GB2097549A - Optical fibre - Google Patents

Abstract

A graded refractive index multi-mode optical fibre which is resistant to radiation has a core of a ternary SiO2/GeO2/P2O5 glass and a preferably cladding of a binary SiO2/B2O3 glass; the composition of the core being: Si approx. 38% by weight Ge 11% by weight (maximum> O approx. 51% by weight P up to 0.34% by weight, preferably 0.02 to 0.17%. The fibre is made by an internal, vapour deposition technique.

Description

SPECIFICATION Optical fibres This invention relates to the manufacture of optical fibre preforms, and to optical fibres produced from such preforms.

Our British Patent Specification No. 1475496 (P.

Lighty et al 12-3-2) describes a method of making optical fibres, in which the bore of a silica substrate tube is coated with a layer of a deposit which is the oxide of a single element, e.g. germania, the oxide being one which is diffusable into silica to form a glass with a higher refractive index than silica. This coating is effected by a reaction involving the deposition from a chemical vapour of, for instance, a halide of the single element, after which the coated tube is subjected to a collapsing operation to produce an optical fibre preform. This preform is then subjected to a drawing operation to produce the optical fibre. Alternatively the glass of the required composition e.g. silica-germania or multiple combinations of oxides can be co-deposited directly.

Optical fibres have been found to increase in attenuation when irradiated with ionizing radiation or neutrons, which is a nuisance. The absorptions which cause such increases in attenuation have been shown to be dose rate and time dependent. Reports in the literature indicate that pure silica fibres offer the prospect of best long-term radiation resistance, but often at the expense of high transients. In our Application No. 8028597 (P. W. Black et al 19-17-6-1) it has been shown that optical fibres with good radiation-resistant characteristics can also be achieved using a binary silica-germania core, in that case with a binary silica-boron trioxide cladding. It is known that doped core fibres containing phosphorous pentoxide can suppress transient loss, but at the expense of long term increments of attenuation.

Thus it has hitherto been considered that optical fibres with germania-doped cores containing phosphorus pentoxide cannot be produced with good long term performance in respect of resistance to the effects of radiation.

An object of the invention is to provide germaniadoped core optical fibre in which the core contains phosphorus pentoxide, and which have good long term characteristics and good transient characteristics.

According to the invention there is provided a graded refractive index multi-mode optical fibre which has a core of a ternary silicon dioxidegermanium dioxide-phosphorus pentoxide glass and a cladding of a binary silicon dioxide-boron trioxide glass, and has an improved resistance to the deleterious effects of irradiation by ionizing radiation or neutrons.

According to the invention there is also provided a method of producing a preform for a graded refractive index multi-mode optical fibre which has an improved resistance to the deleterious effects of irradiation by ionizing radiation or neutrons, which method include: (a) making a binary silicon dioxide-boron trioxide glass by vapour deposition of said oxides on the inner surface of a substrate tube, the deposition being effected from vapours of halides of silicon and boron.

(b) making a ternary silicon dioxide-germanium dioxide-phosphorus pentoxide glass by vapour deposition of said oxides on the inner surface of the binary silicon dioxide-boron trioxide glass which deposition is effected from vapours of halides of silicon and germanium, and (c) applying heat to the results of the depositions to cause collapse thereof to produce a solid preform.

Thus in the present method graded refractive index multi-mode optical fibre has been produced which has a core of a ternary silicon dioxidegermanium dioxide-phosphorus pentoxide (SiO21GeO2/P2O5) glass and a cladding of silicon dioxide-boron trioxide (SiO2/B203). This uses an internal vapour deposition technique, and the resulting optical fibre has an improved radiation performance as compared with hitherto-used fibres.

In one application of the present method the substrate tube on the inner surface of which the vapour deposition takes place (in the manner described in the above-mentioned Patent Specification) is 1.3 metres long, has a 14 mm outside diameter, and a wall thickness of 1.2 mm. The tube is a Heralux W. G.

silica tube, and is cleaned by acid etching, washing in high purity water, and drying.

The first depositions to be made on to the inner surface of the tube are for the cladding, which is a SiO2/B,O3 binary glass, and three layers of cladding glass were deposited, using the following parameters for the vapour deposition: (a) SiCI4 carrier gas flow at 1 33cciminute, using oxygen as the carrier gas.

(b) BBr3 in carrier gas flow at 50cc/minute, again using oxygen as the carrier gas. In both (a) and (b) the bubbler from which the SiCI4 or BBr3 vapour carries is at a temperature of 21 C.

(c) Additional gas flow at 1920cc/minute using oxygen.

(d) Deposition temperature of 16900C. This deposition is followed by the deposition of the SiO2/GeO2/P2O5 core, in the course of which forty layers are deposited, using the following parameters: (a) SiCI4 carrier gas flow at 138cc/minute, using oxygen as the carrier gas.

(b) GeCI4 carrier gas flow at 0-1 50cc/minute, with oxygen as the carrier gas. The rate of this gas flow is increased in forty steps to give graded index fibre with an value of about 2.1.

(c) POCI3 gas flow at a value in the range 0.5-1 0cc per minute, using oxygen as the carrier gas.

(d) Additional gas flow at 1800cc per minute.

(e) Deposition temperature, dependent on POCI3 carrier gas flow rate varying between 1700"C at 10cc per minute and 1760"C at 0.5cue per minute.

The total deposition time is about five hours. The collapse to produce the preform uses two passes of the heat source at 2100 C, the total time needed being approximately 1.5 hours.

The above operations produce a preform which is about 8.5mm in diameter, and this is drawn in a high temperature furnace to give about 2.5km of 125,am outside diameter fibre. This has an optical core diameter of 50,am, and optical cladding thickness of approximately 3,am, the numerical aperture being 0.2.

The attached table summarises experimental results from a series of fibres made using different POCI3 flow rates in the range mentioned. Also included is a fibre made by the method of our above-quoted Application.

Thus it will be seen that the new fibres achieve a superior radiation performance at short times (of the order of 1 Oms) without paying any penalty at times of the order of 30mins.

The core deposition temperature which we use is 20-80 C lower than that used forthe binary core fibre (see the above-quoted application), dependent on POCI3 flow rate.

The range of core compositions, as determined by scanning electron microscope scans of cleaved fibre ends is: Si. about 38% by weight.

Ge about 11% by weight measured at the core centre.

O about 51% by weight.

P in the range (0.02 to 0.34)% by weight for 0.5 to lOcc per minute POCI3 flow rate.

Our tests indicate, see the table, that fibres in the above range of compositions display superior radiation performance. In particular there is an optimum composition in the P, concentration range of 0.02 to 0.17% by weight, giving the widest range of low radiation sensitivity.

Radiation tests were performed using a flash X-ray source giving 600rads in less than one microsec, with the energy spectrum peaking at 0.5MeV with fwhh = 1.9 MeV. The feature unique to the novel fibre is that relatively low incremental loss is poss ible at short times (-10 ms) without having to pay a penalty of increased loss at longer times. Indeed, it is possible to manufacture a fibre having approxi mately the same loss as the binary fibre of our above-quoted application at 30 minutes but with only 60% ofthe loss at 10 msecs.

An advantage ofthe fibres made as described above is that the yield is enhanced due to the lower deposition temperature, which gives less problems with geometry control.

Claims (8)

1. A graded refractive index multi-mode optical fibre which has a core of a ternary silicon dioxidegermanium dioxide-phosphorus pentoxide glass with its composition in the range: Si approx. 38% by weight Ge approx. 11% by weight (maximum) O approx. 51% by weight Pin a range extending up to 0.34% by weight, which fibre has an improved resistance to the deleterious effects of irradiation by ionising radiation or neutrons at short times (s 10 ms) while still retaining good resistance thereto at longer times (a 30 minutes).

2. A graded refractive index multi-mode optical fibre which has a core of a ternary silicon dioxidegermanium dioxide-phosphorus pentoxide glass with its composition in the range: Si approx. 38% by weight Geapprox. by by weight (maximum) Approx. 51% by weight Pin a range extending up to 0.34% by weight, which optical fibre is produced by an internal deposition technique and has an improved resistance to the deleterious effects of irradiation by ionising radiation or neutrons at short times (s 10 ms) while still retaining good resistance thereto at longer times (2 30 minutes).

3. A graded index multi-mode optical fibre as claimed in claim 1 or 2, and wherein the fibre has a cladding over the core, which cladding is of a binary silicon dioxide-boron trioxide glass.

4. A method of producing a preform for a graded refractive index multi-mode optical fibre which has an improved resistance to both the short term and the long term deleterious effects of irradiation by ionising radiation of neutrons, wherein: (a) a binary silicon dioxide-boron trioxide glass is made by internal vapour deposition on the inner surface of a substrate tube, the deposition being effected from vapours of halides of boron and silicon, (b) wherein a ternary silicon dioxide-germanium dioxide-phosphorus pentoxide glass is made by internal vapour deposition of said oxides on the inner surface of the binary silicon dioxide-boron trioxide glass which deposition is effected from vap our of halides of silicon and germanium and from POC13, (c) the proportions of the vapours used in the deposition of said ternary glass are such that the ternary glass thus produced contains phosphorus in a range extending up to 0.34% by weight, the remainder ofthe glass consisting of:: Si approx. 38% by weight Ge 11% by weight (maximum) 0 approx 51% by weight, (d) applying heat to the results of the depositions to cause collapse thereofto produce a solid preform.

5. A method as claimed in claim 4, and wherein the phosphorus concentration is in the range of 0.02 to 0.17% by weight.

6. A method of making an optical fibre which includes performing a drawing operating on a collapsed preform by the method of claim 4 or 5.

7. A method of producing a graded refractive index multi-mode optical fibre which has an improved resistance to the deleterious effects of irradiation by ionising radiation or neutrons, substantially as described herein.

8. A graded refractive index multi-mode optical fibre made by the method of claim 4,5,6 or 7.