BORATE OR ALUMINOSILICATE GLASS COMPOSITION FOR OPTICAL AMPLIFICATION

Abstract: A glass composition suited for use in optical amplification includes a silicate which includes: at least 5o mole percent of SiO2, and at least one group III oxide, which may be selected from the group Al2O3 and B2O3, the ratio (R) of (X2O + YO) to said at least one group III ocide being less than or equal to 1.3, where X2O represents the sum of all alkali metal oxides present and YO represents the sum of all alkaline earth oxides and PbO and ZnO present. Fluorine may be included in the glass compositions. For compositions not including fluorine, it is preferred that the ratio R should be less than or equal to 1.0. The glass compositions are erbium-doped, and may be are co-doped with Yb2O3 and include dispersants such as Y2O3 and/or Gd2O3. This family of glasses possesses a flat gain characteristic in the 1545 nm region.

Description: BORATE OR ALUMINOSILICATE 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 μm. 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 (ZrF -BaF 2 -LaF 3 -AlF 3 -NaF), which has a gain ripple characterised by a "Figure of Merit", FOM (FOM= [(Gain max - Gain min )/Gain min ] 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 μm 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 SiO 2 , and at least one group III oxide, preferably selected from the group AI 2 O 3 and B 2 O 3 , characterised by the following ratio, X 2 O + YO R = A < 1,3, where X 2 O 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, ZnO and PbO. Glass compositions according to an embodiment of the present invention have been found to have a gain characteristic in the 1.5 μm 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 Yb 2 θ 3 in up to 12 molar percent of the base composition. It also may be advantageous to include in the base composition oxides such as Y 2 O 3 and Gd 2 θ 3 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: SiO 2 50.0-90.0 mol.% GeO 2 0.0-10.0 mol.% B 2 O 3 0.0-30.0 mol.% Al 2 O 3 0.0-30.0 mol.% Li 2 O 0.0-15.0 mol.% Na 2 O 0.0-25.0 mol.% K 2 O 0.0-15.0 mol.% MgO 0.0-5.0 mol.% SrO 0.0-10.0 mol.% CaO 0.0-10.0 mol.% BaO 0.0-15.0 mol.% ZnO 0.0-10.0 mol.% PbO 0.0-10.0 mol.% Y 2 O 3 0.0-3.0 mol.% Gd 2 O 3 0.0-3.0 mol.% Yb 2 O 3 0.0-12.0 mol.%, with (B 2 O 3 + Al 2 O 3 ) 5-35.0 mol.% X 2 O 0.0-20.0 mol.% XO 0.0-15.0 mol.%, and YO 0.0-20.0 mol.%, where X 2 O 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 ZnO and PbO 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 1.0. For such compositions, and those according to the invention which include fluorine and having R 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 Er 2 θ 3 , 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: SiO 2 55.0-85.0 mol.% GeO 2 0.0-8.0 mol.% B 2 O 3 0.0-25.0 mol.% Al 2 O 3 1.5-25.0 mol.% Li 2 O 0.0-12.0 mol.% Na 2 O 0.0-20.0 mol.% K 2 O 0.0-12.0 mol.% MgO 0.0-3.0 mol.% SrO 0.0-5.0 mol.% CaO 0.0-8.0 mol.% BaO 0.0-10.0 mol.% ZnO 0.0-5.0 mol.% PbO 0.0-5.0 mol.% Y 2 O 3 0.0-2.0 mol.% Gd 2 O 3 0.0-2.0 mol.% Yb 2 O 3 0.0-10.0 mol.%, with (B 2 O 3 + AI 2 O 3 ) 5-35.0 mol.% X 2 O 0.0-20.0 mol.% XO 0.0-15.0 mol.%, YO 0.0-20.0 mol.%, where X 2 O 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 ZnO and PbO present in the base composition. Oxides such as Tiθ 2 and/or Zrθ 2 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 (X 2 O + YO) to (AI 2 O 3 +B 2 O 3 ) in a typical aluminosilicate glass composition according to the present invention; 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 ; Fig.5 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 μm wavelength region, may be constituted by glass compositions including: at least 50 mole percent of Siθ 2 , and at least one group III oxide, preferably selected from the group AI 2 O 3 and B 2 O 3 , wherein the following ratio is respected, X 2 O + YO R = A < 1,3, Al 2 0 3 + B 2 0 3 where X 2 O 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, ZnO and PbO. Advantageously, the glass compositions according to the invention may include 0.005 to 6 parts by weight of Er 2 θ 3 , 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: SiO 2 50.0-90.0 mol.% GeO 2 0.0-10.0 mol.% B 2 O 3 0.0-30.0 mol.% Al 2 O 3 0.0-30.0 mol.% Li 2 O 0.0-15.0 mol.% Na 2 O 0.0-25.0 mol.% K 2 O 0.0-15.0 mol.% MgO 0.0-5.0 mol.% SrO 0.0-10.0 mol.% CaO 0.0-10.0 mol.% BaO 0.0-15.0 mol.% ZnO 0.0-10.0 mol.% PbO 0.0-10.0 mol.% Y 2 O 3 0.0-3.0 mol.% Gd 2 O 3 0.0-3.0 mol.% Yb 2 O 3 0.0-12.0 mol.%, with (B 2 O 3 + Al 2 O 3 ) 5-35.0 mol.% X 2 O 0.0-20.0 mol.% XO 0.0-15.0 mol.%, and YO 0.0-20.0 mol.%, where X 2 O 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 ZnO and PbO present in the base composition. It is preferred that those compositions according to the invention which include X 2 O+ YO fluorine should have: R — A < 1,3, and those compositions Al 2 0 3 + B 2 0 3 according to the invention which do not contain fluorine should have X 2 O+ YO R = A < 1,3. Al 2 0 3 + B 2 0 3 Yet more advantageously, the glass compositions according to the invention may include 0.005 to 6 parts by weight of Er 2 U 3 , 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: SiO 2 55.0-85.0 mol.% GeO 2 0.0-8.0 mol.% B 2 O 3 0.0-25.0 mol.% Al 2 O 3 1.5-25.0 mol.% Li 2 O 0.0-12.0 mol.% Na 2 O 0.0-20.0 mol.% K 2 O 0.0-12.0 mol.% MgO 0.0-3.0 mol.% SrO 0.0-5.0 mol.% CaO 0.0-8.0 mol.% BaO 0.0-10.0 mol.% ZnO 0.0-5.0 mol.% PbO 0.0-5.0 mol.% Y 2 O 3 0.0-2.0 mol.% Gd 2 O 3 0.0-2.0 mol.% Yb 2 O 3 0.0-10.0 mol.%, with (B 2 O 3 + Al 2 O 3 ) 5-35.0 mol.% X 2 O 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. In Table 1, R represents the ratio of (X 2 O + YO) to (B 2 O 3 + Al 2 O 3 ), where X 2 O represents the sum of all alkali metal oxides which are present and YO represents the sum of all alkaline earth oxides and ZnO and PbO 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 < 1.0. 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 Yb 2 O 3 , Y 2 O 3 and Gd 2 θ 3 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 Y 2 O 3 and Gd 2 U 3 is included in the base composition as dispersants. It has been found that the gain flatness of the glass compositions according to the present invention is improved by including therein at least 0.2 mole percent of Y 2 O 3 and/or Gd 2 O 3 . 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 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 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 < 1.0. 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.5 is a graph illustrating the gain characteristic of the glass composition constituting Example 5 of Table 1 when the ratio R and the fluorine content take the values given in Table 1. The gain spectrum of Fig.5 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. As indicated above, oxides such as ΗO 2 and/or Zrθ 2 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 ΗO 2 and/or in up to 1.0 mol.% of ZrO 2 . 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 SiO 2 , 1 mol.% of each of Y 2 O 3 and Gd 2 O 3 , 2 mol.% of the Na 2 O is in the form of Na 2 θ(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 AS 2 O 3 , and 1 part by weight of Er 2 θ 3 . 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.

Claims: CLAIMS What is claimed is: 1. A glass composition comprising: at least 50 mole percent of Siθ 2 , and at least one group III oxide selected from the group AI 2 O 3 and B 2 O 3 , characterised in that R < 1.3, where: X 2 O + YO R = A < 1,3, Al 2 0 3 + B 2 0 3 and X 2 O 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, ZnO and PbO. 2. A glass composition according to claim 1, characterised in that R is less than or equal to 1.3 if the composition includes fluorine, and less than or equal to 1.0 if the composition does not include fluorine. 3. A glass composition according to claim 2, further comprising over 4 parts by weight of fluorine added for every 100 parts by weight of the base composition. 4. A glass composition according to claim 1, 2 or 3, characterised in that the base composition includes at least 0.2 mole percent of Y 2 O 3 and/or Gd 2 O 3 . 5. A glass composition according to claim 1, 2, 3 or 4, further comprising 0.005 to 6 parts by weight of Er 2 θ 3 , up to 12 parts by weight of fluorine and up to 12 parts by weight of chlorine, added to 100 parts by weight of the base composition, including: SiO 2 50.0-90.0 mol.% GeO 2 0.0-10.0 mol.% B 2 O 3 0.0-30.0 mol.% Al 2 O 3 0.0-30.0 mol.% Li 2 O 0.0-15.0 mol.% Na 2 O 0.0-25.0 mol.% K 2 O 0.0-15.0 mol.% MgO 0.0-5.0 mol.% SrO 0.0-10.0 mol.% CaO 0.0-10.0 mol.% BaO 0.0-15.0 mol.% ZnO 0.0-10.0 mol.% PbO 0.0-10.0 mol.% Y 2 O 3 0.0-3.0 mol.% Gd 2 O 3 0.0-3.0 mol.% Yb 2 O 3 0.0-12.0 mol.%, with (B 2 O 3 + Al 2 O 3 ) = 5-35.0 mol.% X 2 0 0.0-20.0 mol.% XO 0.0-15.0 mol.%, and YO 0.0-20.0 mol.%, where X 2 O 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 ZnO and PbO present in the base composition. 6. A glass composition according to claim 5, characterised in that the base composition comprises less than or equal to 5.0 mole percent of (Y 2 O 3 + Gd 2 O 3 ). 7. A glass composition according to claim 1, 2, 3 or 4, further comprising 0.005 to 6.0 parts by weight of Er O 3 , up to 9.0 parts by weight of fluorine, and up to 9.0 parts by weight of chlorine added for 100 parts by weight of base material including: SiO 2 55.0-85.0 mol.% GeO 2 0.0-8.0 mol.% B 2 O 3 0.0-25.0 mol.% Al 2 O 3 1.5-25.0 mol.% Li 2 O 0.0-12.0 mol.% Na 2 O 0.0-20.0 mol.% K 2 O 0.0-12.0 mol.% MgO 0.0-3.0 mol.% SrO 0.0-5.0 mol.% CaO 0.0-8.0 mol.% BaO 0.0-10.0 mol.% ZnO 0.0-5.0 mol.% PbO 0.0-5.0 mol.% Y 2 O 3 0.0-2.0 mol.% Gd 2 O 3 0.0-2.0 mol.% Yb 2 O 3 0.0-10.0 mol.%, with (B 2 O 3 + AI 2 O 3 ) = 5-35.0 mol.% X 2 O 0.0-20.0 mol.% XO 0.0-15.0 mol.%, and YO 0.0-20.0 mol.%, where X 2 O 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 ZnO and PbO present in the base composition. 8. A glass composition according to any previous claim, further comprising by comprising up to 1 mole percent of Tiθ 2 and/or up to 1 mole percent of Zrθ 2 - 9. A glass composition according to any previous claim, characterised in that after the formation thereof, said composition is subjected to a heat treatment. 10. An optical amplifier apparatus comprising a glass composition according to any one of claims 1 to 9.

Inventors: BROCHETON YVES A, DICKINSON JAMES E JR, JACOB DAVID, PRASSAS MICHEL, BROCHETON, YVES, A., DICKINSON, JAMES, E., JR., JACOB, DAVID, PRASSAS, MICHEL
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2000
Main classification: C03C3
IPC codes: C03C3, C03C4, H01S3