Method of polishing ferrule for optical connector into convex spherical surface

Abstract: A method of polishing a ferrule for an optical connector into a convex spherical surface is provided, which has been developed to greatly reduce the product cost and stabilise the product quality. According to this method, the end face of a ferrule 2 holding an optical fiber 1 is pressed against a polishing sheet 30 with relative movement between the ferrule and sheet. The polishing sheet 30 has inexpensive abrasive grains 31 fixed thereto which have a prescribed hardness necessary to polish the optical fiber 1. The sheet is supplied with a polishing fluid 50 containing abrasive grains 51 which have a prescribed hardness to polish the ferrule 2 and which are, for example, coarser than the abrasive grains 31 of the polishing sheet 30. Thus, two kinds of abrasive grains 31 and 51 differing in hardness polish two kinds of work materials differing in hardness so as to polish the end face of the ferrule 2 into a convex spherical surface.

Description: The present invention relates to an improved method of polishing the end face of a ferrule for use in an optical connector into a convex spherical surface (see for example JP-A-09 300 192). It is a common practice that an optical fiber for transmitting optical signals, which is composed of a core of quartz or the like and a clad covering it, has its end held by a ferrule, and the end face of the ferrule is polished into a convex spherical surface, with its vertex being at the core, such that the resulting surface is mirror-finished or rough finished. The conventional method of polishing the end face of a ferrule into a convex spherical surface consists of pressing the end face of the ferrule holding an optical fiber against a polishing sheet. The sheet is formed of a resin substrate with diamond abrasive grains bonded thereto by a binder and is fixed on an elastically deformable base of a polishing machine. The base is such that the ferrule sweeps a nearly constant path on the polishing sheet which is supplied with water. However, the disadvantage of the conventional method resides in that the layer of abrasive grains of the polishing sheet has a short life. The abrasive grains become deformed, generally deteriorate and drop off, after only several repetitions of the polishing operation. Another disadvantage resides in that the end face of a ferrule polished using a fresh polishing sheet is rougher than the end face of a ferrule polished using a worn-out polishing sheet. This is undesirable even if the difference in surface roughness is within a relatively narrow tolerance. Thus, there is a demand for improvement in the constancy of surface roughness. Moreover, carrying out polishing by moving the base of the polishing machine, to which the polishing sheet is fixed, such that the ferrule holding an optical fiber sweeps a nearly constant path on the polishing sheet is not economical because the polishing sheet remains partly unused (the polishing sheet has a part with which the end face of the ferrule does not come into contact). In addition, simultaneous polishing of the optical fiber and the ferrule necessitates the use of a polishing sheet with expensive diamond abrasive grains because of the hardness of the ferrule. This and the foregoing prevent cost reduction of the product. It is an object of the present invention to provide for polishing the end face of a ferrule for an optical connector into a convex spherical surface, with a view to greatly decreasing the product cost and stabilising the product quality. A first aspect of the present invention to solve the above-mentioned problems resides in a method of polishing the end face of a ferrule with the features of claim 1. A second aspect of the present invention resides in a method of polishing a ferrule for an optical connector into a convex spherical surface as defined in the first aspect of the invention, in which the abrasive grains of the polishing fluid are coarser than those bonded to the polishing sheet. A third aspect of the present invention resides in a method of polishing a ferrule for an optical connector into a convex spherical surface as defined in the first or second aspects of the invention, in which the abrasive grains of the polishing sheet are particles of at least one species selected from the group consisting of alumina, zirconia, chromium, silicon carbide, silicon nitride, CBN sintered body, cerium oxide, and quartz powder. A fourth aspect of the present invention resides in a method of polishing a ferrule for an optical connector into a convex spherical surface as defined in any of the first to third aspects of the invention, in which the abrasive grains of the polishing fluid are particles of at least one species selected from the group consisting of diamond, alumina, zirconia, chromium, silicon carbide, silicon nitride, CBN sintered body, cerium oxide, and quartz powder. Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which:- Figs. 1A and 1B are schematic diagrams showing how to polish a ferrule for an optical connector into a convex spherical surface according to a method of the present invention. Polishing the end face of a ferrule according to a method of the present invention can be embodied as explained below with reference to the accompanying drawings. In one embodiment of the present invention, the end face of a ferrule 2 holding an optical fiber 1 is polished by using a polishing machine of any known type which is made up of a polishing board 3 which supports a polishing sheet 30 via an elastically deformable base 4 and a driving unit (not shown) which simultaneously rotates and revolves the polishing board 3 about its center and about its prescribed eccentric center, respectively. The polishing machine is provided with a device (not shown) which supports the ferrule 2 holding an optical fiber 1. The fiber 1 consists of a core of quartz or the like to transmit optical signals and a cladding. The ferrule 2 is supported such that it can be slid in the vertical direction perpendicular to the polishing sheet 30 and can be pressed against the polishing sheet 30 under a prescribed pressure at the time of polishing. The polishing sheet 30 mounted and fixed to the base 4 is composed of a resin substrate 34 and abrasive grains 31 bonded thereto with a binder 33. The abrasive grains 31 of the polishing sheet 30 should have a hardness high enough to polish the optical fiber 1, such as particles of at least one species selected from the group consisting of alumina, zirconia, chromium, silicon carbide, silicon nitride, CBN sintered body, cerium oxide, and quartz powder, with alumina being economically adequate. Polishing the ferrule 2 with the polishing sheet 30 employs a polishing fluid 50, which is composed of abrasive grains 51 and a medium 52 of water or oil (water being preferable) in which the abrasive grains 51 are dispersed. The grains 51 are coarser and harder than abrasive grains 31 and are capable of polishing the ferrule 2. The medium 52 may contain an optical dispersing stabiliser and the like, if necessary. The abrasive grains 51 should preferably be of diamond or the like in the case where the ferrule is made of ceramics such as zirconia. However, the grains 51 may be those which can be used for the abrasive grains 31 to meet the above-mentioned requirements in the case where the ferrule is made of glass, or the like. To polish the ferrule 2 according to the above-mentioned arrangement, the ferrule 2 is pressed against the polishing sheet 30 which is supported and driven by the polishing board 3, while the polishing sheet 30 is supplied with a predetermined amount of polishing fluid 50, as shown in Figs. 1(A) and (B). Incidentally, the ferrule 2 used in this embodiment has a flat end face and a chamfered periphery. During polishing, the base 4, which is an elastic body of rubber or the like, undergoes elastic deformation under the pressing force, so that the surface of the polishing sheet 30 is bent in conformity with the end face of the ferrule 2. Polishing proceeds in the following manner. First, the end face of the ferrule 2 has its periphery (comer) 2a polished mainly by the abrasive grains 51 of the polishing fluid 50 to become round. The end face is gradually polished into a convex spherical surface with its vertex at the core which is at the center of the optical fiber 1. As polishing into the convex spherical surface proceeds to some extent, contact gradually disappears between the optical fiber 1 and the abrasive grains 51 of the polishing fluid 50, with the result that the end of the optical fiber 1 is polished mainly by the abrasive grains 31 of the polishing sheet 30 while the part of the ferrule 2 surrounding it is polished mainly by the abrasive grains 51 of the polishing fluid 50, as shown in Fig. 1(B). In the way mentioned above, the abrasive grains 31 and 51, which differ in grain size and hardness, polish different work materials and thereby polish the end face of the ferrule into a convex spherical surface with its vertex at the core. In addition, since polishing is accomplished by the abrasive grains 51 which are continuously renewed by the supply of the polishing fluid 50, it is only necessary to supply as much as necessary. This contributes to efficient polishing and economical improvement. The cost of polishing with the polishing sheet 30 (an alumina sheet), which is repeatedly used five time, and the polishing fluid 50 containing the abrasive grains 51 (diamond grains) is estimated to be about one half the cost of polishing with a conventional diamond sheet, which is repeatedly used ten times. In addition, a diamond sheet is almost at the end of it's useful life after it has been used repeatedly ten times and it is likely that the result of polishing becomes poor as the number of repetition increases. However, this is not the case with the method of the present invention, which permits constant polishing at all times. The abrasive grains 31 and the abrasive grains 51 (especially the latter) may have any grain size which is selected according to the stage of polishing, e.g., rough polishing, medium polishing, and finish polishing. The abrasive grains 31 are not specifically restricted so long as they meet the requirement that they should preferably be finer than the abrasive grains 51 and capable of supporting the abrasive grains 51. Usually, they have a particle diameter in the range of about 0.1 to 15 µm. In addition, the polishing sheet 30 supporting such abrasive grains 31 should preferably have an average surface roughness (Ra) of about 0.1 to 1 µm, although this condition is not limitative. In polishing according to the above-mentioned embodiment, the polishing fluid is simply delivered as much as necessary onto the polishing sheet. Alternatively, it is possible to wipe away the polishing fluid after the completion of polishing and to newly supply a proper amount of polishing fluid to the polishing sheet when polishing is resumed. Furthermore, it is possible to recycle the polishing fluid after its use. The embodiment mentioned above is designed to polish a ferrule into a convex spherical surface, the ferrule having a flat end face and a chamfered periphery. It may be modified such that the end face of the ferrule is formed into a rough convex spherical surface by a preliminary step and then it is mirror-finished by the above-mentioned means. If the polishing time is extended, it is possible to polish a flat end face of a ferrule directly into a mirror surface, and this offers the advantage of eliminating the preliminary step for rough polishing. The present invention constructed as mentioned above produces good effects as follows. The polishing is such that the end face of a ferrule holding an optical fiber is pressed against a polishing sheet and with relative movement between the sheet and ferrule, the polishing sheet having inexpensive abrasive grains fixed thereto which have a prescribed hardness necessary to polish the optical fiber with the sheet being supplied with a polishing fluid containing abrasive grains which have a prescribed hardness to polish the ferrule and which are coarser than the abrasive grains of the polishing sheet. This has the advantage that two kinds of abrasive grains differing in hardness polish two kinds of work materials differing in hardness. That is, the abrasive grains of the polishing sheet polish the optical fiber and the abrasive grains of the polishing fluid polish the ferrule to make the end face of the ferule into a convex spherical surface. Fresh abrasive grains are used in a minimum amount necessary at all times so as to realise consistent quality and economical operation.

Claims: A method of polishing the end face of a ferrule (2) holding an optical fiber (1) into a convex spherical surface by pressing the end face of the ferrule against a polishing sheet (30) with abrasive grains (31) bonded to the sheet with relative movement between the sheet and the ferrule, while supplying a polishing fluid (50) containing abrasive grains (51) to the surface of the polishing sheet, characterised in that the abrasive grains (31) bonded to the polishing sheet (30) have a hardness high enough for polishing the optical fiber and that the abrasive grains (51) contained in the polishing fluid (50) have a higher hardness than that of said abrasive grains (31) bonded to the sheet (30) and are capable of polishing said ferrule. A method of polishing a ferrule for an optical connector CBN sintered body into a convex spherical surface using the method as claimed in claim 1, wherein the abrasive grains (51) of the polishing fluid (50) are coarser than those bonded to the polishing sheet (30). A method of polishing a ferrule for an optical connector into a convex spherical surface using the method as claimed in claim 1 or 2, wherein the abrasive grains (31) of the polishing sheet (30) are particles of at least one species selected from the group consisting of alumina, zirconia, chromium, silicon carbide, silicon nitride, CBN sintered body, cerium oxide, and quartz powder. A method of polishing a ferrule for an optical connector into a convex spherical surface using the method as claimed in any of claims 1 to 3, wherein the abrasive grains (51) of the polishing fluid (50) are particles of at least one species selected from the group consisting of diamond, alumina, zirconia, chromium, silicon carbide, silicon nitride, CBN sintered body, cerium oxide, and quartz powder.

Inventors: MINAMI KOUJI, YOSHIKAWA TOMOHIRO, MINAMI, KOUJI, YOSHIKAWA, TOMOHIRO, Minami, Kouji, c/o Nasatekku Precision Co., Yoshikawa, Tomohiro, c/o Nasutekku Precision Co.,
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1999
Main classification: B24B19
IPC codes: G02B6, B24B19