Machine and method for polishing optical fibre end surface

Abstract: A ferrule (1) is held to a ferrule-holding jig (2) such that the ferrule (1) is tilted at an angle of Θ to the axis of rotation of a polishing platen (3). The ferrule (1) is pressed against a polishing sheet (5) lying on the polishing platen (3) via a resilient body (4). The polishing platen (3) is so tilted that the angle between the axis of the ferrule and the normal to the polishing platen increases by Δ from Θ.

Description: The present invention relates to a method and apparatus for polishing the end surface of an optical fibre. In particular, it relates to an optical fibre end surface-polishing machine and method for polishing an end surface of an optical fibre, which is for example used in optical fibre communications, into an oblique convex spherical surface, as per the preamble of claims 1 and 4. An example of such a method and apparatus is disclosed by US 5 351 327 A. Optical connectors used in optical fibre communications are required to have small insertion loss and produce minimal reflected, returning light. Various proposals have been heretofore made to satisfy these requirements simultaneously. The most predominant optical connector which meet these requirements best at this time is an optical connector having a ferrule end surface which has been polished together with an end surface of an optical fibre into a convex spherical surface at an angle to a plane that is vertical to the axis of the optical fibre. This connector is normally known as "oblique PC connector". This oblique angle is so determined that it makes a certain normalized angle Θ to the plane perpendicular to the axis of the optical fibre. In order to reduce the insertion loss and to reduce the reflected, returning light, the optimum angle of the normalized angle is selected, for example, to be 8 degrees, 10 degrees, or 12 degrees, depending on the kind of the optical fibre. In the oblique PC connector, this normalized angle Θ is the angle Θ made between the tangent plane at the intersection of the axis of the optical fibre and the convex spherical surface and the plane perpendicular to the optical fibre, as shown in Figure 2 of the accompanying drawings. The end surface of this connector has been heretofore formed in the manner described below. The prior art method is illustrated in Figure 3 of the accompanying drawings. As shown in Figure 3(a), a ferrule to be polished is pressed against the grinding wheel disk whose surface is flat in such a way that the ferrule is tilted at a given angle of , thus performing oblique polishing. Then, as shown in Figure 3(b), the ferrule is pressed against a grinder while maintaining the angle , to polish the ferrule. The grinder comprises a flat platen on which a resilient body 4 and polishing sheet 5 are placed. At this time, the resilient body 4 warps into a spherical form and so the end surface of the ferrule is polished into an oblique convex spherical surface. In order to make full use of the performance of the oblique PC connector, i.e. low loss and low reflection, it is important that the angle of tilt of the spherical surface formed by the polishing, i.e. the angle Θ' made between a plane tangential to the intersection of the axis of an optical fibre and the convex spherical surface and a plane vertical to the axis of the optical fibre (i.e. the angle between the normal at the central point of the optical fibre and the axis of the ferrule), be equal to the normalized angle Θ. This means that the vertex of the convex spherical surface agrees with the axis of the ferrule (i.e. the centre of the optical fibre) at the normalized angle. The ferrule is normally chamfered. That is, a thinned outer peripheral portion is formed at the front end so that the ferrule is easily inserted into a cylindrical sleeve when the optical fibre is placed in opposition to the ferrule and connected via the sleeve. When the chamfered ferrule is polished by the aforementioned method while tilted at the normalized angle Θ ( = Θ), the ferrule is not polished into a convex spherical surface at the normalized angle Θ, for the following reason. In the polishing method described above, the polishing removal progresses coaxially from the outermost portion of the end surface of the ferrule pressed against the polishing sheet on the resilient body. As a result, at the end of the polishing, as shown in Figure 3(b), the vertex of the convex spherical surface shifts into the middle point P between two points A and B lying on the chamfered portion. Consequently, the vertex deviates from the centre F of the optical fibre. The amount of deviation d is found in the manner described below. In Figure 3, r indicates the radius (normally, 1.25 mm) of the ferrule, α indicates the angle of chamfer of the front end portion of the ferrule, L indicates the length of the chamfer,  indicates the angle made between the axis of the ferrule and the normal to a polishing platen, R is the radius of curvature of the ferrule end surface polished into a convex spherical surface, a point F on the convex spherical surface indicates a point located on the axis of the optical fibre, Θ' indicates the angle made between the normal at the point F on the spherical surface formed by the polishing and the axis of the ferrule, and d indicates the straight distance between points P and F. It can be seen that by geometrical calculations, d and Θ', can be represented by d ≒ (L*tan α-1) (tan α *tan ) / (tan α* tan -1) Θ' = tan -1 {(R*sin -d) / [R 2 -(R*sin -d) 2 ] 1/2 } Normal dimensions of the ferrule, i.e. α = 30 degrees and L = 0.5 mm, are substituted into the formulas. Also, we assume that  = Θ = 8 degrees. Then, the amount of deviation d between the optical fibre axis and the convex spherical surface vertex is about 90 µm. By substituting R = 20 mm into the formula, we have Θ' ≒ 7.75 degrees. This R is determined by the hardness of the resilient body under the polishing sheet and by the polishing conditions including the force applied to the ferrule. The R is empirically found. Accordingly, where optical connectors having ferrules polished as described above are brought into abutment with each other from opposite sides, the optical fibre end surface touches at the point F but the angle made between the normal to the spherical surface at the point F and the optical axis is 7.75 degrees. It substantially follows that the ferrule is polished obliquely at 7.75 degrees. Therefore, with  = 8 degrees, the ferrule cannot be polished at the normalized angle Θ 8 = degrees for the oblique convex spherical surface polishing. This problem is alleviated by eliminating (α = 0) the chamfered portion of the outer peripheral portion at the front end of the ferrule. However, it is impossible to set the oblique polishing angle exactly to 8 degrees. Furthermore, when the ferrule is inserted into the cylindrical sleeve, placed in an opposite relation, and connected to it, the chamfered portion is imperative because of easiness of the insertion, prevention of generation of dust, and for other reasons. It is an object of the present invention to obtain a desired normalised oblique polishing angle Θ when the end surface of an optical fibre and/or ferrule is polished into an oblique convex spherical surface. It is another object of the present invention to obtain a desired normalized oblique polishing angle Θ when a ferrule having a normal shape and having a chamfered portion in the outer peripheral portion at the front end is polished into an oblique convex spherical surface. According to one aspect of the present invention there is provided a method of polishing a respective end surface of at least two optical fibres and/or ferrules into a convex spherical surface having a required normalised angle, the method comprising: positioning the fibres and/or ferrules such that the longitudinal axis of the fibres and/or ferrules are at an angle to the normal to the flat surface of a grinding wheel equal to the normalised angle; rotating the grinding wheel about an axis normal to the flat surface thereof to polish the end surface of the fibres and/or ferrules into an oblique flat surface having the given oblique angle which is an angle between the oblique flat surface and the plane normal to axis of the fibre/ferrule; characterized by positioning the fibres and/or ferrules with respect to a conical polishing platen having a resilient body and a polishing cloth thereon and the conical surface of which forms an angle equal to a compensation angle with the plane perpendicular to its axis of rotation; and rotating the platen to polish the oblique flat surface of the fibres and/or ferrules into the required convex spherical surface. According to another aspect of the present invention, there is provided a machine for polishing optical fibre end surfaces comprising a jig having a plurality of ferrule receiving apertures arranged around the periphery thereof such that ferrules placed in said apertures are tilted at an angle of Θ to the axis of rotation of said polishing platen, and means for providing relative rotation between the jig and a platen characterized by a conical polishing platen having a resilient body and a polishing sheet thereon and the conical surface of which forms an angle equal to a compensation angle with the plane perpendicular to its axis of rotation whereby the ends of optical fibres located in respective ferrules are polished to a required convex spherical surface. Embodiments of the present invention will now be described with reference to the accompanying drawings, of which: Figure 1 is a cross section showing an optical fibre end surface-polishing machine according to the present invention; Figure 2 is a side elevation of a ferrule end portion, illustrating normalized angle Θ of oblique convex spherical surface polishing; and Figure 3 is a side elevation of a ferrule end surface, illustrating the prior art oblique convex spherical surface polishing method. Figure 1 shows a cross section of an optical fibre end surface polishing machine according to the present invention. A ferrule 1 is provided with a minute hole extending through it along the axis of the ferrule. An optical fibre is held in the hole. A ferrule-holding jig 2 holds the ferrule 1 in such a way that it is tilted inwardly by a normalized angle Θ. Indicated by 11 is a base. A polishing platen 3 is mounted over the base 11. A resilient body 4 is stuck to the polishing platen 3. A resilient sheet 5 is stuck to the resilient body 4. The polishing platen 3 is caused to make a rotary motion about its axis and a circular motion along a circular path. The polishing platen 3 assumes an elliptical form which makes a minute angle of Δ to a plane perpendicular to the axis of rotation (the axis of the rotary motion or the axis of the circular motion). The height of the elliptical form increases from the outer periphery toward the centre. The ferrule 1 is pressed against the polishing sheet 5 by the ferrule-holding jig 2 and also by a pressure-applying shaft 40, the jig 2 forming a ferrule-holding portion. A support rod 41 prevents the ferrule-holding jig 2 from being rotated together with the polishing platen 3. In the above-described polishing machine, the ferrule is held to the ferrule-holding holding jig 2 at the angle Θ to the axis of rotation of the polishing platen 3. The polishing platen 3 is tilted in such a way that the angle made between the axis of the ferrule and the normal to the polishing platen 3 increases by Δ from Θ. Therefore, by optimizing this Δ, the end surface of the ferrule is polished into an oblique convex spherical surface at the normalized oblique polishing angle Θ. In the polishing machine described above, the ferrule end surface is previously polished at the angle Θ by the use of a surface polishing grinding wheel machine having a surface perpendicular to the axis of rotation of the polishing platen. Then, the end surface is polished into an oblique convex spherical surface, using a conical polishing machine 3 which is tilted at an angle of Δ to the surface of the surface polishing grinding wheel machine. The vertex lies on the axis of rotation described above. A resilient body and a polishing sheet are placed over the polishing machine 3. In this way, an optical fibre with an oblique convex spherical surface having desired values can be obtained in a short time. The minute angle Δ of the polishing platen is found by finding such a value of  which provides Θ' = Θ from the formulas (1) and (2) above and subtracting the normalized angle Θ from the value of . Therefore, if the chamfer length L, the chamfer angle α, and the radius of curvature R are known, then the value of Δ can be determined. Since the radius of curvature R of the convex spherical surface used in the formulas (1) and (2) are affected by the hardness of the resilient body placed under the polishing cloth and by the polishing conditions such as the force applied to the ferrule, the radius of curvature is found empirically. In the present example, a correcting angle ▵ is imparted to the polishing platen, so that the angle between the ferrule and the polishing platen is  = Θ + Δ. As described thus far, according to the present invention, a ferrule can be polished into an oblique spherical surface at any arbitrary target angle with the above described simple configuration. Consequently, an oblique convex spherical surface-polished optical fibre end surface having an angle normalized (8 degrees, 10 degrees, 12 degrees, or so on) to achieve low loss and low reflection can be easily obtained. Furthermore, an optical fibre with an oblique convex spherical surface having desired values can be obtained in a short time by previously performing surface oblique polishing, using a surface polishing platen having a surface perpendicular to the axis of rotation and then polishing the end surface into an oblique convex spherical surface, using a conical polishing platen tilted at an angle of Δ to the above-described surface. The aforegoing description has been given by way of example only.

Claims: A method of polishing a respective end surface of at least two optical fibres and/or ferrules (1) into a convex spherical surface having a required normalised angle (Θ), the method comprising: positioning the fibres and/or ferrules such that the longitudinal axis of the fibres and/or ferrules are at an angle to the normal to the flat surface of a grinding wheel equal to the normalised angle (Θ); rotating the grinding wheel about an axis normal to the flat surface thereof to polish the end surface of the fibres and/or ferrules into an oblique flat surface having the given oblique angle (Θ) which is an angle between the oblique flat surface and the plane normal to axis of the fibre/ferrule; characterized by positioning the fibres and/or ferrules with respect to a conical polishing platen (3, 4, 5) having a resilient body and a polishing cloth thereon and the conical surface of which forms an angle equal to a compensation angle (Δ) with the plane perpendicular to its axis of rotation; and rotating the platen (3, 4, 5) to polish the oblique flat surface of the fibres and/or ferrules into the required convex spherical surface. A method as claimed in claim 1, comprising the step of providing the fibres an/or ferrules with a respective chamfer to produce a reduced outer peripheral portion formed at the front end thereof. A method as claimed in claim 1 or claim 2, comprising the step of calculating said compensation angle (Δ) by subtracting Θ from  under the condition Θ' = Θ in the following equations: d = (L*tan α-1) (tan α *tan ) / (tan α* tan -1) Θ' = tan -1 {(R*sin  -d) / [R 2 -(R*sin  -d) 2 ] 1/2 } where α is an angle of chamfer of the ferrule, L is length of the chamfer,  is an angle made between the axis of the ferrule and a line normal to the polishing platen, R is a radius of curvature of the end surface of the ferrule polished into the convex spherical surface, F is a point on the convex spherical surface lying on the axis of the optical fibre, Θ' is an angle between the normal at the point F of the spherical surface formed as a result of the polishing and the axis of the ferrule, P is a middle point on a convex spherical surface formed as a result of the polishing, and d is a distance between the points P and F. A machine for polishing optical fibre end surfaces comprising a jig (2) having a plurality of ferrule receiving apertures arranged around the periphery thereof such that ferrules placed in said apertures are tilted at an angle of Θ to the axis of rotation of said polishing platen (3), and means for providing relative rotation between the jig (2) and a platen (3) characterized by a conical polishing platen (3) having a resilient body (4) and a polishing sheet (5) thereon and the conical surface of which forms an angle equal to a compensation angle (Δ) with the plane perpendicular to its axis of rotation, whereby the ends of optical fibres located in respective ferrules are polished to a required convex spherical surface. A machine as claimed in claim 4, wherein said compensation angle (Δ) is the value defined by subtracting Θ from  under the condition Θ' = Θ in the following equations: d = (L*tan α-1) (tan α *tan ) / (tan α* tan -1) Θ' = tan -1 {(R*sin  -d) / [R 2 -(R*sin  -d) 2 ] 1/2 } where α is an angle of chamfer of the ferrule, L is length of the chamfer,  is an angle made between the axis of the ferrule and a line normal to the polishing platen, R is a radius of curvature of the end surface of the ferrule polished into the convex spherical surface, F is a point on the convex spherical surface lying on the axis of the optical fibre, Θ' is an angle between the normal at the point F of the spherical surface formed as a result of the polishing and the axis of the ferrule, P is a middle point on a convex spherical surface formed as a result of the polishing, and d is a distance between the points P and F.

Inventors: ISHIYAMA KEIICHI, IWAKIRI TOYOKAZU, MINAMI KOHJI, SUZUKI NOBUO, TOKITA HIROYUKI, ISHIYAMA, KEIICHI, IWAKIRI, TOYOKAZU, MINAMI, KOHJI, SUZUKI, NOBUO, TOKITA, HIROYUKI
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1995
Main classification: B24B19
IPC codes: B24B19, G02B6