Method of polishing the end surface of an optical connector

Abstract: By a method of oblique spherical polishing of an optical fiber connector a desired contact surface angle on an axis of the optical connector (20) is obtained easily and securely without requiring additional processing. In order to form a dome curved surface of a radius R, which is oblique with the contact surface angle ', on the end surface of the optical connector (20) which has a ferrule (22) of which a tip (24) is beveled, the tip of the optical connector (20) is polished by tilting it by the angle  which fulfills the relation sin  = sin ' + S/R where S is the amount of the deviation of the dome.

Description: The present invention relates to a polishing method of the end surface of an optical connector comprising an optical fiber and a ferrule. In the field of optical communication, connection is often made by abutting of a pair of optical connectors of which the ends are formed in dome-like, namely, convex surfaces of a fixed radius of curvature. This is because the connection loss is minimized by abutting of the domes by which a zero gap between the facing optical fibers can be achieved. However, in the case when the normal of the abutting contact surface coincides with the axis of the optical fibers, the reflection at the contact surface cannot be ignored. Therefore, in order to reduce the adverse reflected return light, optical connectors are used which have a tilted dome with the angle between the normal and the axis of the optical fibers being, for example, 7° or higher. Figure 4 is a side view showing the end of an optical connector polished by the conventional method. According to the conventional method, first the end of the optical connector 1, which supports an optical fiber 3 at the center of a ferrule 2 which has the beveled tip 4, is polished such that its normal is oblique having the desired angle  from the axis of the optical fiber 3. Subsequently, the end surface of the optical connector 1 is polished in dome-like shape by polishing material, etc. The reason why the optical connector 1 has the beveled tip 4 is that it makes it easier to insert the optical connector 1 into an aligning sleeve (not shown), which accepts it, as well as preventing damage on the inner surface of the aligning sleeve. Since the desired angle  is set to be the oblique angle for polishing in this oblique spherical polishing, the center on the dome surface Q is deviated by a significant quantity S from the axis of the optical fiber 3. Furthermore, since the point O on the axis, at which the optical fibers 3 mechanically contact each other, is deviated from the center P on the dome Q, the normal angle (or contact surface angle) ' at the point O is always smaller than the normal angle (that is, oblique angle)  at the center P. Therefore, even if the oblique angle of the dome Q is set at, for example, 7° by which the reflected return light can be reduced to a certain intensity level and its polishing is carried out, the normal angle at the point O on the side of the axis of the optical fiber 3 results in less than 7°. Thus, various connectors having the contact surface angles different from the designed values are abutted against each other, such problems as the insertion loss is increased, or the required amount of reduction of the reflected return light cannot be obtained. As a means to solve this problem, it has been proposed to use a step ferrule for which, as shown in Figure 5 at the tip of the ferrule 12, a cylindrical part 15 of small diameter is formed concentrically beyond the beveled part 14 as disclosed in Japanese Publication No. 3-210509. According to this step ferrule, since the region of the spherical polishing at the oblique spherical polishing does not cover the beveled part 14, the deviation S of the center P on the dome Q is negligibly small if the oblique angle  is not significantly large. Therefore, the normal angle at the point O on the axis of the optical fiber 13 takes an extremely close value to the oblique angle  of the dome Q. However, the aforesaid method, which uses the step ferrule, requires additional processing of the commercial ferrule to form the step ferrule, therefore, there is a problem of the cost increase of the optical connector manufacture. Also, when the oblique angle is large, or when the radius of curvature of the dome is small, the deviation S cannot be a negligible value; therefore, there arises a problem that the normal angle at the point O on the axis of the optical fiber is greatly different from the desired oblique angle. US-A-5140660 cites prior art in its introduction which discloses a method of polishing an end surface of an optical connector with a bevelled tip. The optical connector is held at a polishing angle to a polishing disc which is rotated so as to form a domed end for the optical connector, the polishing angle being set at the desired oblique angle for the contact point on the dome. Consequently, the object of the present invention is to provide a method of polishing an end surface of an optical connector to solve the aforesaid problems which simplifies the manufacturing process, restrains the cost of manufacture, and at the same time, provides an optical connector for which the normal angle of the dome on the axis coincides with the desired oblique angle. To this end the invention consists in a method of polishing an end surface of an optical connector to form a domed surface as defined in claim 1. Here, the contact surface indicates the surface on which two optical connectors are in mechanical contact when they are butt-connected. An embodiment of the invention will now be described, by way of example with reference to the accompanying drawings, in which: Figure 1 shows an end of an optical connector polished spherically according to the present invention. Figure 2 is a magnified showing of the end of the optical connector in Figure 1. Figure 3 shows the polishing condition. Figure 4 shows a conventional optical connector. Figure 5 shows another conventional optical connector. In Figures 1 and 2, optical connector 20 comprises ferrule 22 and an optical fiber (not shown) which is fixed inside a through hole (not shown) of the ferrule 22 by adhesives, etc. The ferrule 22, before its polishing process, is in the form shown by the broken line 22' and available on the market, and for example, the one with the diameter D = 2.499mm and the tip 24 beveled by Ψ = 30° is used. Incidentally, the spherical region outside of a line N is identified as a "dome" wherein line N indicates the bottom surface of the dome Q and  is the angle between line N aid the plane perpendicular to the axis of the optical connector 20.  is also the angle between the normal at the center P of the dome Q and the axis of the optical connector 20. Here, the relation between the normal angle  at the center P of dome Q and the normal angle (contact surface angle) ' at the point O on the axis of the optical connector 20 (or ferrule 22) is considered. The distance L in the x-direction between the center T of the radius of the dome Q to the point P on the dome Q is, assuming the radius of the dome Q is R, L = R sin  On the other hand, since the center P on the dome Q is deviated from the axis of the ferrule 22 by the distance S, assuming the distance in the x-direction from the center of radius T to the axis of the ferrule 22 is L', L' = (R sin ') = L - S = R sin  - S Therefore, sin ' = sin  - S/R or, sin  - sin ' = S/R. Consequently, it should be understood that, since the desired normal angle ' at the point O on the axis is smaller than the oblique angle  of polishing by the angle corresponding to S/R, the target normal angle ' at the point O on the axis can be obtained if the greater oblique angle  of polishing is taken to compensate the desired normal angle  by the angle corresponding to S/R. Also, S is shown by the following equation: S = α + β where, α is the deviation (x-direction) of the intersection of the straight line PT, which connects the center P on the dome Q and the center of radius T, and the straight line N, which connects the ends of the dome Q from the axis of the optical connector 20, and assuming the length of the x-component of the straight line N is w, the height of the beveled part 24 is g, and the polishing depth is h, α = w/2 + (g - h) tan Ψ - D/2. Also, β is the distance in the x-direction from the intersection to the center P on the dome Q, and assuming the half length of the straight line N is m. In Figure 3, by holding the optical connector 20 with a jig 30 at the oblique angle , which is greater than the target contact surface angle, that is, the desired normal angle ' at the point O on the axis, and contacting it with the polishing film 34 on the elastic body 32, and polishing it to the polishing depth h (Figure 2) at which the radius is R, the desired contact surface angle ' at the point O on the axis can be obtained. By monitoring the amount of polishing, namely, the polishing depth, the desired dome radius can be obtained. The method of monitoring the amount of polishing can be, for example, the method of monitoring the amount of polishing by the polishing time by obtaining the coefficient of correlation between the amount of polishing (the polishing depth) and the polishing time beforehand, the method of monitoring the amount of polishing by the stress applied to the polishing jig by obtaining the coefficient of correlation between the amount of polishing and the stress value beforehand, and so forth. An experimental example according to the present method is shown by the following. First, the ferrule 22' used is of diameter D = 2.499mm, the beveling angle Ψ = 30°, the height g of the beveled part 24 = 0.432mm. In case this ferrule 22' is processed in order to obtain a tilted dome of a dome radius R = 8.0mm and the contact surface angle ' = 8.0°, first the ferrule 22' is fixed to the jig at the oblique angle  = 8.85°, and by plane polishing until g = 0.424mm is obtained by contacting with the polishing film on a rigid body (not shown), the oblique plane with the oblique angle  = 8.85° is obtained. Then, while holding the oblique angle  = 8.85° with the oblique jig 30, the spherical polishing is carried out. Since the dome radius R is determined by the polishing depth h as described in the foregoing, the spherical polishing process is monitored by the polishing depth until h = 0.076mm in case R = 8.0mm. Now, various data for obtaining various contact surface angle ' are shown in the following table: '(° ) 7 8 9 10 11  (° ) 7.73 8.85 9.97 11.1 12.24 R (mm) 8 8 8 8 8 h (mm) 0.072 0.074 0.076 0.079 0.082 g (mm) 0.422 0.424 0.426 0.429 0.432 According to the present invention, there is a marked effect that the desired contact surface angle on the axis of the optical connector can be obtained easily and securely without using a special ferrule. Also, since the additional process such as forming a step on the ferrule in advance is not required, there is merit that the optical connector having a tilted spherical surface can be manufactured at low cost.

Claims: A method of polishing an end surface of an optical connector (20) to form a domed spherical surface (Q), the connector comprising a cylindrical ferrule (22) having a bevelled tip (24), a central axis and an optical fibre supported at the centre of said ferrule, the method comprising the steps of: holding the cylindrical ferrule with the optical fiber supported therein in a jig (30) so that the central axis is at an oblique polishing angle  with respect to the normal of a polishing surface of a polishing member (34) wherein the polishing angle  is selected such that a desired angle ' between the central axis and the normal to the domed surface (Q) is formed at the point (O) thereon situated on the central axis which is related to the polishing angle  by the formula sin  = sin ' + S R where: R is the radius of the domed end face (Q); and S is the distance between the central axis and the centre (P) of the domed end face (Q); and forming the dome by polishing with said polishing member (34); wherein prior to polishing the end surface of the optical connector (20) into a domed shape, it is polished to form a plane having a normal disposed at the polishing angle  to the central axis.

Inventors: FURUYA OKITSUGU, MIYACHI AKIHIRO, MIYAZAWA MASAAKI, FURUYA, OKITSUGU, MIYACHI, AKIHIRO, MIYAZAWA, MASAAKI
Applicants: WHITAKER CORP, THE WHITAKER CORPORATION
Application year: 1995
Main classification: G02B6
IPC codes: G02B6, B24B19