Note: Descriptions are shown in the official language in which they were submitted.
CA 02380051 2002-O1-21
WO 01/07374 PCT/US00/16626
BORATE OR ALUM1NOSILICATE GLASS COMPOSITION FOR OPTICAL
AMPLIFICATION
The present invention relates to glass compositions, which are suited for use
in
WDM telecommunication systems employing optical amplification at wavelengths
particularly in the third telecom window, i.e. near 1.5 Vim. More
particularly, the present
invention relates to a family of silicate glasses having introduced therein at
least one oxide
of a group III element, preferably aluminium oxide or boron oxide.
In optical fibre communications systems there is an increasing need for
amplifier
materials which provide a flat gain characteristic, especially in the third
telecommunications window (1525-1560 nm). At present, one of the best optical
amplifier
materials in this wavelength range consists of ZBLAN glass (ZrF4-BaF2-LaF3-
A1F3-NaF),
which has a gain ripple characterised by a "Figure of Merit", FOM (FOM=
[(Gainm~
Gainm;")/Gainm;"] x 100 %), of 10.6% over a bandwidth of 32 nm and a FOM of
18% over
a bandwidth of 35nm, in the 1.5 ~m wavelength region. However, ZBLAN glass is
expensive and requires special processing conditions.
The present invention seeks to provide a family of glasses having a gain
characteristic, in the region of the third telecommunications window, whose
flatness is
comparable to or better than that of ZBLAN.
Incidentally, the FOM is calculated over a fixed portion of the gain
characteristic,
typically a portion 30 or 32 nm wide. The FOM is designated "floating" if the
selected 30
or 32 nm portion is selected, not between fixed wavelength values, but from
whichever
location within the band of interest (here 1525 to 1560 nm) maximises the FOM
value.
The present invention provides a glass composition which includes at least 50
mole
percent of Si02, and at least one group III oxide, preferably selected from
the group A1203
and B203, characterised by the following ratio,
XZO+YO
R = A <_ 1,3,
Alz O3 + B2 O3
SUBSTITUTE SHEET (RULE 26)
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2
where X20 represents the sum of all alkali metal oxides in the composition,
and YO
represents the sum of all oxides in the composition taken from the group
consisting of
alkaline earth oxides, Zn0 and PbO.
Glass compositions according to an embodiment of the present invention have
been found to have a gain characteristic in the 1.5 ~m wavelength region, the
flatness of
which is either comparable to or better than that of ZBLAN over a similar
bandwidth
(around 30nm).
The glass compositions according to the present invention are generally doped
with erbium, in 0.005 to 6 parts by weight added for 100 parts by weight of
the base
composition. Advantageously, these glass compositions may be co-doped with
Yb203
in up to 12 molar percent of the base composition.
It also may be advantageous to include in the base composition oxides such as
Y203 and Gd203 which aid dispersion of erbium in the matrix. These oxides can
be
added in up to 3 mole percent each, up to a total of 5 molar percent for the
sum of these
oxides. It has been found that inclusion of oxides of this type improves yet
further the
gain flatness.
The glass compositions
according to the
present invention
preferably have
a
base composition
comprising:
Si02 50.0-90.0 mol.% 0.0-10.0 mol.% B2O3 0.0-30.0 mol.%
Ge02
A1203 0.0-30.0 mol.% Li200.0-15.0 mol.% Na20 0.0-25.0 mol.%
K20 0.0-15.0 mol.% Mg0 0.0-5.0 mol.% Sr0 0.0-10.0 mol.%
Ca0 0.0-10.0 mol.% Ba0 0.0-15.0 mol.% Zn0 0.0-10.0 mol.%
Pb0 0.0-10.0 mol.% Y2O30.0-3.0 mol.% Gd203 0.0-3.0 mol.%
Yb203 0.0-12.0 mol.%,
with
(B203 X20 0.0-20.0 XO 0.0-15.0 mol.%,
+ A1203) mol.%
5-35.0
mol.%
and
YO 0.0-20.0 mol.%,
where X20 is the sum of all alkali metal oxides present in the base
composition, XO is
the sum of all alkaline earth oxides present in the base composition and YO is
the sum
of all alkaline earth oxides plus Zn0 and Pb0 present in the base composition.
The glass compositions according to the present invention may include up to 12
parts by weight of fluorine, preferably up to 9 parts by weight thereof, added
to every
100 parts by weight of the base composition.
For those compositions according to the present invention which do not include
fluorine, it is preferable that the ratio R should be less than or equal to
1Ø For such
compositions, and those according to the invention which include fluorine and
having R
SUBSTITUTE SHEET (ECULE 26)
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WO 01/07374 PCT/US00/16626
3
less than or equal to 1.3, a FOM less than 25% can be obtained over a 32nm
bandwidth
in the wavelength region of interest.
Also, up to 12 parts by weight of chlorine, preferably up to 9 parts by weight
thereof, may be added to every 100 parts by weight of the base composition, in
order to
dry the glass.
It may be further preferred that the glass compositions according to the
present
invention have 0.005-6.0 parts by weight of Er203, 0.0-9.0 parts by weight of
chlorine
and 0.0-9.0 parts by weight of fluorine, added for 100 parts by weight
composed,as
follows:
Si02 55.0-85.0 mol.% Ge02 0.0-8.0 mol.% B2O3 0.0-25.0 mol.%
A1203 1.5-25.0 mol.% Li20 0.0-12.0 mol.% Na20 0.0-20.0 mol.%
K20 0.0-12.0 mol.% Mg0 0.0-3.0 mol.% Sr0 0.0-5.0 mol.%
Ca0 0.0-8.0 mol.% Ba0 ' 0.0-10.0 mol.% Zn0 0.0-5.0 mol.%
Pb0 0.0-5.0 mol.% Y203 0.0-2.0 mol.% Gd203 0.0-2.0 mol.%
Yb203 0.0-10.0 mol.%, with
(B203 + A1203) 5-35.0 mol.% X20 0.0-20.0 mol.% XO 0.0-15.0 mol.%,
YO 0.0-20.0 mol.%,
where X20 is the sum of all alkali metal oxides present in the base
composition, XO is
the sum of all alkaline earth oxides present in the base composition and YO is
the sum
of all alkaline earth oxides plus Zn0 and Pb0 present in the base composition.
Oxides such as Ti02 and/or Zr02 may be included in the glass compositions of
the present invention, if desired, in order to adjust the refractive index
thereof. Such
oxides would typically be included in up to 1.0 mol.% each.
Furthermore, the fluorescence characteristics of the glass compositions
according to the invention may be further improved by heat-treating the
compositions
after their formation, for example by subjecting the aluminosilicate glasses
of the
invention to temperatures between 500 and 700 °C for one hour.
Other features and advantages of the present invention will become clear from
the following description of preferred embodiments thereof, given by way of
example,
and illustrated by the accompanying drawings, in which:
Fig.l is a graph of gain versus wavelength for a typical borosilicate glass
according to the present invention;
Fig.2 is a graph of gain ripple (as measured by the FOM) versus the ratio of
(X20 + YO) to (A1203+B203) in a typical aluminosilicate glass composition
according
to the present invention;
SUBSTITUTE SHEET (RULE 26)
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WO 01/07374 PCT~LJS00/16626
4
Fig.3 is a graph of gain ripple (as measured by the FOM) versus fluorine
content
in a typical aluminosilicate glass composition according to the present
invention;
Fig.4 is a graph of normalized fluorescence versus wavelength for different
values of fluorine content for the glass composition constituting Example 5 of
Table 1;
S Fig.S is a graph of gain versus wavelength for the glass composition
constituting
Example 5 of Table 1;
Fig.6 is a graph of gain versus wavelength for a glass composition, similar to
Example 5 of Table 1, subjected to heat treatments at different temperatures.
The present inventors have found that optical amplifier materials having gain
flatness comparable to or better than that of ZBLAN, in the 1.5 pm wavelength
region,
may be constituted by glass compositions including: at least 50 mole percent
of Si02,
and at least one group III oxide, preferably selected from the group A1203 and
B203,
wherein the following ratio is respected,
X20+YO
Al2 03 + B203 A < 13
where X20 represents the sum of all alkali metal oxides in the composition,
and YO
represents the sum of all oxides in the composition taken from the group
consisting of
alkaline earth oxides, Zn0 and PbO.
Advantageously, the glass compositions according to the invention may include
0.005 to 6 parts by weight of Er203, up to 12 parts by weight of fluorine, and
up to 12
parts by weight of chlorine (to dry the glass and increase fluorescence
lifetime), added
for 100 parts by weight of the base composition made up, as follows:
Si02 50.0-90.0 mol.% Ge02 0.0-10.0 mol.% B2O3 0.0-30.0 mol.%
A1203 0.0-30.0 mol.% Li20 0.0-15.0 mol.% Na20 0.0-25.0 mol.%
K20 0.0-15.0 mol.% Mg0 0.0-5.0 mol.% Sr0 0.0-10.0 mol.%
Ca0 0.0-10.0 mol.% Ba0 0.0-15.0 mol.% Zn0 0.0-10.0 mol.%
Pb0 0.0-10.0 mol.% Y203 0.0-3.0 mol.% Gd203 0.0-3.0 mol.%
Yb203 0.0-12.0 mol.%, with
(B203 + A1203) 5-35.0 mol.% X20 0.0-20.0 mol.% XO 0.0-15.0 mol.%,
and
YO 0.0-20.0 mol.%,
where X20 is the sum of all alkali metal oxides present in the base
composition, XO is
the sum of all alkaline earth oxides present in the base composition and YO is
the sum
of all alkaline earth oxides plus Zn0 and Pb0 present in the base composition.
SUBSTITUTE SHEET (RULE 26)
CA 02380051 2002-O1-21
WO 01/07374 PCT/US00/16626
It is preferred that those compositions according to the invention which
include
X~_O+YO
fluorine should have: R = A <_ 1,3, and those compositions
Al, 03 + Bz 03
according to the invention which do not contain fluorine should have
X20+YO
A1203+B203A<1,3.
5 Yet more advantageously, the glass compositions according to the invention
may include 0.005 to 6 parts by weight of Er203, up to 9 parts by weight of
fluorine,
and up to 9 parts by weight of chlorine, added for 100 parts by weight of the
base .
composition made up, as follows:
Si02 55.0-85.0 mol.% Ge02 0.0-8.0 mol.% B2O3 0.0-25.0 mol.%
A1203 1.5-25.0 mol.% Li20 0.0-12.0 mol.% Na20 0.0-20.0 mol.%
K20 0.0-12.0 mol.% Mg0 0.0-3.0 mol.% Sr0 0.0-5.0 mol.%
Ca0 0.0-8.0 mol.% Ba0 0.0-10.0 mol.% Zn0 0.0-5.0 mol.%
Pb0 0.0-5.0 mol.% Y2O3 0.0-2.0 mol.% Gd203 0.0-2.0 mol.%
Yb203 0.0-10.0 mol.%, with
(B203 + A1203) 5-35.0 mol.% X20 0.0-20.0 mol.% XO 0.0-15.0 mol.%,
and
YO 0.0-20.0 mol.%.
Some typical compositions and properties of glasses according to the present
invention are given in Table 1 below, together with details of three
comparative
examples.
SUBSTITUTE SHEET (RULE 26)
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CA 02380051 2002-O1-21
WO 01/07374 PCT/US00/16626
In Table l, R represents the ratio of (X20 + YO) to (B203 + A1203), where X20
represents the sum of all alkali metal oxides which are present and YO
represents the
sum of all alkaline earth oxides and Zn0 and Pb0 which are present.
Table 1 shows that the gain flatness of the glass compositions according to
the
present invention, respecting the ratio R <_ 1.3, is good.
It may be preferred that those glass compositions according to the invention
which do not include fluorine have a value of ratio R <_ 1Ø For such
compositions. and
those according to the invention which include fluorine and respect the
condition R
1.3, the FOM (32 nm) in the wavelength region of interest is less than 25%.
Various glasses having compositions which conform to the present invention
have been found to be well suited for use in optical amplification in the
third
telecommunications window. Examples 1 to 4 relate to such compositions, where
the
desired value of the ratio R is primarily obtained by the inclusion of boron
oxide (i.e.
these are borosilicates). Example 1 is a typical sample of Pyrex (TM), Example
2 is a
sample of Vycor (TM), Example 3 is a typical glass composition used for LCDs
and
Example 4 is a typical photochromic glass composition.
These borosilicate glasses give acceptable gain flatness. Chlorine and
fluorine
can be added to these glass compositions (in up to 12, or preferably up to 9,
parts by
weight for 100 parts by weight of the base composition) in order to dry the
glass and to
increase the fluorescence lifetime. Also, the oxides Yb203, Y203 and Gd203 can
advantageously be used to co-dope the borosilicate glass or aid dispersion of
erbium
within the glass matrix.
Of these borosilicates according to the invention, Example 2 may be the
preferred glass composition, not only because of its particularly flat gain
characteristic
but also because this composition is stable and has excellent viscoelastic
characteristics.
The latter feature enables single mode fibres in this material to be drawn,
without
difficulty, using the well-known double crucible technique. The gain
characteristic, for
different degrees of population inversion, of the glass composition
constituting Example
2 of Table 1 is illustrated in Fig. l .
Examples 5 to 9 of Table 1 relate to glass compositions where the desired
value
of the ratio R is obtained primarily by inclusion of aluminium oxide (i.e.
these are
aluminosilicates). For such compositions it is possible to obtain a gain
flatness which is
superior to that of ZBLAN, when the boron oxide content of the base
composition is
less than 4 mol. %, the fluorine content is greater than or equal to 2 parts
by weight
(advantageously, more than 4 parts by weight) added to 100 parts by weight of
base
composition, and at least 0.1 mol.% of each of Y203 and Gd203 is included in
the base
composition as dispersants. It has been found that the gain flatness of the
glass
SUBSTITUTE SHEET (RULE 26)
CA 02380051 2002-O1-21
WO 01/07374 PCT/US00/16626
compositions according to the present invention is improved by including
therein at
least 0.2 mole percent of Y203 and/or Gd203.
Comparative Examples 1 to 3 and Examples 10 to 12 of Table 1 illustrate the
effect on gain flatness of varying the proportion of group III elements in the
glass
5 composition, in other words, the effect of varying the ratio R.
Although they contain appropriate quantities of the component oxides,
Comparative Examples 1 to 3 are outside the scope of the present invention
because the
value of the ratio R is too great to allow a flat gain characteristic to be
obtained.
Comparative Example 1 represents the extreme case where no group III oxides at
all are
10 deliberately included in the composition (the value R?20000 takes into
account possible
impurity levels of 0.1 mol.% of boron or aluminium oxide).
Examples 10 to 12 show how the gain flatness dramatically improves when R <_
1.3, and Examples 11 and 12 correspond to the preferred case where, for
compositions
not including fluorine, R <_ 1Ø
I S The relationship between gain flatness (as measured by the FOM value) and
the
ratio R for glass compositions according to the invention is illustrated
visually in Fig.2.
It has been found that the gain flatness of the glass compositions according
to the
invention is also influenced by the fluorine content thereof. This effect is
demonstrated
by Examples 13 to 17 of Table 1, where the gain characteristic improves as
increasing
quantities of fluorine are added to a constant base composition. This effect
is illustrated
visually in Fig.3, which shows how the FOM in the third telecommunications
window
improves as the fluorine content increases beyond 4 weight percent of the
analysed final
composition. In particular, it may be advantageous that the glass compositions
according to the present invention include over 4 parts by weight of fluorine
added for
every 100 parts by weight of the base composition.
The dependence of the properties of the glass compositions according to the
invention upon the fluorine content, and upon the ratio, R, is further
illustrated in Figs. 4
and 5. Fig.4 is a graph for the glass composition constituting Example 5 of
Table 1,
illustrating how the gain characteristic changes with fluorine content. Fig.S
is a graph
illustrating the gain characteristic of the glass composition constituting
Example S of
Table 1 when the ratio R and the fluorine content take the values given in
Table 1. The
gain spectrum of Fig.S was calculated from bulk measurements.
Examples 18 to 26 of Table 1 illustrate the fact that certain of the oxides in
the
glass composition of the present invention can be changed without
significantly altering
the gain characteristic of the resulting glass, provided that the desired
value of ratio R is
maintained.
SUBSTITUTE SHEET (RULE 26)
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WO 01/07374 PCT/US00/16626
As indicated above, oxides such as Ti02 and/or Zr02 can be included in the
glass compositions of the present invention, if desired, in order to adjust
the refractive
index thereof. Such oxides would be included in up to 1.0 mol.% of Ti02 and/or
in up
to 1.0 mol.% of Zr02.
S It has been found, also, that the fluorescence characteristics of the glass
compositions according to the present invention can be still further improved
by heat-
treating the compositions after their formation, notably by subjecting them to
high
temperatures for a sustained period of time. The duration and temperature of
the heat
treatment are adapted to the particular composition being treated. Moreover,
an
equivalent effect can be obtained from a relatively short heat treatment at
high
temperature and a relatively long heat treatment at a lower temperature.
Fig.6 illustrates the effect of one hour of heat treatment, at each of 4
different
temperatures, on the gain characteristic of a glass composition similar to
Example 5 of
Table 1. Fig.6 includes the gain characteristic of a composition without heat
treatment,
for purposes of comparison.
The glass composition of Fig.6 differs from Example 5 of Table 1 in that it
has
63.1 mol.% of Si02, 1 mol.% of each of Y203 and Gd203, 2 mol.% of the Na20 is
in
the form of Na20(N) and that, in addition to 100 parts by weight of the base
composition, it includes 5 parts by weight of fluorine, 0.3 parts by weight of
As203, and
1 part by weight of Er203.
It will be seen from Fig.6 that it may be advantageous to subject glass
compositions according to the present invention, after formation thereof, to
heat
treatment. Experiments have shown that such heat treatment does not affect the
transparency of the glass compositions.
Although the present invention has been described with reference to certain
specific embodiments thereof, the invention is not limited to the detailed
features of
these embodiments. On the contrary, numerous modifications and adaptations of
the
described embodiments can be made within the scope of the appended claims.
SUBSTITUTE SHEET (RULE 26)
