Improvements in or relating to graded index optical fibre

Claim 2

A graded index optical fibre with a total insertion loss of less than 20 dB/km having a core comprising a glass containing alkali metal oxide, alkaline earth oxide, silica, and boric . oxide; a cladding comprising a glass containing alkali metal oxide, boric oxide, and silica; said core, having a refractive index at least one per cent greater than said cladding, said core and cladding glasses both containing up to 5 mole per cent of other compatible oxides, and no other components, both said glass compositions selected to exclude compositions which undergo phase separation or devitrification: said graded index optical fibre characterised in that said graded refractive index distribution is produced by a heat treatment which induces a thermal diffusion process between said core and said cladding wherein there is a net migration of alkaline earth metal ions from said core to said cladding, and no migration of alkaline earth metal ions or alkali metal ions from said cladding to said core.

Claim 3

A graded index glass optical fibre having a total insertion loss of less than 20 dB/km and having a core and cladding, said core being formed from a first glass consisting of:- a) silica b) boric oxide c) one or more alkali earth_metal oxides selected from the group consisting of sodium oxide and potassium oxide, d) one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide and barium oxide, and optionally, e) up to 5 mole per cent of one or more other compatible oxides, and said cladding being formed from a second glass having a refractive index at least one per cent lower than that of said first glass and consisting of:- f) silica g) boric oxide h) one or more alkali earth metal oxides selected from the group consisting of sodium oxide and potassium oxide, and optionally, i) one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide and magnesium oxide, and also optionally, j) up to 5 mole per cent of one or more other compatible oxides, said first and second glass compositions being selected to exclude compositions which undergo phase separation or devitrification during optical fibre production, and said gradation of refractive index being produced by thermal diffusion between said first and second glasses and being at least partly caused by a conposition gradient of alkaline earth metal oxide: said graded index glass optical fibre characterised in that the concentration of alkali metal oxide in said cladding is substantially equal to or greater than the concentration of alkali metal oxide in said core, and the concentration of alkaline earth metal oxide in said core is equal to or substantially greater than the concentration . of alkaline earth metal oxide in said cladding.

Claim 4

A graded index glass optical fibre having a total insertion loss of less than 20 dB/km and having a core and cladding, said core being formed from a first glass consisting of:- a) silica b) boric oxide c) one or more alkali earth_metal oxides selected from the group consisting of sodium oxide and potassium oxide, d) one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide and barium oxide, and optionally, e) up to 5 mole per cent of one or more other compatible oxides, and said cladding being formed from a second glass having a refractive index at least one per cent lower than that of said first glass and consisting of:- f) silica g) boric oxide h) one or more alkali earth metal oxides selected from the group consisting of sodium oxide and potassium oxide, and optionally, i) one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide and magnesium oxide, and also optionally, j) up to 5 mole per cent of one or more other compatible oxides, said first and second glass compositions being selected to exclude compositions which undergo phase separation or devitrification during optical fibre production, and said gradation of refractive index being produced by thermal diffusion between said first and second glasses and being at least partly caused by a conposition gradient of alkaline earth metal oxide: said graded index glass optical fibre characterised in that the concentration of alkali metal oxide in said cladding is substantially equal to or greater than the concentration of alkali metal oxide in said core, and the concentration of alkaline earth metal oxide in said core is equal to or substantially greater than the concentration . of alkaline earth metal oxide in said cladding.

Claim 5

.A graded index glass optical fibre as claimed in either claim 1 or claim 2 further characterised in that said core is formed from a glass consisting of:- a) 55-65 mole per cent silica b) 10-20 mole per cent boric oxide c) one or more alkaline metal oxides selected from the group consisting of sodium oxide and potassium oxide d) up to 20 mole per cent of one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide and optionally, e) up to 5 mole per cent of one or more other compatible oxides. and said cladding is formed from a second glass consisting of:- f) 55-65 mole per cent silica g) 10-20 mole per cent boric oxide h) one or more alkaline metal oxides selected from the group consisting of sodium oxide and potassium oxide, and, optionally, i) one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide, and magnesium oxide, and also optionally, j) up to 5 mole per cent of one or more other compatible oxides.

Claim 6

A graded index glass optical fibre having a total insertion loss of less than 20 dB/km and having a core and cladding characterised in that said core is formed from a first glass consisting of:- a) 55-65 mole per cent silica b) -10-20 mole per cent boric oxide c) one or more alkaline metal oxides selected from the group consisting of sodium oxide and potassium oxide, d) up to 20 mole per cent of one of more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide and, optionally, e) up to 5 mole per cent of one or more other compatible oxides, and said cladding is formed from a second glass having a refractive index at least one per cent lower than that of said first glass and consisting of:- f) 55-65 mole per cent of silica g) 10-20 mole per cent boric oxide h) one or more alkaline metal oxides selected from the group consisting of sodium oxide and potassium oxide, and optionally, i) one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide, and magnesium oxide, and also optionally, j) up to about 5 mole per cent of one or more other compatible oxides, said first and second glass compositions being selected to exclude compositions which undergo phase separation during optical fibre production, said graded index glass optical fibre further characterised in that the proportions of said components (c), (d), (h), and if present (i) are such as to produce during manufacture a gradation of refractive index by thermal diffusion between said core glass and said cladding glass at least partly caused by the composition gradient of alkaline earth metal oxides, and to exclude ion exchange between alkaline earth metal ions and alkali metal ions during said thermal diffusion.

Claim 7

A graded index glass optical fibre having a total insertion loss of less than 20 dB/km and having a core and cladding characterised in that said core is formed from a first glass consisting of:- a) 55-65 mole per cent silica b) -10-20 mole per cent boric oxide c) one or more alkaline metal oxides selected from the group consisting of sodium oxide and potassium oxide, d) up to 20 mole per cent of one of more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide and, optionally, e) up to 5 mole per cent of one or more other compatible oxides, and said cladding is formed from a second glass having a refractive index at least one per cent lower than that of said first glass and consisting of:- f) 55-65 mole per cent of silica g) 10-20 mole per cent boric oxide h) one or more alkaline metal oxides selected from the group consisting of sodium oxide and potassium oxide, and optionally, i) one or more alkaline earth metal oxides selected from the group consisting of calcium oxide, strontium oxide, barium oxide, and magnesium oxide, and also optionally, j) up to about 5 mole per cent of one or more other compatible oxides, said first and second glass compositions being selected to exclude compositions which undergo phase separation during optical fibre production, said graded index glass optical fibre further characterised in that the proportions of said components (c), (d), (h), and if present (i) are such as to produce during manufacture a gradation of refractive index by thermal diffusion between said core glass and said cladding glass at least partly caused by the composition gradient of alkaline earth metal oxides, and to exclude ion exchange between alkaline earth metal ions and alkali metal ions during said thermal diffusion.

Claim 8

An optical fibre as claimed in claim 3 or 4 further characterised in that the mole percentage of alkali metal oxide in said cladding glass is substantially equal to or greater than the mole percentage of alkaline metal oxide in said core glass.

Claim 9

An optical fibre as claimed in claim 5 further characterised in that said core and cladding glasses each contain a mutually identical single alkali metal oxide.

Claim 10

An optical fibre as claimed in claim 6 further characterised in that said mutually identical alkali metal oxide is sodium oxide.

Claim 11

An optical fibre as claimed in any of claims 3 - 7 further characterised in that the mole percentage of alkaline earth metal oxide in said cladding glass is substantially equal to or less than the mole percentage of alkaline earth metal oxide in said core glass.

Claim 12

An optical fibre as claimed in claim 8 further characterised in that said core and cladding glasses each contain a mutually identical single alkaline earth metal oxide.

Claim 13

An optical fibre as claimed in claim 8 further characterised in that said core glass contains a single alkaline earth metal oxide and said cladding glass contains substantially no alkaline earth metal oxide.

Claim 14

An optical fibre as claimed in any previous claim characterised in that said core glass contains:- 55-65 mole per cent silica, 10-20 mole per cent boric oxide, 14-25 mole per cent sodium oxide, 2-11 mole per cent of an alkaline earth metal oxide selected from the group consisting of calcium oxide, strontium oxide, or barium oxide, and up to 5 mole per cent of one or more other compatible oxides.

Claim 15

An optical fibre as claimed in any previous claim further characterised in that said core glass contains 57-61 mole per cent silica, 14-16.5 mole per cent boric oxide, 19-22 mole per cent sodium oxide, 3-8 mole per cent barium oxide and up to 5 mole per cent of one or more other compatible oxides.

Claim 16

An optical fibre as claimed in any previous claim further characterised in that said cladding glass contains 55-65 mole per cent silica, 10-20 mole per cent boric oxide, 19-25 mole per cent sodium oxide, and up to 5 mole per cent of one or more other compatible oxides, excluding alkaline earth metal oxides.

Claim 17

An optical fibre as claimed in any previous claim further characterised in that said cladding glass contains 59-63 mole per cent silica, 17-19 mole per cent boric oxide, 19-22 mole per cent of sodium oxide, and up to 5 mole per cent of one or more other compatible oxides, excluding alkaline earth metal oxides.

Claim 18

An optical fibre as claimed in any previous claim characterised in that the cladding glass contains up to 3 mole per cent of alumina and said core glass contains no alumina.