WO1996033430A1

WO1996033430A1 - Improvements in and relating to fiber optic cleaving

Info

Publication number: WO1996033430A1
Application number: PCT/GB1996/000919
Authority: WO
Inventors: Ian John Murgatroyd
Original assignee: Oxford Fiber Optic Tools Limited
Priority date: 1995-04-20
Filing date: 1996-04-17
Publication date: 1996-10-24

Abstract

Two pairs of clamping surfaces (8a, 15a, 8b, 15b) trap a section of optical fiber (1) between two points. The clamping surfaces may be made of a compliant material in order to clamp all of the fibers in a multiple fiber ribbon. The fiber or fibers are then tensioned by stretching them by a preset amount by compressing a leaf spring (7) which constitutes an upper pair (8a, 8b) of the clamping surfaces. The lower pair of clamping surfaces (15a, 15b) is provided by a pair of pivoted roller blocks (14a, 14b) which follow the movement of the upper spring to allow longitudinal extension of the fiber or fibers which are clamped therebetween. The tensioned fibers are subsequently displaced laterally by bending them across a two-armed anvil (9) substantially to eliminate the shear stresses therein. The fibers are then scored by a sharp blade (6) on the opposite side of the fiber to the anvil, where the blade is arranged so that it is acutely angled with respect to its direction of motion towards the tensioned fiber or fibers. The very slow penetration of the blade into the fiber due to its acute angling does not give rise to significant extra bending forces and so leads to substantially perpendicular cleaved ends.

Description

IMPROVEMENTS IN AND RELATING TO

FIBER OPTIC CLEAVING

This invention relates to improvements in and relating to methods and devices for cleaving optical fiber materials, in particular, but not exclusively telecommunications fibers.

The relative widespread and ever increasing use of optical fiber materials, usually glass, for use in optical wave guide communication systems, data processing and other signal transmission systems has created demand for satisfactory, easy to use and efficient means for interjoining terminal ends of adjacent fiber lengths without appreciable loss of signal energy through the junction. It has been found that such terminal ends transmit light energy signals with minimum energy loss of signal when the end of the fiber is optically flat with a mirror smooth surface lying in a plane perpendicular to the longitudinal axis of the fiber filament. Such a flat end allows easy and repeatable joining of the two adjacent fiber ends by a splicing technique in which the two fiber ends are fused or otherwise brought into close proximity to each other so that the optical signal suffers minimal loss of power in transiting the junction.

Current art is focused on two principal ways of cleaving an optical fiber which can be conveniently described as the "score-and-bend" technique and the "tension-and-score" technique. In the "score-and-bend" approach, a flaw is created in the optical fiber using a sharp blade made of diamond or tungsten carbide; often this blade is in the form of a wheel which is passed under the fiber and very accurately adjusted in its height so as to nick the fiber as it passes tangentially underneath the fiber, as described in US 5104021. The fiber is then stressed in the area of this flaw, typically by bending, such that the tension created by the bend serves to open up the crack, propagating it across the fiber, so cleaving it. This method has advantages in that there is no need accurately to control the applied tension which serves to propagate the crack and that it can easily cleave several fibers, as might be found in a fiber ribbon, with one pass of the blade. However, the method has certain drawbacks. Firstly, the blade tends to intrude into the fiber creating excess cracking of the fiber at the point of contact. Such cracks can be detrimental to the performance of the splice which is made up of two such cleaved ends. To achieve good cleaving performance, this necessarily implies that the blade must be aligned to the height of the fiber very accurately, typically to within + /-10 microns. During use, this alignment is often lost, for instance because of blade wear, damage or loss of register, leading either to excessive blade damage or to no cleaving at all. Secondly, the size of crack needed to cleave different types of fiber varies and so the necessary amount of the intrusion of the blade into the fiber must be different for different fibers; specifically, Titanium impregnated fiber needs greater blade intrusion than Silica optical fiber, and therefore the blade position must be set higher to achieve the necessary size of crack needed to cleave such a fiber. Thirdly, the mechanism for initiating cleaving is to bend the fiber by depressing an anvil. This leads to cleaved ends which may not be totally planar, due to the variable nature of the cleave initiation site and its propagation, and which may exhibit end angles of greater than 1°.

In the "tension-and-score" approach, the optical fiber is clamped at two points and tensioned with a force of around 200 grams. The tensioned fiber is then nicked with a sharp edge and the resultant crack propagates automatically due to the already applied tension in the fiber to produce a cleave. This method produces cleaved ends which are typically flatter than the scribe-and-bend method with less cracking in the region of the blade strike. However, this method also has disadvantages. The blade must penetrate the fiber a certain distance before the crack propagates as a cleave. Since the blade is normally designed to move perpendicularly into the fiber to create the crack this tends to bend the fiber before a sufficiently deep crack is initiated for cleaving to occur. Such bending will tend to cause non-flat cleaves and excessively large initiation cracks. In previous technologies, vibrating blades (US 4790465) or anvils (US 4565310) have been used to prevent bending of the fiber during cleaving.

Yet another significant disadvantage of the "tension-and-score" technique is that it does not readily lend itself to cleaving multiple fibers at one time, as might be found in a ribbon fiber, because it is difficult to apply tension equally to all fibers at the same time. Instead there is a tendency to tension one or more fibers in the ribbon more than the others. Furthermore, the tensioning in each fiber must be independent from the tensioning in the neighbouring fibers in the ribbon. If the tensioning is not independent, after the first few fibers have been cleaved, the entire applied force will be applied to the remaining fibers, with the result that the remaining fibers support an ever larger tension. This leads to shattering of the ends of the final fibers to be cleaved.

As an alternative to generating perpendicular cleaving, in certain applications an angled cleaved end may be desirable. For instance, in optical communication systems, the back reflection from a cleaved fiber end must be reduced as far as possible, and this can be achieved by cleaving the fiber end at an angle; a 5° end angle on the fiber will reduce the back-reflection to less than -50dB. In this further field both the score-and-break method and the tension-and-score method are unsatisfactory in that, whilst angles produced by the cleaving may be non- perpendicular, the end angle so produced is not controlled. Further, even when the prior art attempts to implement this technique the prior art method using "tension-and-score" techniques are difficult to use and are not well designed as hand tools.

Finally, in all the above approaches to cleaving the apparatuses have been designed principally around bench-mounted devices of relatively high price. With the expanding need to install optical fiber in the field there is a need for tools which are affordable so that they can be made available to every craftsperson all of the time and which are hand- held and rugged whilst producing high quality, accurate and reproducible results.

It is a principal object and purpose of this invention to provide an improved method of cleaving optical fibers which can reliably effectuate optically flat end faces thereon with an end angle less than 1° from the perpendicular.

It is also desirable to provide a method for effectuation of optically perfect end surfaces with end angles of less than 1 ° on multiple optical fibers arranged in the form of a ribbon.

It is also desirable to provide an improved method for cleaving optical fibers in a hand-held or bench-top tool which is rugged to use, easy to adjust and which can be conveniently used in the field.

It is also desirable to provide a method and a cleaving device for effectuation of mirror smooth angled surfaces on single optical fibers or on multiple optical fibers arranged in the form of a ribbon with end angles in the range of 1° to 15° and preferably in the range of 4° to 10°, in order to reduce the strength of the optical signal reflected from the cleaved surfaces. The surfaces should be angled in a specific direction so that the reflection of specific polarisations of the optical signal can be eliminated.

It is also desirable to provide a method and a tool for cleaving optical fibers at specified and precise distance from the end of the coating of the fiber or fibers or other constraining device for the purpose of accommodating different systems of joining optical fibers and different types of optical fiber connection devices. In particular, the cleaving tool should be able to accept standard fiber holders, so that the optical fiber or fibers can be cleaved at a specific position, without the need to take it out of its holder.

It is also important to be able to constrain and support the optical fiber or fibers and its associated coating and cabling prior to and during cleaving so that the fiber can be cleaved conveniently and consistently without danger of the cleaved end being damaged.

According to a first aspect of the invention we provide a method of cleaving an optical fiber comprising the steps of restraining two spaced portions of the fiber, inducing and maintaining tension in the fiber between the two restrained portions, and scoring the tensioned fiber with an edge of a blade in order to create a crack in the fiber which initiates a cleave, the blade being moved relative to the fiber in a direction which makes an acute angle with the edge in the plane of the blade.

This method of cleaving an optical fiber has several significant advantages. The use of an acutely angled blade means that whilst it contacts the fiber creating a crack which is propagated by the applied tension so cleaving the fiber, the movement of the blade radially into the fiber is demagnified by the angling of the blade and so leads to little bending of the fiber and so consistently low end angles. In addition the speed of cleave propagation is much higher than the penetration rate of the blade ensuring cleaving without the presence of extra cracking.

Furthermore, no precise alignment of the blade is needed, for instance via an accurately positioned wheel or bearing, as the extent of scoring is not controlled by the blade's positioning. In this invention, the blade can easily be aligned so that it will always contact a fiber which is held between the two clamps, tolerating some misalignment of the fiber within the clamps or variations in fiber size. Therefore, the cleaver according to this invention does not need accurate alignment, it is easy to manufacture, it will not go out of alignment during use and it will be serviceable in the field.

In a particularly preferred embodiment the method includes laterally displacing the fiber between the two restraining locations prior to scoring with the blade. Preferably this is done by the introduction of an anvil. The bending of the fiber caused by the anvil is designed to resist and further minimise the opposite bending force of the blade as it bites into the fiber and so ensures that the fiber is always under tension at the point of the cut. Most preferably the displacement directly opposes the blade to provide optimum support. The lateral displacement preferably serves as a second mechanism to tension the fiber in addition to axial tensioning arising from an increase in the separation of the restraining locations. Control of the axial tension in the fiber allows control of the propagation of the cleave.

The total tension introduced into the fiber can be (as described with reference to Fig. 12) a superposition of pure tension arising from moving apart the two restraining locations and stress arising from stretching the fiber over the anvil. The stress arising from stretching the fiber over the anvil is tensile on the outside bend of the fiber and zero on the inside bend of the fiber, i.e. the neutral axis lies at the edge of the fiber on the inside of the bend. (It should be noted that this is not the same as simply bending the fiber. Pure bending would give compression on the inside of the bend with a neutral axis half way across the fiber. In stretching the fiber over an anvil, compression on the inside of the bend is countered by the tension generated by the lengthening of the fiber required to stretch it over the anvil.) The resultant total stress is tensile at all points on the fiber's cross-section, highest on the outside of the bend, and smaller, but still non-zero, on the inside of the bend.

When the cleave is initiated at the outside of the bend, the tension is high, so the necessary initiation crack is small, but as the cleave progresses, the tension decreases, preventing the cleave from travelling too fast and producing hackle; by the term "hackle" is meant the characteristic surface irregularity which can be created by uncontrolled energy release in formation of the cleave. This ensures a mirror smooth surface. Any hackle arising from the cleave travelling too fast is confined to the extreme circumference of the cleaved end at 90 and 270 degrees to the point of cleaving; this will not affect the transmission of light through the fiber's core. The cleave will travel perpendicularly across the fiber because it always experiences positive tension. In the previous art as shown in Fig. 2, simply stretching the fiber over an anvil without moving apart the restraining locations gives zero tension in the fiber at the inside of the bend, and this tends to cause the cleave to deviate from the perpendicular as it travels towards the inside of the bend.

In a particularly preferred embodiment, the face of the anvil, which acts to bend the fiber, is parallel with the edge of the acutely angled blade. Therefore, the fiber is trapped between the face of the anvil and the edge of the blade which is caused to score the fiber.

Preferably the anvil has two end portions separated by a recess.

Preferably the restraining locations comprise two pairs of opposing clamps, to provide firm restraint and ready release.

In a possible embodiment, particularly where it is desired to cleave a fiber with a large diameter of 200microns or greater, clamping surfaces at one or both of the restraining locations may comprise a V-groove into which the fiber is pressed by an opposing clamping surface. This has the effect of providing secure clamping of the fiber and decreases the possibility that the fiber will rotate during clamping, which would introduce torsion into the cleaved fiber.

In a possible embodiment, particularly where it is desired to cleave more than one fiber at a given time, clamps may be provided with a compliant surface. This allows cleaving of multiple optical fibers arranged in a linear ribbon fashion. Each fiber in the ribbon is gripped independently by virtue of the compliant surface. Each fiber in the ribbon is placed under a constant extension producing a constant axial tension therein and, consequently, each of the fibers in the ribbon will cleave in the same way with the same angle. This is advantageous compared to alternative tension-and-score methods where the fibers are stretched by a constant force rather than being subject to a consistent extension, where one or more fibers is always tensioned more than the others, particularly as cleaving occurs, leading to different cleaving for different fibers in the ribbon. The compliant material can be made of a uniform material such as a hard rubber or plastic. However, after prolonged use, the material tends to become deformed as it is crushed by the pressure required to clamp the fibers, consequently shortening the lifetime of the clamping surfaces and leading to a deterioration of the cleaving performance.

Most preferably, compliant clamping pads can be made of a matrix structure such as a fiber matting, made up of kevlar, carbon or glass fibers, held together by a bonding agent such as epoxy. As the glass fiber which is to be cleaved is clamped by the compliant surface, the hard fibers in the surface, made of kevlar for example, resist the crushing pressures so that the clamping surfaces is not permanently marked. Simultaneously, as the glass fiber is clamped, the softer bonding material of the clamping surface yields under the clamping force. This ensures that all of the glass fibers in the ribbon are clamped by the compliant pads, even if there is a slight variation in the diameters of the glass fibers in the ribbon. The fibers in the compliant surface are typically 10 microns in diameter, so the glass fiber is clamped by several of them simultaneously, spreading the crushing load. The fibers in the compliant pad are preferably arranged in one direction, parallel to the glass fiber which is to be clamped, so reducing the danger of breaking it. The most successful material fiber material in the compliant pad is comprised of a hard, yet tough, fiber such as Kevlar. The compliance of the compliant surface can be modified by changing the stiffness of the bonding material, where this material could be epoxy resin, polyester, polyethylene, polyetheretherketone (PEEK) , polyphenylenesulphide (PPS), carbon or some other elastic material.

Preferably one and more preferably each of the restraining means at the restraining locations are free to move relative to the other so as to permit the separation of the restraining locations to be increased and tension introduced. This simplifies construction and allows the restraining means and tensioning means to be one and the same.

Most preferably one or both of the clamps comprises a deformable member. It is preferred that the contacting location between the member and the fiber, at the restraining location, should move further from the other restraining location as the member is deformed. Clamping is thus adequately effected with further compression causing tension to be introduced into the fibers. Most preferably a spring forms the clamp and preferably the other clamp is a movable surface, most preferably a pivotally movable surface such as a roller or a second spring. It is particularly advantageous to provide one clamp of each of the pairs by means of a single common spring or to provide both clamps of each of the pairs by means of two opposing springs.

A second anvil may also be introduced, most preferably opposing the first anvil. This also allows the cleaving of controlled angled ends onto single or multiple optical fibers by cutting the fiber at a point where shear stress of a controlled level is present in the fiber. For both single and multiple optical fibers, the fiber is tensioned and the first anvil creates a lateral displacement to resist the cutting of the blade as above, and the secondary anvil is introduced, approximately in opposition to the first anvil, to increase the bending of the fiber to create shear forces which will lead to an angled cleaved end. This embodiment of the invention has the advantage that the same tool can be used either to cleave angled ends onto a fiber or fibers, by the simple means of introducing the secondary anvil, using an actuating device such as a lever, or to cleave perpendicular ends onto a fiber or fibers by not applying the secondary anvil. A further method of cleaving angled ends is optionally provided by introducing a secondary anvil which is moved to displace the fibers laterally in a direction perpendicular to the action of the first anvil. Consequently, the fibers are subject to a shear stress which will lead to angled ends when cleaved. This displacement is preferably achieved by contacting the fibers with the secondary anvil, which is preferably tipped with a compliant material, and moving it with a component of sideways motion with respect to the first anvil, for example by rotating it around a pivot or sliding it. The cleaved angles so produced will be arranged in an approximately perpendicular orientation with respect to the angles produced by the previous method. Therefore the two methods will be effective in decreasing back reflections for opposing optical polarisations. Any rotation of the fibers as they are being displaced will give rise to torsional stresses in the fiber which will also give angled ends when cleaved.

In both of these ways cleaved ends may be produced with angles of between 1 and 15 degrees, and preferably between 4 and 10 degrees, titled in the desired orientation, whilst still producing planar surfaces with minimum damage caused by cleaving.

The angle of the plane of cleavage can optionally be controlled or varied according to the level of shear force introduced. This can in turn be controlled by the extent of displacement generated by the secondary anvil. The level of shear can also be controlled according to the following optional and alternative methods.

An alternative method of cleaving angled ends is optionally provided by rotating the two-portioned anvil about an axis perpendicular to the fiber, either before or after contact, so that the bending of the fiber is non-symmetric, i.e. it is twisted. This maybe done by a simple actuating method whereby a perpendicular setting for the anvil produces perpendicular ends whereas a rotated or part rotated anvil produces angled cleaves.

The ends of the two-portioned anvil should be tipped with a compliant material so that they contact the fiber over a significant fraction of its diameter to insure that, between the two arms of the anvil, a twist is applied to the fiber; the use of a hard tip to the anvil leads to point contact between the fiber and the anvil and so no twisting force will be applied to the fiber.

Similarly, the use of an one-armed anvil to achieve angled cleaves rather than a two-armed anvil to produce perpendicular cleaves can easily be achieved by means, for instance a lever, to change the form of the anvil by withdrawing one of the portions so leaving the fiber unevenly displaced.

In a further alternative, twisting of one or both pairs of clamps relative to the other, for instance about the axis of the fiber, can be used to create non-axial stress in the clamped and tensioned fiber or fibers.

In yet a further alternative angled cleaving can be achieved by providing a two portion anvil and introducing the step of rotating one portion relative to the other such that the support provided by the arms is not parallel, where both portions of the anvil are tipped with compliant material.

Such a tool is preferably manually operable with manual pressure being used to cause the clamping, tensing, optional lateral displacement and scoring leading to cleaving the various stages occurring in progression as the device is progressively compressed. The device is designed to be lightweight and portable, whilst being sufficiently rugged for use in the field. However, the same is easily adapted for bench-mount operation.

Means can also are provided in the tool for regulating the location of the fiber cleavage so as to select the length of fiber remaining between the point of cleavage and the end of the stripped coating selected in accordance with the requirements of the particular application required.

Means can also be provided to cleave an optical fiber or fibers held in various connector devices with either a perpendicular or angled end with a specific distance between the end of the connector device and the cleaved fiber or fibers.

According to a second aspect of the invention we provide a device for use in cleaving optical fibers comprising restraining means whereby two spaced portions of a fiber can be secured, tensioning means whereby tension can be induced and maintained in the fiber between the two spaced portions, and means adapted to hold a scoring blade in engagement with the fiber and to move the blade relative to the fiber in order to create a crack in the tensioned fiber which initiates a cleave, the blade-holding means being arranged to move the blade in a direction which in the plane of the blade made an acute angle with the edge of the blade.

The use of a blade angled acutely to its direction of motion to score the fiber has several advantages. The frictional force between the blade and the glass is substantially less for a near tangential blade motion than for a perpendicular blade motion and it is only required to exert a small force to score the fiber which is therefore not substantially displaced by the blade. In addition, due to the acute angling of the blade, its radial motion relative to the fiber is substantially demagnified compared to the speed of the blade's lateral motion relative to the fiber. Consequently the sharp blade contacts the tensioned fiber gently, minimising the lateral forces which cause bending of the fiber. Furthermore, the motion of the blade relative to the fiber is self-limiting. As the blade advances, it scores the fiber ever deeper until the scratch so created becomes just deep enough that the previously applied axial tension causes the fiber to cleave; the blade does not create an over-deep crack since the fiber cleaves as soon as the crack becomes deep enough for it to be propagated by the tension, stopping any further damage by the advancing blade. This self- limiting behaviour in creating a crack of minimal depth is made possible because the radial movement of the blade into the fiber is very slow due to the demagnification effect on the motion of the acutely angled blade

Preferably the restraining locations are separated by a distance in the range 0.25mm to 100.00mm, or even more preferably in the range of 0.25mm to 30.0mm. In this way the optical fiber does not bend significantly under the influence of the blade during scoring and so the fiber remains under tension at the point of incision such that the crack is propagated as a cleave.

The angle between the edge of the blade and its direction of travel during scoring may be between 0.5 and 45 degrees, and most preferably in the range 2 to 45 degrees, where 90 degrees equates to a blade moving perpendicular to its edge.

The linear section of the fiber is preferably placed under an axial tension by increasing the spacing of the two restraining locations. The restraining means and consequently the restraining locations may be defined by pairs of opposing clamps of various forms. Bringing the clamps together may thus serve to hold the fiber whilst their release allows the cleaved fibers to be withdrawn.

Means may be provided to limit the approach of the clamps to one another so as to prevent overclamping of the fiber.

One or both of the clamp surfaces maybe made of a hard material such as hardened steel.

One or both of the clamp surfaces of one or both of the holding locations may incorporate a V-groove into which the fiber is pressed during clamping to ensure that the fiber is securely clamped without rotation.

Preferably one clamp of each pair is part of a common deformable member. Most preferably this is a spring. Preferably the other part of each clamp is in the form of a roller. The roller is preferably shaped such that it is substantially flat but which can rotate independently around an axis to allow the longitudinal extension of the fiber. Alternatively, the other part of each clamp is also in the form of a common deformable member. Most preferably, this is also a spring.

Preferably compression of the common member or members cause the restraining locations formed thereby to move apart so generating tension in the fiber. The provision of the other clamp as rollers or as a second common member aids in accommodating this movement. Preferably, the clamping surfaces of either or both of the deformable members and/or the rollers are angled inwards prior to clamping, as shown in Fig. 13. As the fiber is clamped and stretched, the clamping surfaces bend or rotate towards the horizontal, as shown by arrows Yl , Y2, Y3, Y4, ensuring that the clamped fiber is always urged outwards, hence tensioning it.

In this way the restraining means and tensioning means are preferably one and the same.

Preferably the device is also provided with means to laterally displace the fiber between the two restraining locations prior to cleaving. This lateral displacement serves to stretch the fiber between its two restraining locations imparting a tension to the fiber in addition to that arising from moving apart the two restraining locations. In addition, the lateral displacement bends the fiber, increasing the tension on the outside of the bend and decreasing it on the inside of the bend. Bending the fiber in this way is designed to resist and further minimise the opposite bending force of the blade as it contacts the fiber. In this way the device ensures that the fiber is always under tension at the point of incision of the blade whatever the distance between the restraining locations

The extent of displacement, for instance caused by an anvil, may be adjustable to allow for optimum displacement for different fiber diameters and materials.

Stop means may be provided to limit the extent of displacement as desired. Preferably in this way the clamped fiber is extended by approximately 2 parts in 1 ,000 of the distance between the two restraining locations.

It is particularly preferred that the anvil be split into two portions with a recess between them. In this way the bending force is applied at two points so the fiber experiences a four-point bending condition (with the clamps making up the remaining two locations) , which gives a substantially zero shear stress in the fiber in the region between the two parts of the anvil. It is in this region that the cleave is preferably initiated. Consequently, the cleave so produced will be perpendicular, as is required in the technology of joining optical fibers.

Preferably the face of the two portions of the anvil will be in the same plane as the edge of the acutely angled blade so that the fiber will be trapped therebetween and so scored by the edge of the blade..

Where an anvil is provided it may pass through the spring and so into contact with the fiber.

The sharp blade maybe made of a hard, tough material such as diamond or Zirconia.

The blade may be adjustable so as to allow different portions of the blade to be presented to the fibers during its working life as the blade wears or gets damaged.

Preferably at least one of each pair of clamps is provided with a compliant surface. The multiple fibers in the ribbon can thus be clamped between the two restraining locations, both of which have some degree of compliance on one or both of their faces so that they can grip each fiber in the ribbon independently. Because each fiber is placed under a constant extension, as each fiber in the ribbon is progressively cleaved, the tension in the remaining fibers is largely unaffected, hence all of the fibers are cleaved under approximately equivalent conditions.

The compliant pads preferably have a surface which comprises a fiber-matrix composite. The matrix is made of epoxy or a similar material to provide compliance. The fibers are made of Kevlar, or similar, to provide hardness and toughness to stop the compliant pad crushing and being damaged under the clamping loads. The Kevlar fibers are preferably laid unidirectionally, parallel to the glass fiber to prevent its breakage during clamping.

Preferably the apparatus also includes a secondary anvil which maybe introduced so as to displace the fiber in a controlled manner. In this way this invention also allows for controllable end angles to be cleaved onto single or multiple fiber ribbons. The cleave in a fiber will propagate in a direction perpendicular to the resultant applied tension in the fiber thus a shear or torsional stress must be induced in the fiber such that when this stress is combined with the applied tension, the maximum stress lies at an angle to the axis of the fiber. This the secondary anvil does.

Most preferably the secondary anvil is provided in opposition to the first and ideally it abuts the fiber between the two arms of the two part anvil. A secondary anvil applied in this way is used to increase the bending forces acting on the fiber in the region of the cleave. The secondary anvil acts with a controlled force on the fiber or fibers, in opposition to the original anvil at a point close to or in between the arms of the first anvil such that the fiber or fibers are significantly bent so creating shear forces. The secondary anvil is preferably tipped with a compliant material which is deformed on contact with the fiber or fibers so exerting a shear force on them. The bent fiber or fibers are then scratched with the blade at a point on the outside of the bent fiber preferably within a few fiber diameters of the point of action of the secondary anvil and this will lead to a cleave which is angled.

Cleaving of the fibers is achieved by scratching each fiber in the ribbon with an acutely angled blade so that they cleave substantially simultaneously with the same cleave length. Subsequently, each fiber in the ribbon is bent equally across the anvil. Consequently, each fiber is equally tensioned and bent, but the stresses are independent, and if one or more fibers in the ribbon are cleaved, the stresses in the other fibers will not be altered. Finally cleaving is initiated by scratching each fiber with an angled blade, where the blade edge is parallel with the plane of the fiber ribbon and also parallel with the end of the two part anvil so that each fiber is scratched and hence cleaves substantially simultaneously.

The position of the secondary anvil and the blade, and the force with which the secondary anvil bends the fiber or fibers, are preferably controlled to produce a cleave end angle in the range of 1° to 15° and preferably an angle of 4°-10°.

This method is also effective in cleaving end angles onto a multiple fiber ribbon, where the secondary anvil is tipped with a compliant material so that it can exert equal bending forces and so produce equal stresses in, and hence cleave end angles on, each fiber in the ribbon. The direction of motion of the secondary anvil can be controlled so that it laterally displaces the fibers with components both in opposition to and perpendicular to the direction of the first anvil. This will serve to control the orientation of the shear created in the fibers and hence the orientation of the cleaved end angles.

This is preferably achieved by contacting the fibers with a secondary anvil which is moved in a direction with a component of motion such that the fibers are displaced perpendicular to the direction of motion of the first anvil, shearing the fiber, but where the direction of motion of the second anvil also has a component of movement acting in opposition to the first anvil so that the fibers are properly gripped by the secondary anvil. This is preferably achieved by rotating the anvil around a pivot. Preferably, the secondary anvil is tipped with a compliant material so that the fibers are gripped. Preferably the secondary anvil is applied after the fibers have been displaced by the first anvil.

The sideways motion of the secondary anvil will also lead to a rolling of the fibers which will induce a torsional force in the fiber which will also lead to angled ends. Preferably the first anvil will also be tipped with a compliant material to allow the fibers to be gripped when subjected to this rolling force.

The extent of movement of the secondary anvil giving rise to lateral displacement and/or rolling of the fiber may be adjustable to allow for optimal cleaved end angles oriented in optimal orientations.

Another way of producing angled cleaves is to bend the fiber or fibers such that the plane of bending is not symmetric with respect to the length of the fiber. Thus the device may be provided with means to rotate the anvil about an axis perpendicular to itself and so perpendicular to the axis of the fibers to be cleaved. In this way as the anvil is rotated so that the arms of the anvil are not perpendicular to the fiber the bending will not be symmetrical, and so shear forces will be created in the fiber, leading to an angled cleave. Preferably, the end of the first anvil is tipped with a compliant material to ensure contact between the anvil and the fiber, over a significant fraction of the fiber's circumference, so that the fiber experiences the shear force arising from the rotated anvil.

This can also be achieved where the device is provided with an anvil having a single arm or two non-symmetrical arms, which result in shear forces when bending and when the fiber or fibers are scored cause the cleaved end or ends to be angled. Preferably, the ends of both parts of the anvil are tipped with a compliant material to ensure contact, over a significant fraction of the fiber's circumference, between the anvil and the fiber.

In yet a further option the device can be provided with a two part anvil, the two parts being rotatable relative to one another. Thus the arms can be arranged parallel to one another and so lead to a straight cleave, alternatively the arms can be rotated relative to one another and so given uneven support leading to an angled cleave. Preferably the extent of rotation is used to control the angle of cleave.

As a further option the device can be provided with means to rotate either or both of the restraining means about the axis of the fiber following clamping so as to introduce a torsional force. The cleaved angle achieved depends upon the relative values of torsion and tension in the fiber. Specifically, for a fiber tension of 200 grams, to achieve an end angle of 5°, the holding sections must be twisted relative to each other by an amount of approximately 5° per centimetre between two points of clamping. This method also allows end angles to be cleaved onto multiple fiber ribbons held in compliant clamps. Torsion is applied to each of the tensioned fibers at the same time by rotating both of the holding sections in opposite directions. The fibers are substantially in a line at the midpoint between the two clamps and so they can be cut by the acutely angled blade. All of the fibers in the ribbon will experience the same tension and torsion and so they will cleave with similar end angles.

Preferably the tool will be provided with channels into which the fiber or fibers can be Iain to constrain them laterally during cleaving. One of these channels will be of slightly larger width than 125microns diameter of the glass fiber. A second co-linear channel will be of width slightly larger than the coating diameter of 250microns or 900microns. This second channel may have two sections of widths corresponding to a choice of coating diameters. The ends of the channels will act as stops for the stripped fiber giving predetermined cleave lengths, as might be required by specific cleaving applications. To allow the user to chose other cleave lengths, a scale will be included.

Preferably the second channel in the cleaving tool will also included a deformable rubber side or rubber insert which will temporarily trap the fiber coating in the channels prior to cleaving to ensure that the fiber coating does not move during cleaving. In addition, the tool will preferably have a removable ledge on which the length of coated fiber, and any associated cabling, can be rested and held during cleaving.

Preferably the cleaving tool will also have a fitting so that a standard holder, containing the fiber to be cleaved, can be clipped onto the tool, without the need to remove the fiber from its holder. This will give a standard cleaved fiber length, as is required by certain applications.

In yet another of its aspects the invention provides a device for use in cleaving optical fibers comprising restraining means whereby two spaced portions of a fiber can be secured, first tensioning means whereby tension can be induced and maintained in the fiber by urging the two restraining portions apart, second tensioning means whereby tension can be induced and maintained in the fiber by laterally displacing the fiber between the two restrained portions, and means adapted to hold a scoring blade in engagement with the fiber and to move the blade relative to the fiber in order to create a crack in the tensioned fiber which initiates a cleave, the first and second tensioning means being independently adjustable to determine the stresses in the fiber prior to engagement of the fiber by the blade.

Various embodiments and examples of the invention will now be described with reference to the accompanying drawings in which :-

Figures 1 and 2 are illustrations of tools which are used in prior art to perform cleaving, manufactured by York Technology Ltd, UK and RXS GmbH, Germany, respectively;

Figure 3 shows schematically an acutely angled blade scoring an optical fiber;

Figure 4 shows the experimental jig used to demonstrate the cleaving principles of this invention; Figure 5a shows an end-on view of one embodiment of the cleaving tool;

Figure 5b shows an side-on view of the open cleaving tool of Figure 5a;

Figure 6 shows the optical fiber bent by the two-armed anvil;

Figure 7a shows an optical fiber bent by both the two arms of the first anvil and the secondary anvil, hence introducing shear stresses in the optical fiber to obtain angled cleaving;

Figure 7b shows a side-view of the action of the secondary anvil in the cleaving tool;

Figure 7c shows an end-view of the lower working head of the cleaving tool showing the action of the secondary anvil;

Figures 8a and 8b show a method to obtain angled cleaving, in side-view and bottom-view, where an optical fiber is bent by a two- armed anvil, where the anvil is, respectively, perpendicular to the fiber giving perpendicular cleaves, or rotated with respect to the fiber giving angled cleaves;

Figures 9a and 9b respectively show a method to obtain angled cleaving where use of an anvil with two arms gives rise to a perpendicular cleave, whereas use of an anvil with one arm gives rise to an angled cleave; Figures 10a and 10b show a modified device for achieving angled cleaves in which parts of the anvil can be rotated relative to one another;

Figure 11 shows a method to obtain angled cleaving where the fibers may be twisted prior to cleaving to give angled cleaves or not twisted to give perpendicular cleaves;

Figure 12 shows the resultant tension present in a fiber 1 , as a superposition of the tensions arising from extending the fiber, by moving apart the clamping means, and stretching the fiber over the anvil 9 which has been advanced a distance h, where the broken lines indicate the position of the fiber l ¹ and anvil 9' before bending;

Figure 13 shows the clamping (bending by the anvil 9) of the fiber 1 arising from preferably both surfaces of the upper and lower clamping means being initially angled inwards such that further clamping force will rotate these clamping surfaces outwards, hence further tensioning the fiber; and

Figure 14 shows multiple fibers in a ribbon clamped between an upper, hard clamp and a lower, compliant pad made up of a composite of fibers and matrix.

Cleaving or fracturing of brittle objects such as optical fibers depends upon control of the stresses present within the optical fiber.

One method of stressing a glass fiber is to place it under tension, as shown in Fig. 1 , for example the cleaving tool manufactured by York Technology Limited. An optical fiber 1 is clamped in two clamps 56a, 56b, 57a, 57b and placed under tension of around 200grams (for a 125micron diameter glass fiber) using a spring 58. Contacting the fiber 1 with a sharp chopping blade 6, which is vibrating as shown by arrow A, will score the fiber and lead to a good quality cleave.

An alternative method of stressing an optical fiber, known from cleaving tools manufactured by Sumitomo, Japan and by RXS, Germany, is to bend the fiber, and consequently, the outside of the fiber will be under tension which can be used to start a fracture. Pure bending creates compressive stresses on the inside of the bend which gives rise to bad cleaved end faces. Therefore the fiber is stretched over an anvil, applying both bending and tension. Fig. 2 shows the RXS tool in which the optical fiber 1 is clamped in two clamps 59a, 59b, 60a, 60b, with one face of each clamp sprung with springs 61a, 61b so that the fiber is first clamped and then stretched over an anvil 62; further closure of the tool brings the outside of the bend of the stretched and tensioned fiber 1 into contact with a sharp blade 6 which damages the fiber, creating an initiation crack, which is propagated by the applied bending and tension forces, causing the fiber to cleave.

In performance of the present invention, the fiber is preferably both tensioned and bent, and so it will experience a stress profile which is the superposition of the tension and bending forces. Therefore, the tensile stress in the fiber will be highest on the outside of the bend where the tension and bending forces are additive, will be less in the centre where the stress is solely due to the tension and will decrease further on the inside of the bend where the tensile force and the bending forces subtract. In prior art cleaving devices, the applied fiber tension must be limited to about 200 grams to obtain a mirror-smooth cleaved surface, otherwise hackle begins to occur on the final part of the surface to cleave because the tension is too high. However, in the present method the propagating cleave will experience a decreasing applied tensile force as it crosses the fiber, and this will reduce the tendency for the cleaved surface to exhibit hackle. Consequently, compared to prior art technologies where only either tension or bending is used to stress the fiber, a higher cleaving tension can be used without the appearance of hackle, giving the advantage of smaller starter cracks and flatter cleaved end faces.

The present method can achieve a very low end angle cleave by substantially eliminating both shear stresses and torsional stresses on the fiber during the cleave in several ways. Firstly, the crack is initiated by scoring the fiber 1 with an angled blade 6, the blade edge being acutely angled at an angle α with respect to its direction of motion towards the fiber, arrow I, as shown in Fig. 3. The use of an acutely angled blade in this invention reduces the force required to score the fiber and so improves the consistency of the cleaves produced. Previous technology uses a chisel or chopping motion to create the starter crack, where the blade moves perpendicular to the fiber. The use of a perpendicular blade requires a greater force on the blade creating stresses within the fiber which lead to uncontrolled cleaving. Secondly, the present method minimises shear and torsional stresses by stretching and subsequently bending the fiber over an anvil which is split into two arms and so provides a four-point bend configuration to the fiber. Consequently shear stresses are substantially eliminated in the fiber at all points between the two parts of the anvil. Thirdly, torsion of the fiber is substantially eliminated by prevention of both sideways movements and any rocking motion of the clamps when the fiber is clamped. The present method can also cleave multiple glass fibers arranged in a ribbon form. This is achieved by clamping each fiber in the ribbon by using compliant clamping surfaces to accommodate any slight variation in the diameter of the glass fibers. The compliance can be achieved by using a clamping surface which is a fiber/matrix composite, in which the compliance is controlled by the properties of the matrix material, such as an epoxy, whereas the crush resistance is determined by the presence of hard, tough fibers, such as Kevlar, in the composite which resist the crushing forces of clamping onto the glass fiber. Current technologies achieve a compliant clamping surface by using a rubber-like surface which is easily damaged by the clamping forces, and consequently has a limited operational life.

Optionally, angled ends can be created on the glass fibers. Firstly, this can be achieved by either rotating the anvil so that the fibers or by rotating the two parts of the anvil in opposite directions, where the end of the anvil is tipped with a compliant material. Both of these methods give non-symmetric stresses in the fibers and lead to angled cleaving. Current devices, using a rotation of the anvil, are only able to cleave angled ends onto a single fiber.

Secondly, this can be achieved by using a secondary anvil to give shear forces in the optical fiber or fibers which give rise to angled ends on cleaving. The secondary anvil can be moved in opposition to the first anvil to bend the fibers, introducing shear forces which will give angled ends on cleaving. Alternatively, the secondary anvil can be moved to bend the fibers in a direction perpendicular to the first anvil, giving shear stresses and angled cleaved ends. The secondary anvil can also be used to roll the fibers, hence giving rise to torsion and angled cleaved ends. In particular, the controlled action of the secondary anvil simplifies the production of angled ends. Furthermore, the direction of motion of the secondary anvil allows the orientation of the angled cleaved end to be altered. Current angled cleaving devices are not able to alter the orientation of the angled ends.

Thirdly, this can be achieved by rotating the clamps so that the fibers experience torsion. Cleaving will lead to angled ends. Current devices can produce angled cleaves for single fibers but not for many fibers in a ribbon simultaneously.

The cleaving performance of the tool will now be described in detail regarding results obtained with an experimental set-up and followed by a description of a number of tools and features of the tools which incorporate the above principles into a tool for achieving such results.

Optical fibers were cleaved using an experimental jig as described below. Standard telecommunications fiber was used, comprising

125micron diameter single mode silica fiber, where its acrylate buffer had been stripped off to a length of about 30 millimetres to allow each cleave to be carried out.

An experimental jig was constructed as shown in Fig. 4, whereby the stripped optical fiber 1 was clamped between two holding sections 63a, 63b and one holding section 63b was urged away from the other by the application of a force originating from a weight W via a pulley system. The holding sections 63a, 63b were restrained from moving apart because of the optical fiber 1 held therebetween, consequently inducing a tension in the optical fiber. In addition, in the region between the two holding sections, the optical fiber was passed over a two-armed anvil 64, such that the fiber was bent upwards by a controlled amount h, where h could be varied by changing the height of the anvil 64 relative to the plane X of the optic fiber. In this way the optical fiber was both tensioned and bent. Scoring of the optical fiber 1 was then achieved using a diamond blade 6. The blade used was a 1.6mm long polished diamond blade incorporating a 60° included angle, obtained from Drukker International, Holland.

The blade 6 was mounted in a Vickers hardness indenter which provided a damped motion of the blade 6 travelling towards the fiber 1 , such that the blade contacted the tensioned and bent fiber at a point between the two arms 65a, 65b of the anvil 64. The blade 6 was mounted so that its edge moved at an acute angle towards the fiber 1 and its direction of motion was adjusted so that the edge of the blade was always parallel to the surface of the anvil.

Cleaves were carried out using different values of the following variables.

1) The weight W used for tensioning was varied between 200grams and 340grams.

2) The distance d between the inside edges of the holding sections 63a, 63b was 18mm; a 12mm spacing was also tested.

3) The angle of the blade with respect to its direction of motion, the angle a in Fig. 1 , was varied between 90°, where the blade edge was perpendicular to its direction of motion, and 8° where it was nearly tangential. 4) The height of the anvil h was varied between lOOmicrons and 570microns.

The cleaves produced were observed in an optical microscope at 200x magnification with the cleaved fiber end mounted vertically in a V- groove inscribed on the side of a steel block and constrained with a flexible magnet. The end angles obtained were assessed using a 20x interference microscope objective (obtainable from Nikon, UK) , operating in green light, which gave 8 interference fringes for a cleaved 125micron fiber end with a 1° end angle..

The best cleaves of 125micron diameter fiber were obtained for a tension of 200 grams. For higher tension, hackle appeared at the circumference of the cleaved fiber end, disrupting the mirror surface of the cleaved fiber. For a tension of 340grams, the hackle extended

20microns inwards from the outside of the fiber.

For a holding section separation d of 18mm, the best cleaves were obtained where the anvil laterally displaced the clamped and tensioned fiber by a distance h of about 400-450microns. For larger anvil displacements of 570microns, circular features were seen across the cleaved fiber end, indicative of microsteps on the cleaved surface. For smaller anvil displacements of lOOmicrons, cleaving did not take place until the blade had been withdrawn, indicating that the blade had placed the fiber under compression at the point of scoring, such that the fiber did not experience net tension, and hence cleave, until the blade had been withdrawn. Such cleaves were characterised by cleaved faces with starter cracks extending up to 30microns into the fiber. The effect of the angle a of the blade's motion was then investigated, using a clamp separation d of 18mm. The best cleaves were obtained for blade angles as small as possible. A 90° (perpendicular) blade gave a starter crack which was approximately lOmicrons deep. A 45° blade gave a 5micron deep crack and a blade at a angle of 15° or less gave a starter crack which was approximately 2microns deep.

From these results, it can be seen that blade motions which are nearly perpendicular to the fiber, i.e. those that use a chisel action, exhibit significantly larger starter cracks than near-tangential blades. This is probably because the perpendicular blade tends to compress the fiber at their point of contact. Conversely, the action of the lower angle blade can be described as more of a scratching action, and this benefits from stress concentration at the trailing edge of the blade. Consequently, an acutely angled blade can be seen to be better suited to scoring the fiber than a perpendicular blade.

The separation of the holding section was also investigated. For a clamp separation d of 12mm, the best cleaves were obtained for an anvil displacement h of around 200-250microns. Smaller anvil displacements gave poor cleaves with large initiation cracks whereas larger displacements gave cleaves with circular features indicative of microsteps on the cleaved surface. The optimum fiber tensions were again found to be around 200grams.

It can be seen from the above that the anvil displacement required to achieve a good cleave is smaller for a smaller clamp separation. We may therefore expect that for a very small clamp spacing, an anvil will not be necessary. In the above experiment, a reduction in clamp spacing by a factor of 2/3 leads to a required anvil displacement which is reduced by a factor of 1/2. 'Therefore, we would expect that a clamp spacing of around 1.58mm would require an anvil displacement of 6.25microns which is of the same order as the blade damage. Under these conditions, therefore, the anvil would not be required, and the fiber would cleave without significant blade damage when held between two holding sections only l-2mm apart using an acutely angled blade.

The end angles of the cleaves produced were investigated by using a microscope to view the cleaved ends which were held vertically under an interference lens, where the separation d of the holding sections was 12mm, the tension applied was 200grams and the displacement of the tensioned fiber by the anvil was 200microns. The end angles achieved from the cleaves were all in the range of 0.25° to 0.75°. These results showed that cleaved ends could be achieved with end angles which were consistently less than 1.0°.

The experimental jig was also modified to be used to study and evaluate the generation of cleave end angles onto an optical fiber. A secondary anvil 66 was introduced to provide a controlled bending force on the fiber at a point between the arms 65a, 65b of and in opposition to the first anvil 64. A clamped fiber 1 tensioned to 200 grams was initially displaced by the first anvil by a distance h of around 400microns, as described above, where the clamp separation d was 18mm and the anvil was centrally located between the clamps 63a, 63b, with a distance between the centre of its two arms 65a, 65b of approximately 2.5mm. A secondary anvil 66, consisting of a narrow edge 67 pressed down upon by a known weight 68, was used to apply a controlled bending force to the fiber in opposition to the bending force applied by the first anvil 64, acting downwards at a point on the fiber approximately equidistant between the two arms 65a, 65b of the first anvil. With the fiber bent by both the first and secondary anvil, the diamond blade 6 was scored against the fiber at a point very close to the secondary anvil and between the points of contact between the fiber and the two arms of the first anvil 65a, 65b. The arrangement of the primary and secondary anvils are shown in Fig. 4.

A weight 68 of 27grams was applied to the secondary anvil 66 and a number of cleaves were obtained with an average end angle of approximately 6.4°, whilst a number of cleaves carried out with weights of 20grams and 12grams gave average end angles of approximately 3.9° and 2.5°, respectively. The end angles were consistent to approximately + /-1°. It would be expected that accurate control of the positioning of the secondary anvil and of the position of its contact with the fiber would decrease the variation in end angle achieved for a given force applied. Furthermore, the end angle achieved would be expected to increase for an increase in the bending force applied by the secondary anvil. We may conclude, therefore, that the application of a force via a secondary anvil to provide shear stresses in the fiber at the point of cleaving is efficient in cleaving controllable end angles onto the end of an optical fiber or fibers in the desired range of 4°-10°.

The results above particularly concern cleaving of optical fiber whose diameter is 125microns. However, this invention can be used to cleave optical fiber over a range of diameters and results above can readily be adapted to other diameters. The tensions required to propagate the cleave will be greater for larger fiber diameters. Furthermore, similarly, the fiber displacement bringing about bending will also be different. However, applying the principles of both tensioning and bending to the fiber or fibers, followed by scoring the fiber or fibers with a sharp blade which is acutely angled with respect to its direction of motion, flat cleaves with a mirror smooth surface, either perpendicular or angled are expected to be achieved over a range of fiber diameters.

A tool which has been constructed according to the cleaving principles which have been determined from the experimental results above, is now described.

Fig. 5a shows an end-on view and Fig. 5b a side-on view of the tool which embodies this invention. A length of optical fiber 1 which has been stripped of its coating 2 is placed in the tool.

The tool consists of a bottom block 3 and a middle plate 4, which are brought together by rotation around a hinge 5 by an operator in use, hence clamping, tensioning and bending the optical fiber 1.

The middle plate 4 acts as a solid mount for a steel clamp spring 7 whose bottom surfaces 8a, 8b serve to clamp the optical fiber 1 when the tool is closed. The clamp spring 7 contains a central hole through which passes an anvil 9 and an anvil bush 10. As the clamp spring 7 is compressed, it extends laterally and therefore stretches the clamped optical fiber 1, so placing it under tension, as required for effective cleaving. Subsequently, the anvil 9 comes into contact with, and serves to bend, the clamped and tensioned optical fiber 1 towards the bottom block 3.

The clamp spring 7 is secured to the middle plate 4 by being clamped between a tilting plate 11 and a securing fastener 12. The orientation of the clamp spring 7 can be adjusted by tilting the plate 11 using one of the adjustment screws 13a, 13b in order to ensure that the clamp spring 7 clamps the fiber 1 simultaneously with both of its legs. The bottom block 3 contains two hardened, ground and polished steel roller blocks 14a, 14b whose upper surfaces 15a, 15b serve to clamp the optical fiber 1. The roller blocks 14a, lb are free to rotate around their ground edges 16a, 16b which are pressed into a location plate 17 which has matching grooves 18a, 18b.

The tool is operated by pressing down on the handle plate 19a which rotates around a hinge 19b and compresses the handle spring 20a, 20b, 20c, where the two outside fingers of the handle spring 20a, 20b exert a downwards force on the middle plate 4. This compresses the tool opening spring 21 which allows the middle plate to move downwards, trapping and clamping the optical fiber 1 between the bottom surfaces 8a, 8b of the clamping spring 7 and the top surfaces 15a, 15b of the rollers 14a, 14b. As the handle plate 19a is further closed, the handle spring 20a, 20b exerts a greater downward force, causing the clamping spring 7 to extend laterally, stretching and hence tensioning the clamped fiber 1 ; the rollers 14a, 14b rotate about their ground edges 16a, 16b to accommodate this movement. The extension of the fiber brought about is approximately 2 parts in 1000, and this is equivalent to applying the 200gramforce of tension found to be optimal for cleaving in the experiments described above.

As the handle plate 19a is closed further, the centre part of the handle spring 20c forces the anvil 9 downwards to contact the clamped and tensioned fiber 1. The anvil 9 is prevented from moving too far and applying more than the required deflection of the optical fiber 1 by its attachment to an anvil plate 22 and an adjustment screw 23 which comes into contact with the top surface of the anvil bush 10. An anvil return spring 24 is also provided. The end of the anvil 9 is slotted, being comprised of two arms, 25a, 25b, which contact the optical fiber 1 at two points and therefore place it under a four-point-bend condition and consequently reduce the effect of any torsion or shear forces in the optical fiber 1 , as sketched in side-view in Fig. 6.

The diamond blade 6 is mounted in a bracket 26a which is pivoted around a pin 27a, 27b; the bracket has an extension 26b and a return spring 26c. The shaft of the pin 27a acts as the pivot on which the bracket 26a is mounted and around which the bracket 26a, and hence the blade 6, rotates. At one end the pin also has a shoulder 27b which is slightly eccentric to the shaft of the pin; this eccentric shoulder 27b is used to mount the pin in a pivot holding block 28. Rotating the pin around its shoulder 27b will raise or lower its shaft 27a and hence the bracket 26a and the blade 6, relative to the pivot holding block 28. This has the effect of setting the height of the blade 6, ensuring that the blade 6 will make contact with and score the fiber 1 to the desired extent. The angular position of the pivot pin 27, and hence its height, is fixed by a locking screw, 29.

The bracket has an extension 26b, on the end of which is attached an adjustment screw 30. A vertical rod 31 is passed through a bush 32 such that it is free to slide vertically in the bottom block 3 but its downward movement is limited by its head coming to a stop against the top surface of the bush 32. When the tool is open, a spring 33a is provided to urge the rod 31 upwards and a collar 33b is present to restrain the rod 31 in the bush 32.

An activation rod 34, mounted on the underside of the handle plate

19a, acts as the activation for the blade 6 to cut the tension and bent optical fiber 1. Closure of the handle plate 19a, causes the activation rod 34 to press down onto the top of the vertical rod 31 which travels downwards and makes contact with the screw 30 which has the effect of rotating the diamond blade 6 around its pivot pin 27a to bring it into contact with and score the clamped, tensioned and bent optical fiber 1. The screw 30 is adjusted to ensure that it is always contacted by the rod 31 when the tool closes, hence moving the blade 6 to score the optical fiber 1 a predetermined distance. The edge of the blade 6 will be at an acute angle to its direction of travel, defined approximately by the magnitude of angle 35 shown in Fig. 5b, and will therefore score the optical fiber 1 gently at an acute angle, causing it to cleave.

When the blade 6 has cleaved the tensioned and bent fiber 1 , the handle plate 19a and the handle spring 20a, 20b, 20c are released. The springs 24 and 21 will, respectively, raise the anvil 9 and the middle plate 4, opening the tool and allowing the cleaved fiber 1 to be removed.

In addition to the height setting obtained from the eccentric shoulder on the pivot pin 27b, the pivot holding block 28, the pivot pin 27a, b, the bracket 26a, b and hence the blade 6, can also be translated in order to move a new part of the blade 6 into contact with the optical fiber 1 during cleaving, as will be required as the blade becomes blunt or chips during use. This has the effect of using several positions on the blade 6 for scoring the fiber, significantly extending the tool's life. The pivot block 28, in which is mounted the pivot 27a, b, the bracket 26a, b and hence the blade 6, is threaded to accept a screw 36. The screw 36a has a shoulder on its end so that it can be positioned in the bottom block 3 such that it is able to rotate; it is securely held by a fastener 36b. When the screw 36a is rotated it will translate the pivot block 28 parallel to the edge of the blade 6 to bring a new part of into use. A modified version of the tool can also cleave multiple fibers arranged in a ribbon, where a ribbon may be made up of between 1 and

16 fibers or more. In order to clamp all of the fibers simultaneously, one or both of the clamping surfaces 8a, 15a and similarly 8b, 15b are coated with a compliant material such as a fiber-matrix composite. This is shown in Fig. 14 where the glass fibers 1 are clamped above by the bottom surfaces 8a, 8b of the clamp spring 7 and below by the compliant pad 71 , comprising of a composite of unidirectional fibers 72, such as

Kevlar, and filler matrix material 73, such as epoxy. As the tool is closed, the clamp spring 7 and the pivoted steel blocks 14a, 14b come together so that the compliant clamping surfaces contact all of the fibers in the ribbon, so clamping them with a substantially equal force.

Each fiber in the ribbon is tensioned by a combination of the fiber extension arising from the extension of the clamp spring 7 with which each fiber in the ribbon is clamped and the displacement of the each fiber by the anvil 9. In the ribbon tool, preferably the tension arising from the displacement of the fibers by the anvil is substantially greater than that arising from the extension of the clamp spring 7. Because the tension in each fiber due to its displacement by the anvil is independent of any other fibers in the ribbon, as each fiber in the ribbon is cleaved, the tension in the remaining fibers is not significantly increased and so they are cleaved in their turn under substantially equal tensioning conditions.

To initiate cleaving, the blade 6 is rotated around its pivot 27a so that its edge contacts all of the fibers in the ribbon at substantially the same time, scratching all of them and causing each of them to cleave. For ribbons with more than 4 fibers, the length of the blade must be increased so that all of the fibers can be contacted at the same time; a 12 fiber ribbon will require a blade which is approximately 3mm long or greater and may be made from diamond, a tough ceramic such as Zirconia or some similar hard and tough material.

The tool here described, in its single or multiple fiber form by suitable design, is able to cleave angled ends onto optical fiber or fibers 1 , with end angles in the range of 1° to 15° and preferably in the range of 4° to 10°. This is achieved by incorporating shear stresses into the fiber. Considering the cleaving of a single fiber in the single fiber tool. As the middle plate 4 is lowered, the optical fiber 1 is clamped and tensioned by the clamp spring 7 and bent by the movement of the anvil 9. However, before the optical fiber 1 is scored with the blade 6, a secondary anvil 38 is applied in direction of arrow C, bending the fiber in the opposite direction to the first anvil 9, as shown in Fig. 7a, and so introduces shear forces into the fiber. When the fiber is scored by the blade on the outside of a bend at a point 37, it cleaves with an angled end.

The secondary anvil 38 consists of a narrow tip 39, mounted on a pivot 40, with an extension arm 41 and a spring activation element 42, as shown in side-view and end-view in Figs. 7b, 7c, respectively. The secondary anvil is activated by depressing the spring 42 which causes the anvil 38 to rotate around its pivot 40 to bring its tip 39 into contact with the fiber 1 , bending it with a force F related to the strength of the spring 42.

To bring about angled cleaving, a lever 43 is moved in the direction of arrow II to introduce a shim 44 underneath a shoulder 45 on the vertical rod 31. Subsequently, the middle plate 4 is lowered and the fiber 1 is clamped and tensioned by the clamp spring 7 and bent by the anvil 9. The activation rod 34 pushes down on the vertical rod 31 such that its shoulder 45 contacts the shim 44, bending the activation spring 42 which causes the secondary anvil 38 to rotate, exerting a shear force F in the optical fiber 1. Further closing the tool will further lower the vertical rod 31 which will cause it to contact the screw 30 and so rotate the bracket 26a, b and hence cause the blade 6 to score the fiber 1 , giving an angled cleave. When the lever 43 is not applied, the shim 44 is not in position, and so the shoulder 45 of the vertical rod 31 does not contact or exert a force on the activation spring 42. Therefore the fiber 1 does not experience any shear forces and so cleaves perpendicularly.

In the case of angled cleaving of single or multiple optical fibers using the multiple fiber tool, the top edge 39 of the secondary anvil 38 is preferably tipped with a compliant material 46 such as a hard rubber; the spring element 42 is replaced by a continuation of the extension arm 41. For angled cleaving, the shim 44 is introduced by the lever 43 so that on closing the casing element 3a, the vertical rod 31 brings about a known displacement of the end of the extension arm 41. This displacement rotates the secondary anvil 38 around its pivot so that its edge 39 is moved a controlled distance towards and into contact with the fibers 1 , with the compliant material 46 on its tip coming into contact with each fiber in the ribbon. The compliant material 46 is compressed by a given amount at its point of contact with each fiber, so applying a bending force F to each fiber in the ribbon which will be equal and independent. Therefore, each fiber in the ribbon will experience the same stresses and will be cleaved by the blade 6 with an equal end angle. Both angled and perpendicular cleaves can be achieved from this tool respectively, either by applying or not applying the secondary anvil 38 using the lever 43.

Another way of creating cleaved ends which are angled is to apply non-symmetric bending to the fiber when it is clamped and tensioned. This can be achieved by rotating the anvil 9 so that its two arms 25a, 25b are not perpendicular to the fiber of fibers, as shown in side- and bottom- views in Fig. 8b. A lever 47 is rotated in the direction of arrow III so that the arms of the anvil 9 are either perpendicular to the fibers 1 (Fig. 8a) , giving perpendicular cleaves, or at an angle to the fibers 1 (Fig. 8b) , giving angled cleaves. Preferably the tip of the two arms 25a, 25b are coated with a compliant material such as rubber.

A further way of cleaving angled ends is to bend the clamped and tensioned fiber or fibers 1 over an anvil 9 which has only one arm, with the blade 6 scoring the fiber or fibers at a point 48 away from the point of application of the anvil 9. This is achieved in Fig. 9 by using a lever 49 to withdraw one arm of the anvil 25a in the direction of arrow IV, with the fiber bent over the remaining arm 25b. The fiber is scored at a point 48 to give angled cleaves, Fig.9b. When the lever 54 is not applied, the anvil 9 possesses two arms 25a, 25b and gives rise to a perpendicular cleave, Fig.9a.

An alternate way of applying non-symmetric bending to cleave angled ends on a clamped and tensioned fiber is to bend the fiber 1 over an anvil 9 which has non-parallel arms 25a, 25b, as shown in bottom view in Fig.10b, preferably whose ends are tipped with a compliant material such as rubber. A separation wedge 48 is applied in the direction of arrow V forcing apart the two annular sections 49a, 49b, so that they rotate around the central pin of the anvil 9. Consequently the arms of the anvil 25a, 25b are rotated with respect to each other and the fibers 1 , leading to shear stresses in the fibers and angled cleaved ends. A spring 50 ensures that the arms of the anvil 25a, 25b, are parallel when the separation wedge is not engaged, as shown in Fig 10a leading to perpendicular cleaved ends. In the case of angled cleaving the shear stresses should consequently be in the plane of fiber ribbon whereas the previously discussed angled cleaving techniques should give shear stresses perpendicular to the plane of the fiber ribbon and perpendicular to the fibers. Consequentially, the angled plane of cleavage is rotated by 90 degrees compared with previously discussed angled cleaving techniques which may offer significant benefits with regard to polarized light and other applications.

Yet another way of cleaving angled ends is to apply torsion to the fiber or fibers 1 in addition to the tension and bending forces, Fig 11. This can be achieved by clamping, tensioning and bending the fiber or fibers 1 by closing the tool as above, and then rotating the clamps with respect to each other before the fiber or fibers 1 are scored with the blade 6. The ground edges 16a, 16b, around which the steel rollers 14a, 14b rotate, are located in two location plates 69a, 69b, with matching grooves 70a, 70b. The opposite ends of the location plates are rested on sharp edges 51a, 51b, around which they can rotate in the direction of the arrows VI, as shown in Fig. 11. The other ends of the location plates are supported on springs 53a, 53b. As the tool is closed, the fiber 1 is clamped onto the clamping faces 15a, 15b and the springs 53a, 53b are progressively compressed, rotating the location plates 69a, 69b such that the faces of the rollers 15a, 15b are rotated in opposite directions, hence rotating the clamped and tensioned fiber or fibers 1. The total angular rotation of the clamps, and hence the end angles cleaved, are controlled by stops 54a, 54b which prevent further rotation of the clamping faces 15a, 15b. Cleaving is brought about by scoring the fiber or fibers with the blade 6, giving an angled cleave when the clamping faces 15a, 15b are rotated. This tool is also capable of cleaving perpendicular ends by preventing the rotation of the clamping faces by introducing shims 55a, 55b in the direction of arrows VII between the bottom faces 52a, 52b of the location plates and the stops 54a, 54b. The ribbon version of this tool will cleave angled or perpendicular ends onto single or multiple optical fibers.

All of the above functions of the tool can be achieved by operating the tool as a benchtop tool, where the lower block 3 is placed on a bench and the handle plate 19a and middle plate 4 are pressed downwards to clamp, tension, bend and score the fiber or fibers 1 , hence cleaving them. Alternatively, the tool can be mounted on a tripod using a fixing in the lower block 3 such as a thread which is compatible with a camera tripod. Alternatively, the tool can be held in the hand and squeezed together to clamp, tension, bend and score the fiber or fibers 1 , hence cleaving them.

Removal of the offcuts of fiber or fibers remaining after the fibers have been cleaved can optionally be achieved by electrostatic attraction between the fiber offcut and a substantially electrically insulating material such as rubber or plastic, whereby optical fibers naturally acquire an electrostatic charge after exposure to the atmosphere for a few seconds. Means can be provided to remove the fiber offcut or offcuts from the tool, whereby a substantially electrically insulating material is pressed gently onto the fiber offcut which electrostatically sticks to the insulating material and is therefore easily removed from the cleaving tool. Preferably the insulating material is in the form of a smooth rubber pad and is preferably made of silicone rubber with a Shore A hardness of between 20 and 70.

Claims

1. A method of cleaving an optical fiber comprising the steps of restraining two spaced portions of the fiber, inducing and maintaining tension in the fiber between the two restrained portions, and scoring the tensioned fiber with an edge of a blade in order to create a crack in the fiber which initiates a cleave, the blade being moved relative to the fiber in a direction which makes an acute angle with the edge in the plane of the blade.

2. A method according to claim 1 in which the step of tensioning the fiber comprises urging the two restrained portions apart.

3. A method according to either of claims 1 and 2 in which the step of tensioning the fiber comprises laterally displacing the fiber between the two restrained portions.

4. A method according to any one of claims 1 to 3 in which the step of tensioning the fiber is completed prior to engagement of the fiber by the blade.

5. A method according to any one of claims 1 to 4 wherein prior to scoring with the acutely angled blade, a primary anvil is used to displace the fiber laterally, hence stressing the fiber by bending and stretching, the anvil acting to resist any tendency of the blade to deflect the fiber.

6. A method according to claim 5 wherein the anvil engages the fiber at two spaced locations between which the blade scores the fiber.

7. A method according to any one of claims 1 to 6, wherein stresses are induced in the fiber or fibers by both tensioning and bending means where these are applied independent of each other.

8. A method according to any one of claims 1 to 7 wherein the angle between the edge of the acutely angled blade and the direction of motion of the blade towards the fiber is not more than 45°.

9. A method according to any one of claims 1 to 8 wherein contacting surfaces of clamps are made of a hard material, such as hardened steel, in order to grip a single optical fiber at the restrained portions.

10. A method according to claim 9 wherein one or both the contacting surfaces of one or both of the clamps are contoured so that they clamp the fiber securely.

11. A method according to any one of claims 1 to 8 wherein one or both contacting surfaces of clamps is made of or coated with a material which is compliant.

12. A method according to claim 11 wherein the compliant contacting surfaces of the clamps are made of or coated with a composite material which is comprised of both hard material and compliant material.

13. A method according to claim 12 wherein the composite compliant contacting surfaces of the clamps are made of or coated with fibers, made of Kevlar, glass, carbon or otherwise and whereby the compliant material is made of rubber, epoxy, plastic, thermoplastic or otherwise.

14. A method according to any one of claims 1 to 13 wherein one or both of the contacting surfaces of one or both of the clamps are free to move apart to extend the fiber or fibers held therein thereby providing tensioning.

15. A method according to claim 14 wherein one or both of the contacting surfaces of one or both of the clamps are pivoted to enable their movement.

16. A method according to any one of claims 1 to 15 wherein tensioning means comprises one or more spring sections which are caused to expand, so extending the distance between clamping sections providing tension to the fiber or fibers.

17. A method according to any one of claims 1 to 16 wherein clamping means and tensioning means are the same.

18. A method according to any one of claims 1 to 17 wherein the fiber or fibers are scored at a point away from the point or points of application of an anvil, where the anvil may be single- or multi-armed, so creating shear forces therein, leading to controlled cleaved end angles in the range of 1° to 15° and preferably in the range of 4° to 10°.

19. A method according to any one of claims 1 to 18 wherein a primary anvil is rotated, so creating shear forces in the fiber or fibers, leading to a controlled cleaved end angle in the range of 1° and 15° and preferably in the range of 4° and 10°, where the anvil is tipped with compliant material.

20. A method according to any one of claims 1 to 19 wherein an anvil is rotated so that its two arms are non-parallel to each other, so creating shear forces in the fiber or fibers, leading to controlled cleaved end angles in the range of 1° to 15° and preferably in the range of 4° to 10°.

21. A method according to any one of claims 1 to 20 wherein holding means is twisted so as to create a torsional stress in the fiber or fibers which are then scored to produce controlled cleaved end angles in the range of 1° to 15° and preferably in the range of 4° to 10°.

22. A method according to any one of claims 1 to 21 wherein a secondary anvil is caused to act on the fiber or fibers in opposition to and in the region of a first anvil, creating further bending of and producing shear forces in the fiber or fibers, and subsequent to which the blade is introduced, acting to score the fiber or fibers at a point where they are under tension, so producing controlled cleaved end angles in the range of 1° to 15° and preferably in the range of 4° to 10°, where the secondary anvil may be stationary whereby the fibers are bent over it by the primary anvil, or where the secondary anvil may be moved against the fiber or fibers either before or after they have been displaced by the primary anvil.

23. A method according to any one of claims 1 to 22 wherein a secondary anvil is introduced, acting on the fiber or fibers with a component of motion lateral to a primary anvil, such that the fibers are displaced laterally with respect to the primary anvil and so experience a component of shear such that they cleave at an angle when they are score with the blade.

24. A method according to any one of claims 1 to 23 wherein a secondary anvil is caused to act on the fiber or fibers with a component of motion lateral to a primary anvil, such that the fibers are rolled and so experience a component of torsion such that they cleave at an angle when they are scored with the blade.

25. A method according to either of claims 23 and 24, wherein the secondary anvil acts on the fiber or fibers such that they are both laterally displaced and rolled giving rise to cleaved ends which are angled in a controlled orientation with respect to the direction of movement of the primary and secondary anvils.

26. A method according to any one of claims 22 to 25 wherein the secondary anvil is sprung so that it presses on or rolls the optical fiber or fibers with a controlled force to achieve a cleave with a controlled end angle in a controlled orientation.

27. A method according to any one of claims 22 to 25 wherein the secondary anvil is coated with a compliant material and advanced to a stop so that the deformation of the compliant material exerts a substantially equal force on all of the fibers, giving rise to cleaves with a controlled end angle in a controlled orientation.

28. A device for use in cleaving optical fibers comprising restraining means whereby two spaced portions of a fiber can be secured, tensioning means whereby tension can be induced and maintained in the fiber between the two spaced portions, and means adapted to hold a scoring blade in engagement with the fiber and to move the blade relative to the fiber in order to create a crack in the tensioned fiber which initiates a cleave, the blade-holding means being arranged to move the blade in a direction which in the plane of the blade makes an acute angle with the edge of the blade.

29. A device according to claim 28 designed to produce cleaved ends which are damage-free and mirror-flat on which any damage arising from the blade does not intrude significantly into the fiber, comprising: two sections which are urged together by rotation around a pivot, hence sequentially clamping, tensioning, bending and scoring the fiber or fibers; a pair of spaced holding means for clamping and supporting a linear section of optical fiber or multiple fibers therebetween, whereby each of the pair of holding means consists of a one half portion which is urged towards a second half portion so that they trap the fiber or fibers therebetween and provide a positive clamping action which will hold the fiber or fibers when they are subjected to a linear tension; a spring means for clamping and extending the section of said fiber or fibers along its or their longitudinal axis so tensioning them, without bending, twisting or otherwise disturbing its or their linear state; a following means which acts to clamp the fiber or fibers, but which moves under the influence of the above spring means, where the following means may be two pivoted rollers or may be a second spring means; a movable scribing blade free to rotate around a pivot, whose edge is caused to contact the fiber or fibers at a glancing angle, scoring them to produce a crack in each of them, initiating cleaving, whereby the blade has an edge which is acutely angled with respect to its direction of motion to produce a scratching action; so causing cleaving of said tensioned and displaced fiber or fibers to produce cleaved end angles of less than 1°.

30. A device according to claim 28 comprising holding means which are separated by a distance in the range of 0.25mm- 100.0mm such that the optical fiber does not significantly bend under the influence of the blade scoring the fiber and therefore does not substantially place the fiber under compression at the point of contact of the blade therewith, and therefore leads to a substantially damage free cleaved end face with an end angle of less than 1°.

31. A device according to claim 28 comprising anvil means for displacing the said fiber or fibers perpendicular to its or their axes, prior to scoring of the fiber with the blade, imparting a force which acts to bend and stretch the fiber or fibers and which acts to resist any tendency of the blade to place the fiber under compression at the point of scoring the fiber.

32. A device according to claim 31 wherein the anvil means is split into two parts, comprising two substantially parallel edges which run perpendicular to the longitudinal axis of the fiber of fibers, so creating the conditions for a four-point bending action giving rise to the substantial elimination of any shear stresses in the fiber at all points between the two parts of the anvil and whereby the blade scores the fiber therebetween.

33. A device according to claim 28 wherein the angle between the edge of the acutely angled blade and the direction of motion of the blade towards the fiber is not greater than 45°.

34. A device according to claim 33 in which the angle is in the range of 2° to 45°.

35. A device according to claim 28 incorporating clamping surfaces, one or more of which comprises a single or multiple V-grooves.

36. A device according to claim 28 wherein the restraining means incorporates compliant clamping surfaces, which may be comprised of a compliant material such as rubber or may be comprised of a fiber-matrix composite.

37. A device according to claim 28 wherein discretionate means are incorporated such that the fiber or fibers can be scored at a point away from the point of application of an anvil, where the anvil may be single- or multi-armed, and which are subsequently cleaved, leading to a controlled end angle in the range of 1° and 15° and preferably in the range of 4° and 10°.

38. A device according to claim 28 wherein discretionary means are incorporated such that a primary anvil is rotated, so creating shear forces in the fiber or fibers, leading to a controlled cleaved end angle in the range of 1° and 15° and preferably in the range of 4° and 10°, where the anvil is tipped with compliant material.

39. A device according to claim 28 wherein discretionate means are incorporated such that an anvil can be rotated so that two arms are non- parallel to each other, so creating shear forces in the fiber or fibers, leading to a controlled cleaved end angle in the range of 1° and 15° and preferably in the range of 4° and 10°, where the anvil is tipped in compliant material.

40. A device according to claim 28 wherein holding sections are discretionately rotated relative to each other to apply twist to the fiber or fibers which are subsequently cleaved, leading to a controlled end angle in the range of 1° and 15° and preferably in the range of 4° and 10°.

41. A device according to claim 28 wherein a stationary or movable secondary anvil is discretionately caused to act on the fiber or fibers in the region of a first anvil further to bend the fiber or fibers, so creating controllable shear and/or torsional forces therewithin, and which are subsequently cleaved to create a controlled end angle in the range of 1° and 15° and preferably in the range of 4° and 10° onto the fibers, where the secondary anvil may be sprung and/or tipped with a compliant material.

42. A device according to claim 28 being in the form of a hand-held tool.

43. A device according to claim 28 being in the form of a bench-top tool.

44. A device according to claim 28 comprising means forming a threaded hole so that it can be mounted on a tripod or other holding means.

45. A cleaving method substantially as hereinbefore described.

46. A cleaving device substantially as hereinbefore described.

47. A device for use in cleaving optical fibers comprising restraining means whereby two spaced portions of a fiber can be secured, first tensioning means whereby tension can be induced and maintained in the fiber by urging the two restrained portions apart, second tensioning means whereby tension can be induced and maintained in the fiber by laterally displacing the fiber between the two restrained portions, and means adapted to hold a scoring blade in engagement with the fiber and to move the blade relative to the fiber in order to create a crack in the tensioned fiber which initiates a cleave, the first and second tensioning means being independently adjustable to determine the stresses in the fiber prior to engagement of the fiber by the blade.