GB2129152A - Optical fibres - Google Patents

Abstract

Graded index optical fibres in which the principal constituent of the optical core is binary SiO2:GeO2 and the balance is constituted by 0.01 to 0.3 wt% SiO1.5F and up to 0.4 wt% P2O5 show improved incremental attenuation resistance to irradiation with ionizing radiation or neutrons.

Description

SPECIFICATION Optical fibres This invention relates to the manufacture of optical fibres, and in particular to the manufacture of graded index fibres that are able to withstand irradiation with ionizing radiation or neutrons without incurring excessively increased attenuation.

According to the present invention there is provided a graded index multimode optical fibre, wherein the principal constituent of the material of the optical core is binary SiO2:GeO2 of a graded composition providing all or substantially all of the refractive index grading of the core, and wherein the balance of the material of the optical core is constituted by 0.01 to 0.3 wt % SiO, 5F, and P205 in an amount up to 0.4 wt %.

There follows a description of the background to the invention and of embodiments thereof. The description refers to the accompanying drawings in which Figure 1 is a graph showing, at 5 minutes after irradiation, the incremental loss as a function of wavelength for a set of optical fibres with differing amount of P205 in their cores, Figures 2, 3 and 4 are graphs showing respectively at -31 CC, +230C, and +51 CC the recovery characteristics of some of the fibres of Figure 1, and Figure 5 is a graph showing the recovery after irradiation of a fibre embodying the invention.

Optical fibres have been found to exhibit increased attenuation after irradiation with ionizing radiation or neutrons, and for some applications this is a significant problem. These attenuation increments have been shown to be dose rate and time dependent. Early reports in the literature indicated that pure silica fibres without any internal waveguiding structure offerred the prospect of the best long term radiation resistance, but often at the expense of high temperature.

In our Patent Specification No. 2083921 A it has been shown that fibres with good radiation resistant characteristics can be obtained using silical fibres that have an internal waveguiding structure provided by an optical core region in which the silica is doped with germania and an optical cladding region in which the silica is doped with boric oxide. A preferred method for making such fibres is substantially as one of the methods described in our Patent Specification No.

1475496 in which silica together with the appropriate dopant is deposited upon the bore of a silica substrate tube as the reaction product of a chemical vapour reaction from which hydrogen and its compounds are excluded, following which the bore of the internally coated tube is collapsed to form an optical fibre prefrom from which optical fibre is produced by a subsequent drawing operation.

It has also been known that the addition of phosphorous pentoxide to the optical core of a silica fibre can at least partially suppress the short term irradiation induced attenuation increment, but that this is at the expense of an increased long term increment. However the letter entitled 'Effects of Radiation on Doped Silica Core Optical Fibres' by A. Rosiewicz et al appearing in Electronics Letters, 6th November 1 980 Vol. 1 6 No. 23 pp 866-7 disclosed that, within certain limits, the use of phosphorous pentoxide in the optical core of silica fibres designed to operate in the wavelength region of 850 nm can be beneficial in respect of resistance to the effects of irradiation, both in the short term and in the long term.This is also described in our Patent Specification No.

2097549A to which attention is directed.

Figure 2 of that letters shows that these irradiation resistance effects are wavelength sensitive and in particular exhibit reduced increments as the wavelength of operation is increased out to a wavelength slightly in excess of 900 nm. It has however since been found that this reduction is not maintained, but shows an increase by the time the wavelength of operation is extended into the region of 11 50 to 1350 nm and through towards the 1 600 nm. This is clearly evident in the graph of Figure 1 which shows the measured results for seven fibre samples 5 minutes after irradiation. Fibre 1 contained no phosphorous pentoxide in its core, while fibres 2 to 7 had increasing amounts in the measured range of approximately 0.02 wt % to 0.34 wt%.

One of the reasons for wanting to include phosphorous pentoxide in the optical cores of such fibres is that it facilitates their manufacture by reducing the temperature necessary for deposition of the core material when, for instance, the fibres are made by the previously referred to method involving deposition by chemical vapour reaction upon the bore of a tube. It is known the boric oxide and fluorine each produce similar effects upon deposition temperature, but the inclusion of boric oxide in the optical core greatly increases the irradiation induced attenuation, while the inclusion of fluorine has been found, while operating in the wavelength region of 850 nm, to reduce the short-term increment at the expense of an enhanced long-term increment.

However at longer wavelengths, particularly in the operating wavelength range of 1150 to 1350 nm, the long term incremental penalties caused by the use of fluorine doping are much smaller, and thus in this range we have found that fluorine can advantageously be used to replace part of the phosphorous pentoxide doping, particularly in view of the previously mentioned impaired performance of phosphorous pentoxide in this wavelength range.

It is also postulated that there is a correlation between large short-term irradiation induced attentuation increments at room temperature and a large temperature dependence of irradiation induced increments. Thus, particularly in the case of short term response, a large short term increment at room temperature is generally associated with a significantly larger short term increment if the operating temperature is raised for instance to 51 0C or lowered to -31 OC, whereas this temperature sensitivity is reduced when the room temperature short-term increment is small.An example of this effect is seen in the graphs of Figures 2, 3 and 4 which depict the recovery characteristics at 850 nm of some of the fibres of Figure 1 at respectively -31 0C +23 0C and +51 0C. It is therefore postulated that an additional effect of introducing fluorine into the core will be to reduce the temperature sensitivity of the irradiation induced attenuation increment.

A particular application for these radiation resistance optical fibres is in multimode telecommunication systems requiring the use of graded index fibres with numerical apertures lying in the range 0.18 to 0.24, and preferably within the range 0.2 to 0.22, these ranges being determined primarily by the need on the one hand to have a large enough numerical aperture to avoid the problems of excessive cabling and microbending losses, while on the other hand having a low enough numerical aperture to avoid the problems of excessive attenuation resulting from the need to use high doping levels.

In one specific embodiment of the invention optical fibre was drawn from a preform mode by the previously identified method involving deposition upon the bore of a substrate tube. For this purpose the substrate tube consisted of a Heralux W.G. silica tube about 1.3 metres long with a 14 mm outside diameter and a wall thickness of 1.2 mm. Before the first deposition, the bore of this tube was cleaned by acid etching, washing in high purity water, and drying. The first layers to be deposited on its bore were layers of constant composition material destined to form the optical cladding of the fibre. The material chosen for this optical cladding was silica doped with germania, phosphorus pentoxide and fluorine.Subsequent spectural analysis revealed the following composition in weight %: SiO2 99.5 GeO2 0.16 P205 0.39 SiO1~5F 0.24 100.29 total The reagents for depositing this cladding were:- silicon tetrachloride vapour entrained at 21 0C in oxygen carrier gas glowing at a rate of 250 cc/min.

germanium tetrachloride vapour entrained at 21 0C in oxygen carrier gas flowing at a rate of 33.3 cc/min.

phosphorus oxychloride vapour entrained at 21 OC in oxygen carrier gas flowing at a rate of 20.0 cc/min.

dichlorodifluoromethane flowing at a rate of 10.5 cc/min.

and additional oxygen flowing at a rate of 2000 cc/min.

Deposition took place in the region of a iocalised hot zone produced by an external oxyhydrogen flame at a temperature of 1 5900C which was slowly traversed along the iength of the tube. Five traversals were employed to provide five layers of constant composition for the optical cladding material, and then the composition was progressively modified by increasing the flow rate of the entrained germanium tetrachloride vapour, so as to grade the composition of the deposit over the next forty layers in such a way as to produce a grading of refractive index in the finished fibre giving it a numerical aperture of 0.22 and an t value of 2.1.

The reagents for depositing this core were therefore: silicon tetrachloride vapour entrained at 21 C in oxygen carrier gas flowing at a rate of 1 38 cc/min.

germanium tetrachloride vapour entrained at 21 OC by an oxygen carrier gas flowing at a rate from zero increasing to 200 cc/min over the forty layers, dichorodifluoromethane flowing at 10.5 cc/min.

additional oxygen gas at 1700 cc/min.

Deposition temperature changed from 1 6600C to 1 5800C over the forty core layers.

Temperatures were measured using an infra red pyrometer which detects energy radiated from a surface in the wavelength region of 4 microns.

Analysis of fibre compositions was by scanning electron microscopy and microprobe. In view of the problems associated with analysing the small areas of material available within a fibre sample, the microprobe measurements were performed on larger area samples which were fabricated under identical flow rate, temperature and deposition conditions.

The flame temperature was then increased to 20500C so that a further two traverses was effective to bring about a full collapse of the bore to produce an optical fibre preform having an entirely solid cross-section. Subsequently the preform was mounted in a vertical drawing tower and fibre was drawn from its lower end as this end was lowered at a controlled rate through a furnace, to give fibre with a 50 microns diameter optical core, a 5 micron thickness optical cladding, within a total diameter of 125 microns.

Figure 5 shows the decay of irradiation induced incremental attenuation as a function of time at 1.3 microns for a 240 metre length of this fibre after irradiation with a total gamma-ray dose of 1106 rads at 1010 rads/sec, at 200C. At this dose rate it has been demonstrated that the response in linear with doses over the range extending at best as far as 1200 rads.

Claims (6)

1. A graded index multimode optical /fibre wherein the principal constituent of the material of the optical core is binary SiO2:GeO2 of a graded composition providing all or substantially all of the refractive index grading of the core, and wherein the balance of the material of the optical core is constituted by 0.01 to 0.3 wt % SiO1F, and P205 in an amount up to 0.4 wt %.

2. An optical fibre as claimed in claim 1 wherein the index grading provides a numerical aperture lying in the range 0.18 to 0.24.

3. An optical fibre as claimed in claim 1 wherein the index grading provides a numerical aperture lying in the range 0.20 to 0.22.

4. An optical fibre as claimed in any preceding claim, wherein the optical core is surrounded by an optical cladding whose refractive index is not greater than that of silica.

5. An optical fibre as claimed in any preceding claim, wherein its optical cladding consists of a material being constituted of SiO2 doped with materials with the following ranges: GeO2 00.2 wt % P205 0--0.4 wt S/o Si015F 0--0.3 wt %

6. An optical fibre substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.