GB2215082A

GB2215082A - Optical fibre gland insert for bulkhead feedthrough

Info

Publication number: GB2215082A
Application number: GB8803507A
Authority: GB
Inventors: Brian Arthur Eales
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-02-16
Filing date: 1988-02-16
Publication date: 1989-09-13
Anticipated expiration: 2008-02-16
Also published as: GB2215082B; GB8803507D0

Abstract

An optical fibre gland insert, for leading an optical fibre through a bulkhead dividing regions of widely different pressure, consists of a sleeve 20 provided with a longitudinal slot 21 and two blind holes 22, 23 for receiving respectively an intermediate length of preferably metallised, bare fibre 13 and the two adjoining regions of the fibre with its optionally plastics protective sheathing 12 still remaining. The fibre may be soldered to the insert in the region of the slot between the blind holes, the rest of the voids being resin filled. <IMAGE>

Description

OPTICAL FIBRE GLAND This invention relates to the provision of glands on optical fibres for feeding such fibres through bulkheads, and is particularly concerned with pressure resistant designs of glands suitable for use in applications where there is significant difference in pressure of the ambient between the two sides of the bulkhead.

One form that a gland of this sort can take is illustrated in Figure 1. This has a flanged body portion 10 to which is fitted a cap portion 11, both these parts being made of metal. A plastics sheathed optical fibre 12 threads a central aperture, and an intermediate portion of its sheathing is removed to expose bare fibre 13 which is then metallised. Before the cap portion 11 is fitted to the body portion 12, a pair of collet pieces 14a, 14b are fitted in position around the fibre substantially to fill the space between the region of bare fibre embraced by the collet pieces and the internal wall of the gland body member 10.

Molten solder 15 is then introduced to fill the residual spaces around the collet pieces and up to the level of the top of the body portion. The void beneath the collet pieces 14 is then filled with resin to form a resin plug 16. The cap portion 11 is fitted to the body portion 10, and more resin is applied to form a resin plug 17 within the cap portion and resin fillets 18 where the sheathed fibre 12 emerges from the top and bottom ends of the gland.

Glands of this sort have been made and used satisfactorily in service, but their manufacture has proved relatively difficult, and the manufacturing yield has not been good. One of the problems is believed to be associated with the fact that considerable heating of the top end of the body portion 10 is required to ensure that the solder is sufficiently molten to flow down the sides of the collet pieces 14. On the other hand the stem of the body portion 10 beneath the flange has to be cooled to ensure that the plastics sheathing 12 of the still sheathed portion of the optical fibre is not damaged by the requisite heating of the upper part of the body portion 10. Another problem is associated with the provision of the resin plugs 16 and 17, for if bubbles become entrapped in either of these plugs they are liable to distort the bare fibre and introduce unacceptable microbending losses.

According to the present invention there is provided an optical fibre gland insert in which an optical fibre provided with protective sheathing has that sheathing removed from an intermediate portion of its length which threads a longitudinal slot extending from end to end in a gland insert member which has a pair of blind holes accommodating still sheathed portions of the optical fibre lying beyond the extremities of the intermediate portion of the fibre from which the protective coating has been removed, wherein the fibre is secured within the insert member with fixative subtantially filling said slot and blind holes. In forming an optical gland such an insert is inserted into an aperture in a gland body member and hermetically secured to it.

This type of gland insert is particularly suited to applications requiring their fibres to be secured in their glands by metal seals, though it is to be appreciated that the invention is applicable also to constructions of gland that rely upon resin seals between fibre and gland.

There follows a description of an optical fibre gland incorporating a gland insert embodying the invention in a preferred form. The description refers to the accompanying drawings, in which: Figure 1 depicts a schematic longitudinal section through a prior art gland to which reference has been made previously Figure 2 depicts a part sectioned perspective view of the preferred form of gland insert member at an early stage of assembly Figure 3 depicts the insert member at a slightly later stage of assembly Figure 4 depicts the insert member at a still later stage, and Figure 5 depicts the completed insert member being inserted into a gland.

Referring now to Figure 2, a plastics sheathed optical fibre 12 has an intermediate portion of its sheathing removed to expose a length 13 of bare fibre which is metallised so as to render it solderable. A metal insert member 20 for an optical fibre gland consists of a cylinder provided with a longitudinal slot 21 extending from end to end and a pair of co-axial blind holes 22 and 23. The slot is wide enough to accommodate the metallised fibre and the blind holes are large enough to accommodate the sheathed fibre. A slight curve is put into the fibre in the region from which its sheathing has been removed and this region is introduced into one end of the slot. The fibre is then moved along the slot in the direction of arrow 24 thereby carrying a still-sheathed portion of the fibre into blind hole 23.This hole is deep enough for the sliding operation to be continued until the other sheathed portion is clear of the far end of the insert member. At this stage the fibre is straightened by bending movement of the fibre in the direction of arrow 25 in order to allpw the fibre to be moved axially in the reverse direction so bring the unsheathed portion to rest symmetrically within the insert member with both sheathed portions engaged symmetrically within their respective blind holes.

As depicted in Figure 3, a low melting point solder preform 30 is introduced into the slot 21 in the region extending between the two blind holes, and this solder is caused to flow and form a solder plug 31 filling the slot in this region and just entering the regions where the slot opens out into the root of each blind hole. The residual voids in the interior of the insert member are filled by applying drops 40 of liquid uncured resin, for instance an epoxy resin, to each end of the insert member 20 as depicted in Figure 4 and allowing this resin to be drawn in by capillary action before curing it. Figure 5 shows how an externally flanged gland body member 50 is advanced along the figure 12 in the direction of arrow 51 until the end of the insert member 20 comes to rest against an internal flange (not shown) of the gland body member. This internal flange is located at a depth equal to approximately half the length of the insert member so that the mouth 52 of the body member is positioned at the level of the solder plug 31 of Figure 3. A solder preform ring 53 is then advanced over the exposed end of the insert member to come to rest against the mouth 52 of the gland body member where it is fused to form a solder fillet securing and sealing the insert to the body member.

In a specific example of optical fibre gland the optical fibre had a diameter of 125um and was acrylate sheathed to a diameter of 650um. A lOmm length of this acrylate sheathing was mechanically removed having applied a quantity of dichloromethane to cause the acrylate to swell, soften, and release its grip on the underlying fibre. Residual traces of acrylate were removed with acid from the resulting exposed bare fibre which was then rinsed with water before being metallised. Conventional evaporation techniques were used for metallisation which comprised an initial thin chromium layer of about lOnm thickness, followed by copper to a depth of about lum, which was itself covered with a flash of gold. Both the insert and the gland body member were made of gold plated EN3B mild steel.

The insert was 16mm long and about 1.8mm in diameter, its slot being 0.25mm wide, and its blind holes being 6.5mm deep and 0.7mm in diameter. The metallised fibre was secured to the insert with a standard low melting point lead tin silver solder, and the insert to the gland body member with a still lower melting point tin bismuth solder.

Claims

CLAIMS:

1. An optical fibre gland insert in which an optical fibre provided with protective sheathing has that sheathing removed from an intermediate portion of its length which threads a longitudinal slot extending from end to end in a gland insert member which has a pair of blind holes accommodating still sheathed portions of the optical fibre lying beyond the extremities of the intermediate portion of the fibre from which the protective coating has been removed, wherein the fibre is secured within the insert member with fixative subtantially filling said slot and blind holes. In forming an optical gland such an insert is inserted into an aperture in a gland body member and hermetically secured to it.

2. An optical fibre gland including a gland insert as claimed in claim 1.

3. An optical fibre gland insert as claimed in claim 1 wherein in the region between the blind holes the fibre is secured by solder to the insert member.

4. An optical fibre gland including a gland insert as claimed in claim 3 secured by solder to the gland.

5. An optical fibre gland insert substantially as hereinbefore described with reference to Figures 2, 3 and 4 of the accompanying drawings.

6. An optical fibre gland substantially as hereinbefore described with reference to Figures 2, 3, 4 and 5 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. An optical fibre gland insert in which an optical fibre provided with protective sheathing has that sheathing removed from an intermediate portion of its length which threads a longitudinal slot extending from end to end in a gland insert member which has a pair of blind holes accommodating still sheathed portions of the optical fibre lying beyond the extremities of the intermediate portion of the fibre from which the protective coating has been removed, wherein the fibre is secured within the insert member with fixative subtantially filling said slot and blind holes.

2. An optical fibre gland including a gland arses as claimed in claim 1.