GB2134101A - Method of and apparatus for cleaving optical fibres - Google Patents

Abstract

A method of cleaving an optical fibre 5 to produce optically flat ends orientated to within a degree of a radial plane of the fibre (and therefore suitable for subsequently forming a low-loss fused joint) comprises the steps of tensioning the fibre over a supporting surface 6, radially applying a lightly loaded cutter 10 moving under low momentum to the fibre surface and subsequently increasing the load on the cutter to apply a correspondingly increasing radial pressure to the tensioned fibre until it cleaves. In a tool for carrying out the method the cutter 10 is carried by a tube 7 partially filled with liquid 8 which is approximately balanced about a horizontal axis 2. In use the tube is tilted through the horizontal with the cutter a few mm above the fibre surface. Consequently a bubble 12 travels along the tube and the tube/cutter gently and reproducibly topples onto the fibre and applies a gradually increasing radial pressure to the fibre until it cleaves. <IMAGE>

Description

SPECIFICATION Method of and apparatus for cleaving optical fibres.

The present invention relates to the cleaving of optical fibres and is particularly though not exclusively applicable to the cleaving of monomode optical fibres, which operation is necessary, for example, prior to forming a fused joint between fibre ends. Particularly when joining the ends of monomade optical fibres (which have a relatively small-diameter light-conducting core and a relatively thick refractive cladding) it is necessary to form cleaved surfaces which are optically flat and perpendicular (to within a degree or less) to the respective fibre axes in order to achieve losses of 0.2dB or less at the joint.

An object of the present invention is to provide a method and apparatus for cleaving optical fibres sufficiently cleanly and accurately for low-loss fused joints to be made between the fibre ends.

Hitherto optical fibres have commonly been cleaved by tensioning the fibre and making a slight cut (a few ym deep) in the fibre surface with a diamond or similar cutter which descends onto the fibre surface. A crack then propagates in a radial plane and the fibre cleaves. If necessary the tension in the fibre is increased sufficiently to cleave the fibre if the initial cut does not cause a crack to propagate. It has been found that the depth and shape of the cut have a critical influence on the axial tension required to initiate fracture. If the cut is too shallow or too rounded, the increased axial tension required for fracture results in a misted or hackled surface finish. If the cut is too deep it causes a deviation from optical flatness over a significant proportion of the fibre cross section.Particularly when cleaving fibres having a diameter greater than about 110 Mm, it has been found advantageous to provide a stress gradient in the fracture plane of the fibre by tensioning the fibre over a convex surface. The bent fibre is cut at the stressed radially outer part of its bent portion and causes a crack to propagate in the plane radial to the fibre axis. Generally the said convex surface is made with a radius of curvature of about 100 mm.

Various types of cleaving tool have been made which embody the principles outlined above, but it has proved difficult to reliably achieve cleaved fibre surfaces orientated to within one degree of the radial plane with such tools.

According to one aspect of the present invention, a method of cleaving an optical fibre comprises the steps of axially tensioning the fibre over a supporting surface; applying a lightly loaded cutter moving radially with respect to the fibre axis under low momentum to the surface of the fibre; and subsequently increasing the load on said cutter to apply a correspondingly increasing radial pressure to the tensioned fibre until the fibre cleaves.

By low momentum it is meant that the momentum is sufficiently low to ensure that any cut made by the cutter as it initially contacts the fibre surface is shallower than about 2 ym (this depth being typical of the cuts produced by conventional optical fibre cleaving tools). It may be desirable or even necessary in some cases that the cutter should contact the fibre surface with a momentum so low that the surface is not appreciably marked at all until the load on the cutter is increased beyond a certain value.

However it may well be difficult in some cases to determine the precise instance at which the cutter contacts the surface and at which the surface is permanently indented by the cutter. The cutter may be a diamond or a tungsten carbide blade for example.

The increasing load may be applied by means of a tiltable tube partially filled with liquid, which tube is linked to the cutter in such a manner that when the tube is tilted through the horizontal a bubble in the liquid travels along the tube and the resulting changing couple increases the load on the cutter. The tube is preferably approximately half-filled with liquid. The load on the cutter may suitably increase at between 10-' and 10-3 No~1.

The cutter is preferably arranged so that a lowering means may be operated to tilt the tube in a direction to bring the cutter towards the optical fibre-supporting surface, the lowering means being arrested by stop means with the cutter closely adjacent to but spaced from said surface with the tube tilted through the horizontal, and the lowering means also being such as to permit further tilting movement of the tube in said direction due to the flow of liquid within it.

By closely adjacent to the optical fibresupporting surface, it is meant that the cutter is close to the surface but at a distance greater than the diameter of the fibre so that the cutter is slightly spaced from the fibre.

The supporting surface over which the fibre is tensioned is preferably convex with a radius of curvature in the plane defined by the fibre axis of less than 200 mm.

According to another aspect of the present invention, apparatus for cleaving optical fibres comprises a supporting surface for supporting, in use, an optical fibre, located between a pair of separable optical-fibre gripping means, biasing means tending to separate said gripping means, a cutter located over said supporting surface and mounted for travel in a direction approximately perpendicular to said supporting surface and damped loading means for applying a slowly increasing load to the cutter. The loading means is preferably arranged to load the cutter only when the cutter is closely adjacent (e.g. up to 5 mm away from) the optical-fibre supporting surface.

The loading means preferably comprises a tiltable tube partially filled with liquid, the rest of the tube having gas therein forming a bubble, the cutter being attached to the tube at a positon offset from the tilt axis so that when the tube is tilted through the horizontal to bring the cutter closely adjacent the optical-fibre supporting surface, the liquid travels along the tube with the rate of flow limited by viscosity, and increases the load on the cutter. The tube may be necked to further limit the rate of flow of the liquid.

Alternatively the tiltable tube may be filled with fluid and provided therein with a hall having a diameter smaller than the bore of the tube, and travelling along the tube as the tube is tilted to increase the load on the cutter. The ball may be of greater or lesser density than the fluid.

The principles of operation of optical-fibre cleaving apparatus in accordance with the present invention will become clearer on consideration of the accompanying drawing, which is a schematic sketch perspective view, partly in section, of one particular optical fibre cleaving tool embodying the present invention. The apparatus, which is shown in relation to x,y and z references axes (the x-y plant being assumed to be horizontal) comprises an arm 1 pivoted on accurately located ball races (not shown) about an axis 2 parallel to they axis and a pair of grips 3 and 4 which hold an optical fibre 5 in the y-z plane against the convex upper surface of an anvil 6. The arm 1 is shown partly in section and tilted in either sense (orientations A and B) with respect to the horizontal plane.The arm 1 comprises a metal tube 7 approximately 1 50 mm long and having a bore approximately 4 mm in diameter, and a cutting diamond 10 mounted at one end of the arm with a counter balance 11 mounted at the opposite end of the arm. The tube 7 is partially filled with a silicone liquid 8 (for example a Dow Corning 200 series silicone) which has a low temperature coefficient of viscosity.

An air bubble 12 remains at the upper end of tube 7 and ensures that the total moment of the arm is such that the arm is held against the upper arm of a stop 9.

An optical fibre 5 is tensioned over anvil 6 between grip 3 (which is fixed) and grip 4 (which is mounted for sliding movement of the y direction) and tensioned by a linkage 1 3. The tension in the fibre may be adjusted by an adjusting screw 14, connected to linkage 1 3 via a pivoted dial gauge 1 5 which indicates the tension in the fibre. For a monomode fibre 125 ym in diameter, the tension is suitably between 1.0 and 2N. The fibre is held in position on the surface of the anvil 6 by four accurately located slotted guides, an inner pair 1 6 and 1 7 of which are located against the end faces of an anvil 6 and an outer pair 18 and 19 of which ensure that fibre 5 is not curved in the x-y plane.The anvil 6 is mounted for sliding motion in the vertical direction and is initially lowered before the fibre 5 is inserted into grips 3 and 4. After the fibre has been inserted and tensioned the anvil 6 is raised and locked into the position shown.

With the arm in orientation A and the fibre 5 appropriately tensioned across the anvil, stop 9 is lowered manually to tilt arm 1 into orientation B, so that the cutter diamond 10 lies a few mm above the fibre 5. Initially the arm 1 bears against the upper arm of stop 9, but since the arm has been tilted through the horizontal, bubble 12 travels slowly towards axis 2 as shown by the arrow and displaces liquid 8 towards the end of the arm. Consequently the cutter diamond 10 is gently lowered by the arm 10 onto the fibre 5 and exerts a gradually increasing cleaving force on the fibre 5, as bubble 1 2 continues to travel toward axis 2. The load exerted by arm 1 on cutter diamond 10 suitably increases at a rate of 1 0-2/NS. Typically, the fibre cleaves in less than one second. It will be apparent that the rate of increase of cleaving force will be a function of the viscosity of liquid 8, and it is therefore desirable to use a liquid (such as a silicone) which has a low temperature coefficient of viscosity.

Although it is convenient for the tube 7 to form part of the arm 1, it will be apparent that other methods of coupling such a tube to a cutting arm may be used within the scope of the invention.

However, it is advantageous to mount the cutter diamond on a pivoted arm rather than a sliding carriage, since an arm may be pivoted with very little friction, and is easily counterbalanced.

Claims (18)

1. A method of cleaving an optical fibre comprising the steps of: axially tensioning the fibre over a supporting surface; applying a lightly loaded cutter moving radially with respect to the fibre axis under low momentum (as hereinabove defined) to the surface of the fibre; and subsequently increasing the load on said cutter to apply a correspondingly increasing radial pressure to the tensioned fibre until the fibre cleaves.

2. A method according to any preceding Claim wherein the cutter is a diamond or a tungsten carbide blade.

3. A method according to any preceding Claim wherein said load is slowly increased by means of a tiltable tube partially filled with liquid, which tube is linked to the cutter in such a manner that when the tube is tilted through the horizontal, a bubble in the liquid travels along the tube and the resulting changing couple increases the load on the cutter.

4. A method according to Claim 3 wherein the tube is approximately half-filled with liquid.

5. A method according to any preceding Claim wherein the load on the cutter is increased at between 10-' and 10-3 Ns-l.

6. A method according to any one of Claims 3, 4 or 5 wherein the cutter is arranged so that a lowering means is operated to tilt the tube in a direction to bring the cutter towards the optical fibre, the lowering means being arrested by stop means with the cutter closely adjacent but spaced from said fibre with the tube tilted through the horizontal, and the lowering means also being such as to permit further tilting movement of the tube in said direction due to the flow of liquid within it.

7. A method according to any preceding Claim wherein the supporting surface is convex with a radius of curvature in the plane defined by the fibre axis of less than 200 mm.

8. Apparatus for cleaving optical fibres comprising a supporting surface located between a pair of separable optical fibre gripping means, biassing means tending to separate said gripping means, a cutter located over said supporting surface and mounted for travel in a direction approximately perpendicular to said supporting surface and damped loading means for applying a slowly increasing load to the cutter.

9. Apparatus according to Claim 8 wherein the loading means is arranged to load the cutter only when the cutter is closely adjacent (as hereinabove defined) to the optical fibresupporting surface.

10. Apparatus according to any one of Claims 8 or 9 wherein said loading means comprises a tiltable tube partially filled with liquid, the rest of the tube having gas therein, forming a bubble, the cutter being attached to the tube at a position offset from the tilt axis so that when the tube is tilted through the horizontal to bring the cutter closely adjacent to the optical fibre-supporting surface the liquid travels along the tube, with the rate of flow being limited by viscosity, and increases the load on the cutter.

11. Apparatus according to Claim 10 wherein the tube is necked to further limit the rate of flow of the liquid.

12. Apparatus according to any one of Claims 8 or 9 wherein said loading means comprises a tiltable tube filled with fluid and provided therein with a ball having a diameter smaller than the bore of the tube, the cutter being attached to the tube at a position offset from the tilt axis so that when the tube is tilted through the horizontal to bring the cutter closely adjacent to the optical fibresupporting surface, the ball travels along the tube and increases the load on the cutter.

13. Apparatus according to Claim 1 2 wherein the density of the ball is greater than that of the fluid.

14. Apparatus according to Claim 12 wherein the density of the ball is less than that of the fluid.

1 5. Apparatus according to any one of Claims 8 to 14 wherein the supporting surface is convex with a radius of curvature in the plane defined in use by the fibre axis of less than 200 mm.

16. Apparatus according to any one of Claims 8 to 1 5 wherein the cutter is a diamond or a tungsten carbide blade.

1 7. Apparatus for cleaving optical fibres substantially as hereinbefore described with reference to the accompanying drawing.

18. A method of cleaving an optical fibre substantially as hereinbefore described with reference to the accompanying drawing.