WO1994011768A1

WO1994011768A1 - Optical fibre termination

Info

Publication number: WO1994011768A1
Application number: PCT/GB1993/002299
Authority: WO
Inventors: Simon George Preston Meadowcroft
Original assignee: BT&D Technologies Ltd
Current assignee: BT&D Technologies Ltd
Priority date: 1992-11-06
Filing date: 1993-11-08
Publication date: 1994-05-26
Anticipated expiration: 1995-05-06
Also published as: GB9223307D0; AU5427994A

Abstract

A termination for an optical fibre which comprises a (metal) tubular member (10) which is adhesively bonded to both the glass filament and the sheath whereby the relative movement of these two is restricted. Preferably the glass filament passes through the bore (17) of a plug (16) to which the glass filament is adhesively bonded and the plug is adhesively bonded to a first portion (11) of the tubular member. The sheath (14) of the fibre is preferably adhesively bonded to a second portion (12) of the tubular member which conveniently has indentations (26) which pass through the adhesive into the sheath whereby movement of the sheath is further restricted.

Description

OPTICAL FIBRE TERMINATION

This invention relates to optical fibre terminations, i.e. to terminations which are applied to the end of an optical fibre to protect the end portion and to make the end available for optical connection. Optical fibres take the form of a glass filament (having a glass core for conveying optical signals and a glass cladding surrounding said said core) which is protected by a plastic sheath. At the end of a fibre the sheath may restrict access for optical connections so the end portion of the sheath is usually removed to expose a short length, e.g. 0.5 - 5 mm, of glass filament.

The end of an optical fibre is particularly liable to damage, e.g. damage during storage or while the fibre is being connected to an optical device. It is, therefore, good practice to provide extra protection for the end portion and this protection often takes the form of a rigid tube, e.g. a metal tube, fitted onto the end portion of the fibre.

Sometimes it is difficult to fit terminations onto the end of a fibre and, when fitted, the termination may not provide adequate protection during the life of the fibre. It is an object of this invention to achieve adequate protection for the end of the fibre during the whole of its lifetime with a termination which is convenient to fit.

In accordance with this invention a fibre termination comprises a protective tubular member which surrounds the end portion of the fibre. The end portion, especially the tip, of the glass filament is exposed to facilitate optical connections and this exposed portion is secured to a first portion of the tubular member whereas the sheath is separately secured to a second portion of the tubular member. Thus the glass filament and its sheath are separately secured to the tubular member and this means that relative displacement of the sheath and the glass filament is restricted. This invention is based on the recognition that relative displacement of the glass filament and its sheath tends to occur at the end of an optical fibre and this displacement causes problems during the life of the device. This invention reduces the effects of the problem because the relative displacement is hindered. In addition the ridged tubular member provides extra protection for the end portion of the fibre and this extra protection is also valuable.

It has been stated that the sheath of the fibre is secured to the second portion of the tubular member. This is conveniently achieved by an adhesive which is located in the annulus between the sheath of the fibre and the inner wall of the second portion. This adhesive forms bonds with both items so that longitudinal movement is restricted. The longitudinal movement can be further restricted by indentations in the wall of the tubular member. These indentations pass through the adhesive layer into the outer regions of the sheath of the fibre. It will be apparent that these indentations constitute a mechanical restriction on both the fibre sheath and on the film of adhesive whereby longitudinal movement is further restricted.

Conveniently, the exposed portion of the glass filament is indirectly secured to the first portion of the tubular member. A convenient technique for achieving this indirect connection takes the form of a plug, preferably a ceramic plug, which is located in the first portion of the tubular member. The plug has a fine bore through which the exposed glass filament passes, preferably the tip projects beyond the exposed face of the plug. Both the bore and the annular space between the plug and the inner wall of the first portion contain adhesive which bonds the glass filament to the plug and the plug to the first portion of the tubular member. It will be apparent that this indirectly secures the glass filament to the first portion whereby relative longitudinal displacement between the glass filament and the tubular member is hindered. It is also desirable that the first portion of the tubular member has indentations which pass through the adhesive film and bear against the outer surface of the plug. These indentations constitute a mechanical restriction on the movement of the plug and also on the movement of the adhesive film.

The technique for assembling a termination in accordance with the invention will now be described. It is convenient to distinguish between the two ends of the tubular member. The end nearer the tip of the glass filament will be called the "outer" end whereas the end furthest from the tip will be called the "inner" end. The basis of the assembly technique is that the end portion of the sheath is removed to expose the glass filament and then the prepared fibre is introduced into the tubular member from the inner end whereby the exposed glass filament passes through the whole length of the tubular member until it reaches the outer end. Where an adhesive is used, a suitable amount of liquid adhesive is placed on the end of the sheath and the inner portion of the exposed filament but the outer end of the filament, especially the tip, is kept adhesive free. When the fibre is introduced into the tubular member the outer end of the glass filament is always in advance of the adhesive so that no adhesive is transferred onto the outer end. However the fibre carries the necessary quantity of adhesive into the tubular member so that the adhesive films are formed as described above.

In embodiments in which a plug is used, the inner end of the plug conveniently has a tapered guide so that the tip of the glass filament is guided into the bore of the plug. The plug is introduced into the first portion of the tubular member before the fibre and indentations are formed to hold said plug in position during the remainder of assembly. Conveniently the fibre is introduced into the tubular member until the end of its sheath contacts the end of the plug. It will be appreciated that this has the effect of squeezing the adhesive against the plug so that the adhesive is forced into the two annular spaces.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings wherein:-

Figure 1 is a longitudinal cross section through the component parts of a termination according to the invention;

Figure 2 is a longitudinal cross section showing the parts of Figure 1 assembled into a termination; Figure 3 is a transverse cross section along the line 3 - 3 of Figure

2; and

Figure 4 is a transverse cross section along the line 4 - 4 of Figure 3. As can be seen from Figure 1 the component parts of the assembly are a tubular member 10, a fibre 13 to which the termination is applied and a ceramic plug 16.

For the purposes of example, the fibre 13 is a conventional fibre having a glass filament 22 with an external diameter of 400μ. The optical signals travel in the core of the filament 22 which is a region of higher refractive index having a diameter of 9μ. The glass filament 22 is surrounded by a plastic sheath and the external diameter of the plastics sheath 14 is approximately 900μ. As can be seen from Figure 1 the end portion of the sheath 14 has been removed leaving approximately 1 - 2 centimetres of the glass filament exposed. The purpose of the termination is to protect the end of the fibre. During the life of the fibre the sheath 14 is liable to move relative to the glass filament 22 whereby the length of the exposed tail 15 can vary. This variation can have adverse effects upon a device into which the fibre is connected and it is a special purpose of the termination to restrict this movement.

The tubular member 10 is conveniently made of metal and it has a first portion 11 which has a wider diameter than a second portion 12. It is particularly appropriate for the diameter of the second portion 12 to be about 100 - 200μ larger than diameter of the fibre sheath 14. This makes it easy to introduce the fibre sheath 14 into the second portion 12 leaving an annulus which is 50 - lOOμ thick. A shoulder 20, substantially normal to the axis of the tubular member 10, separates the first portion 11 from the second portion 12. The plug 16 has an outside diameter which is about 100 - 200μ less than the internal diameter of the first portion 11 whereby the plug 16 can be introduced into the first portion 11 leaving an annulus which is about 50 - lOOμ thick. The shoulder 20 prevents the plug 16 entering the second portion of the tubular member 10. The plug 16 has a central bore 17 which is about 50 - lOOμ in diameter so that it can accommodate the exposed glass filament 15. The inner end of the plug 16 has a tapered guide 18 which, during the assembly of the termination, guides the exposed filament 15 into the bore 17.

The technique for assembling the components shown in Figure 1 into a termination will now be described.

As a first step the plug 16 is introduced into the second portion 12 so that its inner end abuts against the shoulder 20 whereby the plug 16 is located within the first portion 11. After the plug 16 has been inserted, indentations (not shown in Figure 1) are produced in the wall of the first portion 11 so that the indentations press against the outer surface of the plug 16. The pressure of these indentations is sufficient to hold the plug 16 in position during the assembly of the termination and they assist in retaining the plug after the termination has been assembled. After the plug 16 has been located as described, a portion of adhesive 19 is placed on the inner end of the fibre tail 15 against the outer end of the sheath 14. When the adhesive is applied it is important that the outer end of the filament 15 be kept adhesive free because contamination of the tip 21 by adhesive might degrade the performance of the fibre.

After the application of the adhesive 19, the fibre 13 is introduced into the inner end of the second portion 12 and it is inserted until the end of the sheath 14 meets the inner end of the plug 16. When the tip 21 of the filament 15 reaches the plug it is guided into the bore 17 and the tip 21 passes through the bore 17. Preferably the tip 21 projects beyond the bore 17 so that it is presented for making optical connections.

When the sheath 14 meets the plug 16 the adhesive 19 is caused to flow into the two annuli around the plug 16 and the sheath 14. The amount of adhesive is selected so that these two annuli are conveniently filled. As the adhesive flows into the annulus around the plug 16 it will pass around the indentations which were made when the plug had been inserted. Finally indentations are made in the wall of the second portion 12 and these indentations pass through the film of adhesive (which is still fluid at this stage) so that the indentations bite into the plastic sheath 14. After a few minutes (e.g. 2 or 3) the adhesive will set into a solid film and it will form bonds to the adjacent surfaces.

The termination produced by the method described above results in a termination as illustrated in Figures 2, 3 and 4 and this termination will be further described with reference to these Figures. For convenience of description the thickness of the adhesive films will be exaggerated in the drawings.

The assembled termination comprises the plug 16 which is located in the first portion 11 with its inner end in contact with the shoulder 20. The inner portion of the glass filament 15 is located within the bore 17 of the plug 1 and the extreme end is either level with the outer surface of the plug 16 or it extends a few millimetres, e.g. 2 or 3 millimetres, beyond the outer face.

It is convenient that the outer end of the tubular member 10 extends beyond the plug so that the projecting end of the filament 15, if any, is located within a protective cavity.

As can also be seen in Figure 3, the bore 17 of the plug 16 contains a film of adhesive 27 which adheres the filament 15 to the plug 16 whereby relative longitudinal movement between the two is hampered. The wall of the first portion 11 has four indentations 25 which bear against the outer surface of the plug 16 and the annulus between the plug and the first portion contains a second film of adhesive

28 which bonds the plug 16 to the wall of the first portion. This hinders the relative movement between the plug 16 and the tubular member 10 and this longitudinal movement is further hindered by the pressure of the indentations 25 on the plug 16 and the fact that the four indentations 25 penetrate the solid film of adhesive whereby the movement of the film of adhesive is hindered. It will be apparent that the glass filament 15 is indirectly secured to the first portion 11 whereby relative longitudinal movement between the glass filament 15 and the tubular member 10 is hampered.

As can also be seen in Figure 2, the sheath 14 is located in the second portion 12 and the four indentations 26 (as can be seen from

Figure 4) penetrate into the sheath 14 creating a mechanical restriction to the longitudinal movement of the sheath. In addition the annulus between the sheath and the second portion 12 contains a film of adhesive

29 which is adhesively bonded to both the outer surface of the sheath 14 and the inner surface of the second portion 12. This adhesive bonding further restricts relative movement between the sheath 14 and the tubular member 10. In addition the indentations 26 penetrate through the solid film of adhesive so that the movement of the adhesive film is also restricted. It is clear that the ultimate effect is that relative movement between the sheath 14 and the second portion 12 is also restricted. From the above description it will be apparent that the glass filament 15 and the sheath 14 are separately bonded to the tubular member 20. The relative displacements of the sheath 14 and the glass filament 15 are severely restricted and the dimensional stability of the fibre at the end is enhanced. Thus the tubular member 10 not only protects the end portion of the fibre but it enhances the dimensional stability of the fibre. When the fibre is connected to optical devices mechanical connections can be made to the tubular member 10 whereby good mechanical stability is achieved over the lifetime of the device and this lifetime is effectively prolonged.

Claims

1 A fibre termination comprising a protective tubular member (10) surrounding the end portion of an optical fibre (13) which fibre comprises a glass filament (22) and a protective sheath (14) surrounding said filament wherein the end portion of said filament (15) is exposed to facilitate optical connection thereto; wherein the exposed portion of the glass filament (15) is secured to a first portion (11) of said tubular member and the sheath is secured to a second portion (12) of the tubular member whereby relative displacement between said glass filament and said sheath is restricted.

2 A termination according to claim 1, wherein the annular space between the sheath and the second portion of the tubular member contains an adhesive layer bonded to the inner wall of the second portion and the outer surface of the sheath whereby relative longitudinal movement between the sheath and the tubular member is restricted.

3 A termination according to claim 2, wherein said second portion has indentations which pass through the adhesive layer and into the sheath whereby longitudinal movement of the sheath relative to the tubular member is further restricted. 4 A termination according to any one of the preceding claims, wherein the exposed glass filament is indirectly secured to the first portion of the tubular member by means of a plug located in said first portion, said plug having a bore through which a glass filament extends said filament being adhesively bonded to said plug and said plug being adhesively bonded to said first portion whereby relative movement between the glass filament and the tubular member is restricted.

5 A termination according to claim 4, wherein said second portion has indentations which pass through said adhesive layer and press against the outer surface of said plug whereby relative longitudinal movement between said plug and said first portion is further restricted.

6 A method of assembling a fibre termination according to claim 2, wherein a fluid adhesive is applied to the zone where the glass filament emerges from its sheath leaving the tip of the glass filament free of adhesive and introducing said fibre into the open end of the second part of said tubular member so that the fibre introduces adhesive into the tubular member whereby adhesive bonds are produced between the sheath and the second portion of the tubular member and wherein the tip of the glass filament proceeds in advance of the adhesive whereby contamination of the tip with adhesive is avoided.

7 A method of producing a termination according to claim 5 which method comprises :-

(a) Introducing the plug into the first portion of the tubular member; (b) Producing indentations in said first portion to bear against the outer wall of said plug whereby said plug is located during assembly process;

(c) Applying fluid adhesive to the zone where the glass filament emerges from its sheath leaving the tip of the glass filament free of adhesive;

(d) Introducing said fibre into the open end of the second part of said tubular member so that the glass filament passes through the bore in the plug and adhesive is introduced in the bore of the plug;

(e) Pressing said adhesive against the inner end of said plug whereby the adhesive is caused to flow into the two annuli whereby the plug becomes adhesively bonded to the first portion and the sheath becomes adhesively bonded to the second portion; and

(f) Producing indentations in the second portion of the tubular member said indentations passing through the liquid adhesive to engage with the sheath.