GB2217871A

GB2217871A - Optical fibre seals in walls of a chamber

Info

Publication number: GB2217871A
Application number: GB8810136A
Authority: GB
Inventors: Thomas Oswald; Duncan Alexander Gunn; Roy Allen
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-04-28
Filing date: 1988-04-28
Publication date: 1989-11-01
Also published as: GB8810136D0

Abstract

A sealing gland for an optical fibre 3 comprising a pair of optical fibre seals 1, 2 spaced along a length of optical fibre, the seals being located in opposing walls 6, 7 of a chamber 8 forming a reservoir through which the fibre intermediate the seals passes. The chamber 8 may then be either evacuated or pressurised through sealable aperture 10 to provide zero operating pressure gradient across one of the seals. The chamber may be fitted in a bulkhead 9, e.g. of a submerged repeator. The seals 1 and 2 may be soldered in place. <IMAGE>

Description

SEALING GLAND FOR OPTICAL FIBRE.

This invention relates to a sealing gland for an optical fibre, such as may be used in a modern submerged repeater.

A problem in optical fibre systems in undersea cables is the provision of adequate long term sealing of the fibres where they enter a submerged repeater housing,usually through a bulkhead. In particular a seal is required that has no or negligible leakage of contaminants such as moisture or gases such as hydrogen, the presence of which can degrade the performance of the active semiconductor devices in the repeater.

Hitherto various forms of optical fibre seal have been proposed. In British application No. 8803507 there is disclosed a seal structure consisting of a metal sleeve provided with a longitudinal slot and two blind axile holes, one at each end of the sleeve, for receiving a length of bare fibre intermediate two adjoining regions of the fibre with its plastics protective sheath still remaining, the adjoining ends being positioned within the respective blind boles. The bared portion of the fibre is soldered to the sleeve in the region of the slot between the blind holes, the remaining voids in the sleeve being filled with an epoxy resin material. Such a sleeve/fibre structure can then be readily incorporated in a gland for fitting in an aperture in a bulkhead.

Another form of seal, developed in 1977 by Bell Laboratories, Murray Hill, N.J. comprises a copper tube through which a bared portion of a fibre is threaded.

The remaining void on the tube is then filled with a molten tin/lead solder material. The solder wets the copper tube and, when cooled and solidified, squeezes against the fibre forming a strong hermatic seal.

However, although such seals have a long effective life there is always the probability that they will suffer a stress failure in use, which may be manifested as an increase in leakage rate over a period of time or a sudden failure.

The present invention seeks to provide a sealing gland arrangement for an optical fibre that provides a significant improvement in leakage performance and operating reliability.

According to the present invention there is provided a sealing gland for an optical fibre comprising a pair of optical fibre seals spaced along a length of optical fibre, the seals being located in opposing walls of a chamber forming a reservoir through which the fibre intermediate the seals passes.

Embodiments of the invention will now be described with reference to the accompanying drawing which is a section through a sealing gland.

The sealing gland comprises in essence a pair of seals 1, 2 in series spaced along a length of optical fibre 3. The seals are located in bores 4, 5 in opposing walls 6, 7 of the chamber 8 which forms a reservoir between the seals 1, 2. The chamber 8 is adapted to be fitted and secured in an aperture in a bulkhead 9, e.g.of a submerged repeater. The chamber 8 is provided with a sealable aperture 10 by which the chamber may be evacuated or, alternatively, gas pressurised with, say, an inert gas such as helium.

The performance of some types of seals, such as solder seals as referred to above, is very good. Seals have been tested at pressures of 5MPa and have been -11 shown to have leakage rates as low as 1 X 10 11 cc/sec using helium as a test gas. The rationale of the present invention is that initially both seals are wgoodw and should each have a lifetime in excess of 25 years under normal operating pressure differentials between the two sides of the bulkhead. If that could be proven to be the case for all seals then the "downstream' seal quite clearly would be redundant.However, should failure of the upstream seal occur prematurely (failure being defined as sudden gross leakage rate) the "downstream" seal should then be able to provide the necessary continuing sealing of the fibre. This is because until failure of the upstream seal occurs the downstream seal has been present in a zero pressure differential environment and has therefore not been subjected to the working stresses which may have contributed to or caused the failure of the upstream seal.Should the failure mode of the upstream seal be of gradually increasing leakage over a long period of time then the reservoir pressure will change slowly over a period of time that will ultimately be less than the required lifetime of the whole gland structure,with considerably reduced pressure gradient over the downstream seal during much of the failure period. Thus the whole assembly will operate as two seals in series with a much reduced sealing burden on the downstream seal.

If the failure mode of a seal is such that both time and pressure differential are contributory factors (as test experience to date is suggesting) then the fact that the downstream seal has been relieved of pressure for some considerable time means that its failure hazard will be dated from the time of failure of the upstream seal. Thus the lifetimes of the two seals can be considered to be cumulative.

Fabrication of the sealing gland structure described above is as follows. First two short lengths of the plastics sheathing 3a are removed from the fibre 3 to accomodate the seals 1 and 2. The seals 1 and 2 are then soldered in place on the bared length of fibre.

Seal 1 is made with a smaller outside diameter than seal 2. The bore 4 in wall 6 is a clearance fit for seal 1.

The bore 5 in wall 7 is primarily a blind hole which is a clearance fit for seal 2. At the inner end of bore 5 a clearance hole 5a is formed through which the length of fibre bearing seal 1 can be drawn. The fibre bearing seal 1 is threaded through hole 5a and hole 5 and then into bore 4 until seal 2 is fully entered into bore 5.

Seal 2 is soldered in place first and then seal 1 is soldered in place while suitable tension (if necessary) is applied to the fibre to maintain the intermediate portion taut in the reservoir to prevent kinking. The chamber 8 is provided with a sealable aperture 10 by which the chamber can be evacutated or pressurised. The aperture 10 can also be used to facilitate leak testing of the seals before the gland is installed in other equipment. One method of testing not only the two seals but also the quality of sealing of aperture 10 is readily available where the gland is to be used with a pressurised chamber 8. The chamber is filled with helium under pressure and the aperture 10 is sealed. Thereafter helium gas detection techniques, which are well known, can be used to detect leaks at either of the fibre seals or from the chamber aperture. Likewise, if the chamber is fabricated from two machined parts which are then welded together, as is indicated in the drawing, the quality of the joint can also be tested for leaks.

Claims

CLAIMS.

1. A sealing gland for an optical fibre comprising a pair of optical fibre seals spaced along a length of optical fibre, the seals being located in opposing walls of a chamber forming a reservoir through which the fibre intermediate the seals passes.

2. A sealing gland according to claim 1 wherein the reservoir is evacuated.

3. A sealing gland according to claim 1 wherein the reservoir is gas pressurised.

4. A sealing gland according to any preceding claim wherein the seals are metallic members surrounding bared lengths of fibre and are soldered thereto and to bores through the chamber walls.

5. A sealing gland according to claim 4 wherein the seals and their respective bores are of different diameters.

6. A sealing gland for an optical fibre substantially as described with reference to the accompanying drawing.