Aluminosilicate optical glass

Details Aluminosilicate optical glass Aluminosilicate optical glass

1985-11-07

1988-03-15

Capella, Steven

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

501 37, 501 55, 501 68, C03C 3062, C03C 3083

Patent

active

047313480

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to aluminosilicate optical glass containing at least one oxide of a monovalent modifying metal chosen from among Tl and Ag. The glass, which may also contain cesium, is characterised by its composition which, comprises 60 to 95 mole % silica, at least 2 mole % of alumina and at least 2 mole % of one or more oxides of the aforementioned metals. In a variant, the molar proportions of Al.sub.2 O.sub.3 and oxides of modifying metals are above 3 mole % and the proportion of SiO.sub.2 is below 94 mole %.
Glass of this kind is particularly useful in the sector of waveguides for optical transmission, since under certain conditions it is adapted to exchange the monovalent metal cation for a foreign cation, which operation results in a change in the refractive index of the thus-treated glass. The term waveguide is used to mean e.g., both optical fibres and microlenses capping optical fibres and used for injecting light signals into them.
As is known, numerous studies have recently been made to develop optical glass having a variable refractive index for manufacturing microlenses and optical fibres having low loss in transmission. This is because when the index of an optical fibre decreases radially from the centre towards the exterior in accordance with a given function, the transmitted signals undergo very little attenuation or phase-shifting. To obtain these conditions, the variation in the index preferably conforms to a function such as n=n.sub.o (1-ar.sup.2) where n.sub.o is the index at the centre of a fibre of radius r and a is a constant associated with the kind of glass used.
Waveguides having the aforementioned properties are usually made starting with glass having a uniform index and given a suitable shape (e.g., a rod which can subsequently be moulded into a micro-lens or drawn in the form of a fibre). The composition of the glass comprises one or more metals capable of diffusing towards the exterior with said one or more metals being gradually replaced by another metal, the presence of which results in a refractive index different from that of the original glass. Research has shown that the effect of a metal on the refractive index of a given glass is related to the electronic polarizability (.ANG.) divided by the ionic radius (A.sup.3) of the metal. The quotient is relatively low for certain metals which have only a slight influence on the refractive index, e.g., the alkali and alkaline-earth metals (0.06 for Li; 0.56 for K; and 0.2 for Mg), but is appreciably higher for metals such as cesium (0.744), thallium (1.572) and silver. The effect of the oxides of these metals on the refractive index is of considerably greater significance. If a rod of glass containing suitable proportions of the latter metals, which have a relatively high molecular weight, is placed under suitable conditions, e.g., in a bath of molten salt at a temperature sufficient to promote migration of the metals in cationic form (a temperature in the order of 400.degree.-800.degree. C.), the metals are exchanged for the metal or metals contained in the molten salt bath. If the salt bath contains cations for which the ratio .ANG./.ANG..sup.3 is below the corresponding ratio of the metal in the glass subjected to exchange, the introduced cations will lower the index of the glass, with the amount of reduction depending on the degree of exchange in the zone under consideration. Since the diffusion rate progressively decreases with distance from the periphery towards the centre of the rod, the progressive variation in index will follow a similar function, increasing towards the centre and approaching the aforementioned ideal function to an extent depending on the nature of the glass, the diffusion medium and the operating conditions.
Detailed information on these questions will be found in the following documents: 1092-95; Thin Solid Films (1976), 36(2), 493-6; Applied Physics (1975), 6(2), 223-8; Journal of the American Ceramic Society 54 (7), 321-26 (1971); Japan Society of Applied Physics 39 (1970), 63-70; FR

REFERENCES:
patent: 3843228 (1974-10-01), Yoshiyagawa et al.
patent: 4495298 (1985-01-01), Yamagishi et al.

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