GB2083921A

GB2083921A - Optical Fibres

Info

Publication number: GB2083921A
Application number: GB8028597A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-09-09
Filing date: 1980-09-09
Publication date: 1982-03-31
Also published as: GB2083921B

Abstract

One of the hazards to which optical fibres may be subjected is that of irradiation by ionizing radiation of neutrons, which causes undesirable increases in attenuation. Evidence in the literature indicates the only prospects of achieving radiation hardness, i.e. reduced susceptibility to radiation, was to use pure silica, which has certain demerits. However, we have found that a graded refractive index multi-mode optical fibre with a core of a binary SiO2/GeO2 glass and a cladding of a binary SiO2/B2O3 glass, made by an internal vapour deposition technique has improved radiation resistance characteristics.

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 can be codeposited 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. Using doped core fibres containing phosphorus 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 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 germania-doped core optical fibre with good long term characteristics.

According to the invention there is provided a graded refractive index multi-mode optical fibre which has a core of a binary silicon dioxidegermanium dioxide 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 binary silicon dioxide-germanium dioxide glass by vapour deposition of said oxides on the inner surface of the binary silicon dioxideboron 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 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 binary silicon dioxidegermanium (SiO2/GeO2) dioxide glass and a cladding of silicon dioxide-boron trioxide (SiO2/B2O3). 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 teh 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 14mm.

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/B2O3 binary glass, and three layers of cladding glass were deposited, using the following parameter for the vapour deposition: (a) SiCI4 carrier gas flow at 133 cc/minute, using oxygen as the carrier gas.

(b) BBr3 carrier gas flow at 50 cc/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 OC.

(c) Additional gas flow at 1 920 cc/minute using oxygen.

(d) Deposition temperature of 1 790cC.

This deposition is followed by the deposition of the SiO2/GeO2 core, in the course of which forty layers are deposited, using the following parameters: (a) SiCI4 carrier gas flow at 1 38 cc/minute, using oxygen as the carrier gas.

(b) GeCI4 carrier gas flow at 0--1 50 cc/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 a value of about 2.1.

(c) Additional gas flow at 1 800 cc/minute using oxygen.

(d) Deposition temperature of 1 7800C.

The total deposition time is about five hours.

The collapse to produce the preform uses two passes of the heat source at 21 00cC, the total time needed being approximately 1.5 hours.

The above operations produce a preform which is about 8.5 mm in diameter, and this is drawn by processes similar to conventional wire-drawing processes to give about 2.5 km of 125 ,um outside diameter fibre. This has an optical core diameter of 50 Hm, and optical cladding thickness of approximately 3 ym, the numerical aperture being 0.2.

The attached table summarises experimental results indicative of various types of fibres including a fibre produced by a process according to the present invention.

Thus it will be seen that the new fibre, the first example in the above table, achieves superior radiation performance as compared with other fibres, albeit at some cost in fabrication problems such as relatively high deposition temperatures.

Radiation tests were performed using a flash X-ray source giving 600 rads in less than one microsec, with the energy spectrum peaking at 0.5 MeV with fwhh=1.9 MeV. The features unique to the novel fibre are that incremental loss is relatively low at times above about 30 secs., with a relatively low recovery rate at room temperature but higher recovery rates at reduced temperatures. Another interesting feature is that such results for 23 OC and -300C were virtually the same after about five minutes.

Neutron damage tests were carried out using a pulsed reactor and showed the effect of neutrons to be of the order of one-tenth that of the equivalent dose of ionizing radiation.

Claims

1. A graded refractive index multi-mode optical fibre which has a core of a binary silicon dioxidegermanium dioxide 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.

2. A graded refractive index multi-mode optical fibre which has a core of a binary silicon dioxidegermanium dioxide glass and a cladding of a binary silicon dioxide-boron trioxide glass, which optical fibre is produced by an internal deposition technique, and has an improved resistance to the deleterious effects of irradiation by ionizing radiation or neutrons.

3. 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.

4. A method of making an optical fibre which includes performing a drawing operation on a collapsed preform produced by the method of claim 3.

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

6. A graded refractive index multi-mode optical fibre, made by the method of any one of claims 2, 3, 4 or 5.

New Claims or Amendments to Claims filed on 15th December 1981 Superseded Claims None New or Amended Claims:

7. A method of producing a preform for a graded refractive index multi-mode optical fibre which has an improved resistance to the longterm deleterious effects of irradiation by ionising radiation or neutrons as compared with hitherto known optical fibres, which method includes:: (a) making a binary silicon dioxide-boron trioxide glass by vapor deposition of the oxides on the inner surface of a substrate tube, which deposition is effected from vapours of halides of silicon and boron at a relatively high deposition temperature such as 1 7000C, the glass thus produced ultimately forming the cladding of the fibre; (b) making a binary silicon dioxide-germanium dioxide glass by vapour deposition of the oxides on the inner surface of the binary silicon dioxideboron trioxide glass made in step (a), which deposition is also effected at a relatively high temperature such as 1 7800C, the glass thus produced ultimately forming the core of the fibre; and (c) applying heat to the result of the deposition to cause collapse to produce a solid preform, which heating involves subjecting the results of the depositions to a temperature higher than the temperatures used in staeps (a) and (b), for instance 21O00C.