattering. This is easily shown by the transmission of H e N e light (0.63 Ixm) t h r o u g h a fiber which subsequently illuminates m a n y defect centers which a p p e a r as pin points of light. Optical and s c a n n i n g / transmission electron microscopic analysis of these scattering centers reveals that these defects are p r e d o m i n a n t l y sub-micron platinum particles
Table 1 TM and rare earth ion content in fluoride glass Content (ppb-wt):
Fe
Co
108
<7
Ni <7
Cu <6
Nd 5
Loss/ppb a): (dB/km/ppb) Contribution: (dB/km)
0.015
0.017
0.016
< 0.119
0.0024 < 0.017
a): Ref. [11]. 0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
0.003
0.022
< 0.018
0.110
J.S. Sanghera et al. / Obtaining low loss fluoride glass fibers
147
Table 2 Scattering centers found in fluorozirconate glasses, preforms and fibers
Table 3 Bulk scattering loss of RAP melted fluoride glass (dB/km)
Crystals/defects
Bulk glass Fiber
Source
ZrF4
Contamination from sublimed particles ZB, 2ZB (Na) Pt and oxyfluoride nuclei Z(Na) Surface growth (bubbles) LaF3, LaF 3 .2ZrF4 [8] Incomplete dissolution Surface growth (container walls) Solubility limit exceeded a l F 3 (Al-rich) Incomplete dissolution Moisture, hydroxides and oxide ZrO 2 [12], ZrO~Fy impurities Fe, Ni phosphides [13] Phosphate impurities in precursors Pt (Au, Ir, Rh) Crucible reactions Carbon Organic impurities, crucible erosion Bubbles Contraction, cavitation, reactions, sublimation and gas precipitation
[3], oxyfluoride particles [4,5], bubbles [6,7] and fluoride microcrystals [7-10]. The fluoride microcrystals grow heterogeneously on the sub-micron sized defect centers. Table 2 lists the most fre-
Loss at 0.63 Ixm
Loss at 2.55 jxm
5.47_+ 0.2 7,5
0.02 a~ 0.025
~) Extrapolated.
quently found (and some not so common) defects in fluoride glasses, preforms and fibers and their possible origin. Our analysis reveals that the elimination of platinum, bubbles and oxyfluoride scattering/ nucleating centers is imperative to attaining ultralow loss. To this extent, RAP processing of anhydrous chemicals using vitreous carbon crucibles and SF6 atmosphere has been used and the processing conditions optimized for reproducible preparation of high quality and stable glasses. The thermal stability is characterized by the lack of crystallization on reheating at the crystal growth temperature [4]. Table 3 shows the scat-
Table 4 Advantages and problems associated with different fiberizing techniques Fiberizing Techniques
Advantages
Problems
Clean interface Core bubbles Clean interface, single mode Simple technique
Tapered preform
Bulk casting techniques Modified-built-in-casting Suction technique Built-in-casting Clad-over-core Rotational casting
No core bubbles Uniform core/clad Uniform core/clad
Tapered preform Tapered preform, interracial and core bubbles Interfacial crystallization and bubbles Core contraction bubbles, interfacial bubbles, cladding crystals by interface
Tubular structures / collapse techniques Rod-in-tube Spin-spin casting
No core bubbles, single mode Clean interface, no core bubbles, single mode Graded index, single mode
Interfacial bubbles and crystals Non-uniform collapse
In situ purification, single mode
Soot consolidation, Non-uniform collapse
Double crucible
Long lengths, single mode
Triple crucible
Long lengths, single mode, chemical durability
Poor concentricity of core and cladding Interfacial bubbles and crystals Interracial bubbles and crystals, poor concentricity
Reactive Vapor transport Chemical vapor deposition
Non-uniform collapse
Crucible techniques
148
J.S. Sanghera et al. / Obtaining low loss fluoride glass fibers
tering loss obtained for a typical fluoride glass melted in a vitreous carbon crucible and under SF6 atmosphere. Note the narrow distribution in scattering values obtained from approximately 10 random regions in the glass, highlighting the glass homogeneity. Further, this is the lowest scattering loss reported for a fluoride glass obtained by melting in vitreous carbon crucibles and under SF6 atmosphere. Hence, while the glasses possess high stability and quality, long lengths of fibers still possess higher losses than would be expected from the short lengths of fiber and bulk glasses alone. The answer to this problem lies in the difficulty in fabricating a high quality preform from which the fiber is subsequently drawn. Inherently, most preform processing techniques give rise to defects in the core glass, cladding glass and core/cladding interface. Table 4 reviews the different preform/ fiber fabrication techniques and highlights both the advantages and problems associated with each technique. For convenience, the techniques are grouped under the following categories, namely, bulk casting, tubular structures and crucible drawing techniques. The first of these preform fabrication techniques, bulk casting, can be subdivided into two groups. The first group consists of the modified-built-in-casting and suction casting techniques and arises from intimate contact between core and cladding melts in the liquid phase and without atmospheric exposure to the interface. The advantage of these techniques is that they lead to fewer crystals at the core/clad interface but unfortunately produce a tapered preform and so long lengths are limited. The second group consists of the built-in-casting, rotational casting and clad-over-core techniques and results in reheating of the solidified glass from the casting of a melt onto a glass (rod or tube) during exposure of the core/cladding interface to atmosphere. The advantage of the latter two techniques is the fabrication of a uniform core/clad geometry. However, both techniques give rise to interfacial bubbles and crystals. The built-in-casting technique gives neither a clean interface nor a uniform core/clad geometry. The second category, namely tubular structures, came about primarily for the purpose of
single mode fiber fabrication. Despite the obvious advantages of each technique, all these techniques are limited by the inability, at this point in time, to perform uniform collapse/consolidation of the tubes. The third category, namely crucible drawing techniques, allows for the fabrication of significantly longer lengths of fiber than is currently possible with the preform techniques. However, optical loss data are scarce and this is presumably related to the higher losses obtained compared to preform fabrication techniques. The mechanisms responsible for the high losses are the lack of concentricity of the core and cladding and interfacial bubbles and crystals. From table 4, it is quite evident that an extensive amount of work has been carried out by numerous research groups to fabricate high quality preforms. While each technique offers several advantages over other techniques, all seem to possess the same underlying problems with respect to attaining ultra-low loss in long fiber lengths. These problems can be summarized as follows: the presence of defects such as microcrystals, sub-micron precious metal nuclei/scattering centers, oxyfluoride nuclei and micro-bubbles. It is now accepted that the fluoride microcrystals grow on heterogeneities and that homogeneous nucleation is not limiting ultra-low loss [7]. Further, the microcrystal presence has been eliminated by using anhydrous chemicals, a clean room environment and RAP processing. However, irrespective of this, one defect still remains and is common to practically every fiberizing technique, namely bubbles. Therefore, the question arises as to what is the origin of the bubbles. Table 5 lists the different types of bubbles and their origins. Based on the frequent occurrence of bubbles Table 5 Origin of the different types of bubbles in glass Bubble types
Bubble Origin
Vacuum bubble
Glass contraction during cooling
Gas filled
Cavitation due to turbulent melt flow Outgassing of inert gas Chemical reactions Sublimation of volatile fluorides
J.S. Sanghera et aL / Obtaining low loss fluoride glass fibers i
. •
. .
\\
\\
CladdingTube~ \ \ IL \ \ ~kP1 \ \ \\ \\ \\
to turbulent flow. Preliminary results look extremely promising as evidenced by a clean, bubble-free core/cladding interface• Optimization of this process should produce high quality preforms leading to low loss fibers.
\\
'\\_ \\ \\ \\ \\ \\ CoreGlass- ~
149
~
\\--
k\l
\\
Fig. 1. Schematic representation of the core injection technique for bubble-free preform fabrication (P2 > P~).
using the different techniques, the present authors have developed a new preform fabrication technique which can potentially eliminate the bubbles• This approach is called the core injection technique (CIT) and is shown schematically in fig. 1. The cladding tube is first prepared by the conventional rotational technique but is then immersed in the core melt. The application of a differential pressure allows the core melt to travel up the cladding tube in a controlled manner depending upon the temperature of the melt and the differential pressure. By performing this at low temperatures and conditions, where P1 is greater than the pressure of the gas in the core melt, one eliminates sublimation of volatile components, cavitation bubbles and outgassing of inert gas. In addition, laminar flow conditions are easily obtained thereby inhibiting cavitation due
References [1] S. Shibata, M. Horiguchi, K. Jinguji, S. Mitachi, T. Kanamori and T. Manabe, Electron. Lett. 17 (1981) 775. [2] I. Aggarwal, G. Lu and L. Busse, Mater. Sci. Forum 32&33 (1988) 495. [3] G. Lu and J.P. Bradley, J. Am. Ceram. Soc. 69 (1986) 585. [4] J.S. Sanghera, I. Aggarwal, B. Harbison, L. Busse, P. Pureza, P. Hart, M.G. Sachon, L. Sills and R. Miklos, in: Optical Fiber Materials and Processing, Mater. Res. Soc. Proc. 172 (1990) 157. [5] J.S. Sanghera, P. Hart, M.G. Sachon and I.D. Aggarwal Mater. Lett. 10 (1991) 181. [6] P. McNamara and D.R. MacFarlane, J. Non-Cryst. Solids 95&96 (1987) 625. [7] I. Aggarwal, J.S. Sanghera, B. Harbison, L Busse and P. Pureza, in: Proc. 6th. Int. Syrup. on Halide Glasses, Clausthal-Zellerfeld, W. Germany (1989). [8] J. Parker, D. Seddon and A. Clare, Phy. Chem. Glasses 28 (1987) 4. [9] J. Bradley, G. Lu, C. Fisher and M. Robinson, Mater. Sci. Forum 19&20 (1987) 545. [10] J.S. Sanghera, P. Hart, M.G. Sachon, L. Busse and I.D. Aggarwal, J. Am. Ceram. Soc. 73 (1990) 2677 [11] P.W. France, S.F. Carter, M.W. Moore and J.R. Williams, Proc. SPIE (Infrared Optical Materials and Fibres IV) 618 (1986) 51. [12] H. Hattori, S. Sakaguchi, T. Kanamori and Y. Terunuma, Appl. Opt. 26 (1987) 2683. [13] G. Lu, C. Fisher and J.P. Bradley, J. Non-Cryst. Solids 94 (1987) 45.
Journal of Non-Crystalline Solids 140 (1992) 146-149 North-Holland
L OF
NON-CRYSTALLISOLIDS NE
Challenges in obtaining low loss fluoride glass fibers J.S. S a n g h e r a b, B.B. H a r b i s o n a a n d I.D. A g g a r w a l a a Naval Research Laboratory, Washington, DC 20375, USA b Geo-centers Inc., Ft. Washington, MD 20744, USA
The contribution from the different mechanisms to the total loss at 2.55 ~xm is reviewed. The observations suggest that extrinsic scattering induced by the different fiberizing techniques is the biggest limitation to achieving low loss in fluoride fibers. Extrinsic scattering is mainly due to microcrystals which grow on heterogeneities which can be subsequently eliminated and bubbles which are frequently present in all fiberizing techniques. A new fiberizing technique called the core injection technique (CIT) is discussed which potentially eliminates the presence of bubbles.
T h e optical attenuation of fluoride glass fibers is still over two orders of m a g n i t u d e higher than the theoretically predicted value of approximately 0.01 d B / k m at 2.55 p.m [1]. Despite this, advances have b e e n m a d e on short lengths of fibers where scattering losses of 0.025 d B / k m have b e e n r e p o r t e d at 2.55 fxm [2]. These results indicate that low loss is feasible. However, the question arises as to what is the factor limiting ultralow loss on longer lengths. T h e optical attenuation of a fiber is determ i n e d by the s u m m a t i o n of three terms, namely the waveguide, absorption and scattering losses. By a p p r o p r i a t e control of fiber processing techniques and drawing, the waveguide losses can be minimized. O n the o t h e r hand, the absorption losses arise from transition metal and rare earth ion impurities in the precursors a n d / o r during their processing into opical fibers. Table 1 shows
the transition metal ion c o n t e n t of a fluoride glass by G F A A direct-solid analysis in ppb-wt. T h e N d content was d e t e r m i n e d by photoluminescence spectroscopy. Also shown is the contribution from each ion to the loss at 2.55 Ixm. T h e contribution of extrinsic absorption to the total loss at 2.55 txm is therefore calculated to be < 0.280 d B / k m . Therefore, extrinsic absorption is not the major contributor to the total loss observed in current fibers. T h e major contributing m e c h a n i s m to total loss at 2.55 Ixm is extrinsic scattering. This is easily shown by the transmission of H e N e light (0.63 Ixm) t h r o u g h a fiber which subsequently illuminates m a n y defect centers which a p p e a r as pin points of light. Optical and s c a n n i n g / transmission electron microscopic analysis of these scattering centers reveals that these defects are p r e d o m i n a n t l y sub-micron platinum particles
Table 1 TM and rare earth ion content in fluoride glass Content (ppb-wt):
Fe
Co
108
<7
Ni <7
Cu <6
Nd 5
Loss/ppb a): (dB/km/ppb) Contribution: (dB/km)
0.015
0.017
0.016
< 0.119
0.0024 < 0.017
a): Ref. [11]. 0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
0.003
0.022
< 0.018
0.110
J.S. Sanghera et al. / Obtaining low loss fluoride glass fibers
147
Table 2 Scattering centers found in fluorozirconate glasses, preforms and fibers
Table 3 Bulk scattering loss of RAP melted fluoride glass (dB/km)
Crystals/defects
Bulk glass Fiber
Source
ZrF4
Contamination from sublimed particles ZB, 2ZB (Na) Pt and oxyfluoride nuclei Z(Na) Surface growth (bubbles) LaF3, LaF 3 .2ZrF4 [8] Incomplete dissolution Surface growth (container walls) Solubility limit exceeded a l F 3 (Al-rich) Incomplete dissolution Moisture, hydroxides and oxide ZrO 2 [12], ZrO~Fy impurities Fe, Ni phosphides [13] Phosphate impurities in precursors Pt (Au, Ir, Rh) Crucible reactions Carbon Organic impurities, crucible erosion Bubbles Contraction, cavitation, reactions, sublimation and gas precipitation
[3], oxyfluoride particles [4,5], bubbles [6,7] and fluoride microcrystals [7-10]. The fluoride microcrystals grow heterogeneously on the sub-micron sized defect centers. Table 2 lists the most fre-
Loss at 0.63 Ixm
Loss at 2.55 jxm
5.47_+ 0.2 7,5
0.02 a~ 0.025
~) Extrapolated.
quently found (and some not so common) defects in fluoride glasses, preforms and fibers and their possible origin. Our analysis reveals that the elimination of platinum, bubbles and oxyfluoride scattering/ nucleating centers is imperative to attaining ultralow loss. To this extent, RAP processing of anhydrous chemicals using vitreous carbon crucibles and SF6 atmosphere has been used and the processing conditions optimized for reproducible preparation of high quality and stable glasses. The thermal stability is characterized by the lack of crystallization on reheating at the crystal growth temperature [4]. Table 3 shows the scat-
Table 4 Advantages and problems associated with different fiberizing techniques Fiberizing Techniques
Advantages
Problems
Clean interface Core bubbles Clean interface, single mode Simple technique
Tapered preform
Bulk casting techniques Modified-built-in-casting Suction technique Built-in-casting Clad-over-core Rotational casting
No core bubbles Uniform core/clad Uniform core/clad
Tapered preform Tapered preform, interracial and core bubbles Interfacial crystallization and bubbles Core contraction bubbles, interfacial bubbles, cladding crystals by interface
Tubular structures / collapse techniques Rod-in-tube Spin-spin casting
No core bubbles, single mode Clean interface, no core bubbles, single mode Graded index, single mode
Interfacial bubbles and crystals Non-uniform collapse
In situ purification, single mode
Soot consolidation, Non-uniform collapse
Double crucible
Long lengths, single mode
Triple crucible
Long lengths, single mode, chemical durability
Poor concentricity of core and cladding Interfacial bubbles and crystals Interracial bubbles and crystals, poor concentricity
Reactive Vapor transport Chemical vapor deposition
Non-uniform collapse
Crucible techniques
148
J.S. Sanghera et al. / Obtaining low loss fluoride glass fibers
tering loss obtained for a typical fluoride glass melted in a vitreous carbon crucible and under SF6 atmosphere. Note the narrow distribution in scattering values obtained from approximately 10 random regions in the glass, highlighting the glass homogeneity. Further, this is the lowest scattering loss reported for a fluoride glass obtained by melting in vitreous carbon crucibles and under SF6 atmosphere. Hence, while the glasses possess high stability and quality, long lengths of fibers still possess higher losses than would be expected from the short lengths of fiber and bulk glasses alone. The answer to this problem lies in the difficulty in fabricating a high quality preform from which the fiber is subsequently drawn. Inherently, most preform processing techniques give rise to defects in the core glass, cladding glass and core/cladding interface. Table 4 reviews the different preform/ fiber fabrication techniques and highlights both the advantages and problems associated with each technique. For convenience, the techniques are grouped under the following categories, namely, bulk casting, tubular structures and crucible drawing techniques. The first of these preform fabrication techniques, bulk casting, can be subdivided into two groups. The first group consists of the modified-built-in-casting and suction casting techniques and arises from intimate contact between core and cladding melts in the liquid phase and without atmospheric exposure to the interface. The advantage of these techniques is that they lead to fewer crystals at the core/clad interface but unfortunately produce a tapered preform and so long lengths are limited. The second group consists of the built-in-casting, rotational casting and clad-over-core techniques and results in reheating of the solidified glass from the casting of a melt onto a glass (rod or tube) during exposure of the core/cladding interface to atmosphere. The advantage of the latter two techniques is the fabrication of a uniform core/clad geometry. However, both techniques give rise to interfacial bubbles and crystals. The built-in-casting technique gives neither a clean interface nor a uniform core/clad geometry. The second category, namely tubular structures, came about primarily for the purpose of
single mode fiber fabrication. Despite the obvious advantages of each technique, all these techniques are limited by the inability, at this point in time, to perform uniform collapse/consolidation of the tubes. The third category, namely crucible drawing techniques, allows for the fabrication of significantly longer lengths of fiber than is currently possible with the preform techniques. However, optical loss data are scarce and this is presumably related to the higher losses obtained compared to preform fabrication techniques. The mechanisms responsible for the high losses are the lack of concentricity of the core and cladding and interfacial bubbles and crystals. From table 4, it is quite evident that an extensive amount of work has been carried out by numerous research groups to fabricate high quality preforms. While each technique offers several advantages over other techniques, all seem to possess the same underlying problems with respect to attaining ultra-low loss in long fiber lengths. These problems can be summarized as follows: the presence of defects such as microcrystals, sub-micron precious metal nuclei/scattering centers, oxyfluoride nuclei and micro-bubbles. It is now accepted that the fluoride microcrystals grow on heterogeneities and that homogeneous nucleation is not limiting ultra-low loss [7]. Further, the microcrystal presence has been eliminated by using anhydrous chemicals, a clean room environment and RAP processing. However, irrespective of this, one defect still remains and is common to practically every fiberizing technique, namely bubbles. Therefore, the question arises as to what is the origin of the bubbles. Table 5 lists the different types of bubbles and their origins. Based on the frequent occurrence of bubbles Table 5 Origin of the different types of bubbles in glass Bubble types
Bubble Origin
Vacuum bubble
Glass contraction during cooling
Gas filled
Cavitation due to turbulent melt flow Outgassing of inert gas Chemical reactions Sublimation of volatile fluorides
J.S. Sanghera et aL / Obtaining low loss fluoride glass fibers i
. •
. .
\\
\\
CladdingTube~ \ \ IL \ \ ~kP1 \ \ \\ \\ \\
to turbulent flow. Preliminary results look extremely promising as evidenced by a clean, bubble-free core/cladding interface• Optimization of this process should produce high quality preforms leading to low loss fibers.
\\
'\\_ \\ \\ \\ \\ \\ CoreGlass- ~
149
~
\\--
k\l
\\
Fig. 1. Schematic representation of the core injection technique for bubble-free preform fabrication (P2 > P~).
using the different techniques, the present authors have developed a new preform fabrication technique which can potentially eliminate the bubbles• This approach is called the core injection technique (CIT) and is shown schematically in fig. 1. The cladding tube is first prepared by the conventional rotational technique but is then immersed in the core melt. The application of a differential pressure allows the core melt to travel up the cladding tube in a controlled manner depending upon the temperature of the melt and the differential pressure. By performing this at low temperatures and conditions, where P1 is greater than the pressure of the gas in the core melt, one eliminates sublimation of volatile components, cavitation bubbles and outgassing of inert gas. In addition, laminar flow conditions are easily obtained thereby inhibiting cavitation due
References [1] S. Shibata, M. Horiguchi, K. Jinguji, S. Mitachi, T. Kanamori and T. Manabe, Electron. Lett. 17 (1981) 775. [2] I. Aggarwal, G. Lu and L. Busse, Mater. Sci. Forum 32&33 (1988) 495. [3] G. Lu and J.P. Bradley, J. Am. Ceram. Soc. 69 (1986) 585. [4] J.S. Sanghera, I. Aggarwal, B. Harbison, L. Busse, P. Pureza, P. Hart, M.G. Sachon, L. Sills and R. Miklos, in: Optical Fiber Materials and Processing, Mater. Res. Soc. Proc. 172 (1990) 157. [5] J.S. Sanghera, P. Hart, M.G. Sachon and I.D. Aggarwal Mater. Lett. 10 (1991) 181. [6] P. McNamara and D.R. MacFarlane, J. Non-Cryst. Solids 95&96 (1987) 625. [7] I. Aggarwal, J.S. Sanghera, B. Harbison, L Busse and P. Pureza, in: Proc. 6th. Int. Syrup. on Halide Glasses, Clausthal-Zellerfeld, W. Germany (1989). [8] J. Parker, D. Seddon and A. Clare, Phy. Chem. Glasses 28 (1987) 4. [9] J. Bradley, G. Lu, C. Fisher and M. Robinson, Mater. Sci. Forum 19&20 (1987) 545. [10] J.S. Sanghera, P. Hart, M.G. Sachon, L. Busse and I.D. Aggarwal, J. Am. Ceram. Soc. 73 (1990) 2677 [11] P.W. France, S.F. Carter, M.W. Moore and J.R. Williams, Proc. SPIE (Infrared Optical Materials and Fibres IV) 618 (1986) 51. [12] H. Hattori, S. Sakaguchi, T. Kanamori and Y. Terunuma, Appl. Opt. 26 (1987) 2683. [13] G. Lu, C. Fisher and J.P. Bradley, J. Non-Cryst. Solids 94 (1987) 45.