Note: Descriptions are shown in the official language in which they were submitted.
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STABLE CLADDING GLASSES FOR SULPHIDE FIBRES
Field of the Invention
This invention relates generally to glasses for use in optical fibers, and
more specifically to cladding glasses which exhibit improved thermal stability
and a low refractive index.
Background of the Invention
U.S. Patent 5,389,584, NGa-and/or In-Containing AsGe Sulphide
Glasses" describes the addition of either Ga or In to GeAs Sulphide glasses,
which when doped with a suitable rare earth metal, can be used for the
fabrication of efficient amplifier, laser and/or upconverter devices. In the
particular application of 1300 nm optical amplification, such glasses are
excellent hosts for Pr, and are characterized by a high quantum efficiency for
the desired 'G4 >'Hs emission. These glasses also have sufficient thermal
stability to be drawn into fibre, and are therefore suitable for use as the
core
glass of an optical waveguide to amplify 1300 run signals.
In order to fabricate such a sulphide glass waveguide, it is necessary to
clad the core glass with another chemically and physically compatible glass
that
has a lower refractive index. In the basic GeGaAsS or GeInAsS systems, lower
index glasses can be obtained by reducing the As content, andlor increasing
the
- Ge content, respectively, relative to that of a given core glass. However,
such compositional
changes typically degrade the thermal stability of these
materials, e.g. as measured by the temperature interval TX T8, resulting in an
increased tendency towards crystallization. It can therefore be seen that
there
is a need for a method to both lower the refractive index and maintain or
improve the thermal stability of such sulphide glasses so as to be able to
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fabricate a waveguide with suitable light-guiding properties. The present
invention is based on the discovery that the addition of silicon or phosphorus
to
GeAs sulphide glasses provides a means to achieving the above goals.
Summary of the Invention
In one embodiment of the present invention, the glass composition
consists principally of Ge, As and S, t Ga and/or In, with small but necessary
additions of Si. Other metals, including Ca, Sr, Ba, Ag, Tl, Cd, Sn, Hg, Pb,
Y,
La and other rare-earth metals from the lanthanide series and Sb, as well as
optional anionic components such as Se, Te and the halogens F, Cl, Br and I,
can be added to optimize various other physical properties such as thermal
expansion, viscosity, etc., but are not essential constituents. In a second
embodiment of the present invention, the addition of phosphorus to GeAs
sulphide glasses can be used instead of silicon to accomplish the same
objectives. Glasses of these compositions provide for a cladding glass which
exhibits improved thermal stability and a lower refractive index relative to
that
of a GeGaAsS or GeInAsS core.
Brief Description of the Drawings
For a fuller understanding of the nature and objects of the invention,
reference should be made to the following detailed description of a preferred
mode of practicing the invention, read in connection with the accompanying
drawings, in which:
FIG. 1 is a perspective view of a segment of an optical fiber made of a
glass composition of the present invention.
FIG. 2 is a cross sectional view of the fiber of Fig. 1 taken along line 2-2.
FIG. 3 is a plot of the refractive index based on the concentration of Si
(as expressed in terms of atomic %) in a GeAs sulfide glass.
FIG. 4 is a plot of the thermal stability of GeAs sulfide glasses with
varying concentrations of Si as expressed in atomic %.
FIG. 5 is a plot of the refractive index based on the concentration of P as
expressed
in terms of atomic ~ in a GeAs sulfide glass.
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Detailed Description of the Invention
Fig. 1 illustrates a segment of an optical fiber 10 suitable for use in an
amplifier, laser andlor upconverter device. The fiber comprises an inner glass
core 14 which is clad with an outer glass cladding 12 which is a chemically
and physically
S compatible glass that has a lower refractive index than core glass 14
(see Fig. 2).
The present invention, in one embodiment, is based on the discovery
that the incorporation of Si in a GeAs sulphide glass results in a progressive
decrease of the refractive index, as illustrated in Fig. 3 of the drawings.
The
data in Fig. 1 show that substitution of 2.5% At% of Si for Ge lowers the
refractive index by about 0.025 for glasses with the (Ge, Si)uAs,oSbs
stoichiometry. Therefore, if glasses Nos. '7 and 1 were utilized as core and
cladding glasses, respectively, the numerical aperture (NA) of the resultant
waveguide would be about 0.35, which is sufficiently high for an efficient
amplifier fibre.
Tables 1 and 2 report a group of glass compositions expressed in terms of
atomic
percent (At% ) , illustrating the subject inventive glasses. Because the
glasses were prepared in the laboratory, the glasses were typically prepared
by
melting mixtures of the respective elements, although in some cases a given
metal was hatched as a sulfide. As can be appreciated, however, that practice
is
not necessary. The actual batch ingredients can be any materials which, upon
melting together with the other batch components, are converted into the
desired sulfide in the proper proportions.
The batch constituents were weighed, loaded and sealed into silica
ampoules which had been evacuated to about 10-5 to 10-6 Torr. The ampoules
were placed into a furnace designed to impart a rocking motion to the batch
during melting. After melting the batch for about 1-3 days at 850°-
950° C., the
melts were quenched to form homogeneous glass rods having diameters of about
7-10 mm and lengths of about 60-70 mm, which rods were annealed at about
325 °-425 °C.
Table 1 also records the glass transition temperature (Tg), the
temperature at the onset of crystallization (Tx), and the difference between
those measurements (TX - Tg), which quantity is commonly used to gauge the
thermal stability of a glass, as well as the refractive index at the sodium D
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line (nD).
It will be appreciated that the above-described procedures represent
laboratory practice only. That is, the batches for the inventive glasses can
be
melted in large commercial glass melting units and the resulting melts formed
into desired glass shapes utilizing commercial glass forming techniques and
equipment. It is only necessary that the batch materials be heated to a
sufficiently high temperature for an adequate period of time to secure a
homogeneous melt, and that melt thereafter cooled and simultaneously shaped
into a body of a desired configuration at a sufficiently rapid rate to avoid
the development of
devitrification.
Examples of Si-containing glasses of the present invention that are
useful for the purpose of cladding a core consisting of GeGaAsS or GeInAsS
glass are tabulated below in Table 1 in At%, along with an example of a
representative GeGaAsS core glass (Example 7).
Table 1
Glass 1 2 3 4 5 6 7
Ge 22.5 20 17.5 25.3 22.5 19.7 25.0
Si 2.5 5 7.5 2.8 5.6 8.4 -
As 10 10 10 6.3 6.3 6.3 9.8
S 65 65 65 65.6 65.6 65.6 65.0
Ga - __ __ __ __ __ 0.2
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Tg 312 348 339 364 368 361 -
Tx > 650 -630 585 640 625 630 -
TX Tg > 335 --280 246 275 255 270 -
np 2.283 2.265 - 2.264 2.252 2.241 2.308
In order to achieve corelcladding structures with a comparable NA in the
basic GeGaAsS or GeInAsS systems, the thermal stability (TX TB) of typical
cladding glasses is on the order of 230-250°C. However, the Tx Tg of Si-
5 substituted glasses can be maintained at a value in excess of 250°C
over a wide
range of compositions, and in some cases can be in excess of that of the base
GeAs sulphide glass, as illustrated in Fig. 2 of the drawings.
The composition of the Si containing cladding glasses comprise the
following approximate ranges in terms of mole percent on the sulfide basis
(see
Tabie 2): 50-95 % GeS2, 2-40% AsZS3, 0.1-30% SiS2, 0-20% Ga2S3 and /or InZS3,
0-10 % MSX, where M is selected from Ca, Sr, Ba, Ag, Tl, Cd, Hg, Sn, Pb, Y, La
and other rare-earth metals of the lanthanide series, or Sb, 0-5 % of the
corresponding metal selenide andlor telluride, 0-20 % of the corresponding
metal
halide, and wherein the sulfur and/or selenium and/or tellurium content can
vary between 85-125 % of the stoichiometric value.
Table 2 (Mole %)
Glass 1 2 3 4 5 6 7
GeS2 75.0 66.7 58.3 81.0 72.0 63.0 83.3
SiSz 8.3 16.7 25.0 9.0 18.0 27.0 -
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Ga2S3 _ _ _ _ _ _ 0.3
As2S3 16.7 16.7 16.7 10.0 10.0 10.0 16.3
prZS3 _ _ _ _ _ _ 0.03
In a second embodiment of the present invention, the glasses consist
principally of Ge, As and S, t Ga andlor In, with a small but necessary
addition
of P. Other metals, including Ca, Sr, Ba, Ag, TI, Cd, Hg, Sn, Pb, Y, La and
other rare-earth metals from the lanthanide series and Sb, as well as optional
anionic components such as Se, Te and the halogens F, CI, Br and I, can be
added to optimize various other physical properties such as thermal expansion,
viscosity,
etc., but are not essential constituents. Compositions (in atomic %) of
suitable P containing glasses that are useful for the purpose of cladding a
core
consisting of GeGaAsS or GeInAsS glass are given below in Table 3:
Table 3 (Atomic %)
Glass 8 9 10 11 12 13 14
Ge 24.4 23.8 23.6 23.3 22.2 19.5 16.9
P 2.4 4.8 5.7 7.0 2.5 4.9 7.2
As 7.3 4.8 3.8 2.3 9.9 9.8 9.6
S 65.9 66.7 67.0 67.4 65.4 65.8 66.3
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Glass 15 16 17 18 19 20 21
Ge 23.0 22.1 21.3 22.8 21.9 21.1 23.6
P 4.8 4.8 4.8 5.7 5.7 5.7 2.4
As 5.7 6.7 7.7 4.7 5.7 6.7 7.1
S 66.5 66.4 66.2 66.8 66.7 66.5 66.9
In this embodiment, the incorporation of P in a GeAs sulphide glass
results in a progressive decrease of refractive index, and a reduced tendency
of
GeS2 to crystallize, leading to enhanced thermal stability. Accordingly, when
Example 8 is used as cladding for a core glass with the composition of Example
?, the resultant fibre is expected to have a numerical aperture of 0.32 which
is
more than adequate for a sulphide 1.3 Eun amplifier fibre.
The compositions of these phosphorous containing cladding glasses
comprise the following approximate ranges in terms of mole percent on the
sulfide basis (see Table 4); 50-95% GeS2, 2-40% As2S3, 0.1-25% P2S5, 0-20%
Ga2S3 andlor
In2S3, 0-10% MSx, where M is selected from Ca, Sr, Ba, Ag, Tl, Cd, Hg, Sn, Pb,
Y, La and
other rare-earth metals of the lanthanide series, or Sb, 0-
S % of the corresponding metal selenide and/or telluride, 0-20% of the
corresponding metal halide, and wherein the sulfur and/or selenium andlor
tellurium content can vary between 85-125 % of the stoichiometric value.
Table 4 (Mole %)
Glass 8 9 10 11 12 13 14
GeSz 83.3 83.3 83.3 83.3 78.3 72.7 66.7
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PZSs 4.2 8.3 10.0 12.5 4.4 ~ 9.1 14.3
ASzS3 12.5 8.3 6.7 4.2 17.4 18.2 19.1
T8 331 337 315 317 280
Tx > 583 > 577 > 557 > 544 > 512
TX Tg > 252 > 240 > 242 > 227 > 232
np 2.286 2.259 2.239 2.314 2.316 2.317
Glass 15 16 17 18 19 20 21
GeS2 81.4 79.3 77.2 81.3 79.3 77.2 83.3
PISS 8.5 8.6 8.8 10.2 10.3 10.5 4.2
AsZS3 10.2 12.1 14.0 8.5 10.3 12.3 12.5
Excess - - - - - - 105
S
np 2.280 2.278 2.298 2.275 2.285 2.289 --
Fig. 5 illustrates that the substitution of 2.5 At% P for Ge lowers the
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refractive index by about 0.022 for glasses with the (Ge,P)uAs,oSbs
stoichiometry.
While the present invention has been particularly shown and described
with reference to the preferred mode as illustrated in the drawing, it will be
understood by one skilled in the art that various changes in detail may be
effected therein without departing from the spirit and scope of the invention
as
defined by the claims.
