Polishing member and method for polishing end faces of optical fibers

Abstract

A polishing member for polishing an end face of an optical fiber comprises a substrate and a polishing layer overlaid on the substrate, the polishing layer having been formed with application of a coating composition, which contains a binder and polishing particles as principal constituents, onto the substrate. The polishing layer contains first particles, which act as the polishing particles and are constituted of silica particles having a mean particle diameter falling within the range of 5 nm to 50 nm, and second particles, which are constituted of silica particles having a mean particle diameter 50 to 2,500 times as large as the mean particle diameter of the first particles.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a polishing member, such as a polishing sheet or a polishing disk, for polishing an end face of an optical fiber, which polishing member is suitable for finish polishing of a ferrule end face of an optical fiber connector. This invention also relates to a method of polishing an end face of an optical fiber.

[0003] 2. Description of the Related Art

[0004] Optical fiber connectors are utilized for connecting optical fibers with each other. The optical fiber connectors comprise zirconia ferrules, which cover regions around connection end areas of the optical fibers. Ferrule end faces of the optical fiber connectors, which ferrule end faces are to be connected with each other, are subjected to mirror polishing, and the polished end faces of the ferrule end faces of the optical fiber connectors are brought into abutment with each other by use of a fixture. In this manner, the ferrule end faces of the optical fiber connectors are connected with each other. The optical fiber connectors have heretofore been used widely in optical communication fields for easiness of processing.

[0005] The mirror polishing of the ferrule end faces of the optical fiber connectors is performed in a plurality of steps ranging from coarse polishing to finish polishing, in which particle sizes of polishing particles are reduced successively. As for the optical fiber connectors utilized for connecting the end areas of the optical fibers, through which light passes, processing accuracy and optical characteristics of the ferrule end faces of the optical fiber connectors are important. Also, the accuracy of the finish polishing, which accuracy have influences upon the optical characteristics of the end faces of the optical fibers, are particularly important. If smoothness (i.e., the mirror-smooth characteristics) of the ferrule end faces of the optical fiber connectors, particularly the smoothness (i.e., the mirror-smooth characteristics) of the end faces of the optical fibers connected with each other by the utilization of the optical fiber connectors, is insufficient, the problems will occur in that good optical characteristics cannot be obtained, and in that return loss characteristics (i.e., light quantity transfer loss characteristics) become bad. Also, if a large difference in level occurs between the polished end faces of the optical fiber and the ferrule, which are made from different materials, or if flaws occur on the polished end faces of the optical fiber and the ferrule due to the polishing operation, the problems will occur in that the optical characteristics and the durability of the optical fibers become bad.

[0006] A technique for polishing an end face of an optical fiber connector has been proposed in, for example, Japanese Unexamined Patent Publication No. 8(1996)-336758. With the proposed technique for polishing an end face of an optical fiber connector, a polishing member having a polishing layer, in which cerium oxide particles, silica particles, or the like, are utilized as polishing particles, is utilized, and finish polishing of an end face of an optical fiber connector is performed only with polishing force of the polishing layer.

[0007] The polishing member described above comprises a substrate and a polishing layer overlaid on the substrate, the polishing layer containing a binder and polishing particles as principal constituents. The polishing members whose polishing layers have different coarseness are utilized in accordance with the polishing accuracy required of the optical fiber. Ordinarily, as the polishing particles, particles having a Mohs hardness of at least 6 and constituted of, for example, chromium oxide, alumina, or diamond are employed. As the polishing member for a finish polishing operation, the polishing member containing polishing particles having a small particle size is utilized, such that the polishing operation is capable of being performed with a desired accuracy.

[0008] However, with the conventional polishing member described above, the problems occur in that, during the operation for polishing the end face of the optical fiber connector, polishing flaws are apt to occur on the end face of the optical fiber or the end face of the ferrule due to, for example, intervention of scraped fragments of the optical fiber or the ferrule and wear debris of the polishing layer between the end face of the optical fiber connector and the polishing member. As a result, the optical characteristics of the polished end face of the optical fiber connector are affected adversely. A certain polishing member has a surface having an altered shape such that the polishing flaws may not occur readily. However, the effects of preventing the flaws from occurring are not sufficient, and sufficiently good optical characteristics cannot be obtained.

[0009] Also, as for the operation for polishing the end face of the optical fiber connector with the polishing member, in cases where the ratio of the binder to the polishing particles is set at a small value, the polishing accuracy is capable of being enhanced, and the optical characteristics of the end face of the optical fiber connector are capable of being enhanced. However, in such cases, the coating film strength of the polishing layer becomes low. As a result, the problems occur in that the polishing particles are apt to separate from the polishing layer during the polishing operation, and peeling of the polishing layer is apt to occur due to breakage of the coating film.

[0010] In view of the foregoing, it may be considered that the ratio of the binder to the polishing particles be set at a large value. However, in such cases, since the amount of the polishing particles per unit volume becomes small, uniform scraping cannot be achieved. Therefore, the problems occur in that sufficient flatness and sufficient mirror-smooth characteristics of the end face of the optical fiber after being polished cannot be obtained, good optical characteristics cannot be obtained, and the return loss characteristics become bad. Also, the problems occur in that a large difference in level occurs between the polished end faces of the optical fiber and the ferrule, and flaws are apt to occur.

[0011] A different technique for polishing an end face of an optical fiber connector has been proposed in, for example, Japanese Unexamined Patent Publication No. 2000-354944. With the proposed technique for polishing an end face of an optical fiber connector, in cases where the end face of the optical fiber connector is brought into contact with a polishing sheet having a polishing layer, which comprises a binder and polishing particles dispersed in the binder, and the end face of the optical fiber connector is subjected to sliding movement on the polishing sheet, a polishing liquid, in which free polishing particles have been dispersed, is supplied to the slide polishing area. In this manner, a wet polishing operation is performed by the utilization of the polishing sheet and the polishing liquid.

[0012] However, with the proposed technique for polishing an end face of an optical fiber connector, the problems occur in that the optical characteristics of the end face after being polished are affected by a combination of the material of the polishing particles contained in the polishing sheet, the material of the polishing particles contained in the polishing liquid, and the particle size of the polishing particles. Also, the problems occur in that the flatness and the mirror-smooth characteristics of the end face of the optical fiber after being polished cannot be kept good, the optical characteristics become bad, and the return loss characteristics become bad. Therefore, it is not always possible to obtain the return loss characteristics markedly better than −50 dB. Also, the problems occur in that a large difference in level occurs between the polished end faces of the optical fiber and the ferrule, and in that scratch flaws occur on the polished end faces of the optical fiber and the ferrule due to the polishing operation.

SUMMARY OF THE INVENTION

[0013] The primary object of the present invention is to provide a polishing member for polishing an end face of an optical fiber, which polishing member has an enhanced coating film strength of a polishing layer without a proportion of a binder with respect to polishing particles being set to be high, has a high durability, and is capable of achieving finish polishing of an end face of an optical fiber connector, such that flaws do not occur on polished end faces of the optical fiber and the ferrule, such that optical characteristics (return loss characteristics and mirror-smooth characteristics) of the polished end face of the optical fiber connector are enhanced, and such that little difference in level occurs between polished end faces of the optical fiber and the ferrule.

[0014] Another object of the present invention is to provide a method of polishing an end face of an optical fiber, wherein finish polishing of an end face of an optical fiber connector is capable of being performed by use of a polishing member and a polishing liquid, which have good polishing characteristics, such that optical characteristics of the polished end face of the optical fiber connector are enhanced, such that return loss characteristics are kept to be better than −57 dB, e.g. to be approximately −65 dB, such that little difference in level occurs between polished end faces of the optical fiber and the ferrule, and such that scratch flaws do not occur on the polished end faces of the optical fiber and the ferrule.

[0015] The present invention provides a polishing member for polishing an end face of an optical fiber, the polishing member comprising:

[0016] i) a substrate, and

[0017] ii) a polishing layer overlaid on the substrate, the polishing layer having been formed with application of a coating composition, which contains a binder and polishing particles as principal constituents, onto the substrate,

[0018] wherein the polishing layer contains:

[0019] a) first particles, which act as the polishing particles and are constituted of silica particles having a mean particle diameter falling within the range of 5 nm to 50 nm, and

[0020] b) second particles, which are constituted of silica particles having a mean particle diameter 50 to 2,500 times as large as the mean particle diameter of the first particles.

[0021] In the polishing member for polishing an end face of an optical fiber in accordance with the present invention, the second particles, which are utilized together with the first particles acting as the polishing particles, should preferably be used in a proportion falling within the range of 1 part by weight to 5 parts by weight per 100 parts by weight of the first particles. Also, the second particles should preferably exhibit oil absorption falling within the range of 100 ml/100 g to 400 ml/100 g. Further, the second particles should preferably have a spherical shape.

[0022] Also, in the polishing member for polishing an end face of an optical fiber in accordance with the present invention, the binder contained in the polishing layer should preferably contain a polyurethane resin. In such cases, the polyurethane resin should preferably have the characteristics, such that a glass transition temperature Tg is at most 40° C., and/or an elongation is at least 200%.

[0023] The polishing member for polishing an end face of an optical fiber in accordance with the present invention should preferably be formed in the manner described below. Specifically, the first particles, the second particles, and the binder are mixed together, such that the ratio of the first particles and the second particles to the binder takes a value between 1:0.2 and 1:2. Also, the solid concentration is adjusted to a value falling within the range of 5% by weight to 30% by weight. In this manner, a coating composition for the formation of the polishing layer is prepared. Thereafter, the coating composition is applied onto the substrate with a coating process, such as a micro gravure coating process, a comb coating process, or a doctor coating process. The coating composition having been applied on the substrate is then dried in an atmosphere at a temperature falling within the range of 110° C. to 130° C. In this manner, the polishing member is formed.

[0024] Such that good dispersibility of the silica particles may be obtained, the coating composition for the formation of the polishing layer should preferably be prepared by subjecting the first particles, the second particles, and the binder to a dispersing process utilizing a mechanical type of kneader.

[0025] Polishing of the end face of the optical fiber with the polishing member in accordance with the present invention may be performed in the manner described below. Specifically, an end face of an optical fiber connector, which comprises a ferrule having a center hole and an optical fiber having been inserted and secured in the center hole of the ferrule, is brought into contact with the polishing layer of the polishing member. Also, the end face of the optical fiber connector is subjected to sliding movement on the polishing layer of the polishing member, while the end face of the optical fiber connector is being in contact with the polishing layer. Further, distilled water, which acts as a polishing liquid, or a slurry, which acts as the polishing liquid and contains free polishing particles and water, is supplied to the sliding contact area, at which the end face of the optical fiber connector is in sliding contact with the polishing layer. In this manner, the end face of the optical fiber connector is finish-polished into a convex spherical surface shape. In such cases, the slurry should preferably be a silica slurry, which contains silica particles having the characteristics such that the mean particle size (D50) of the primary particles falls within the range of 5 nm to 50 nm. Alternatively, the free polishing particles contained in the slurry may be the particles of cerium oxide, titanium oxide, or the like.

[0026] The present invention also provides a first method of polishing an end face of an optical fiber, the method comprising the steps of:

[0027] i) bringing an end face of an optical fiber connector, which comprises a ferrule having a center hole and an optical fiber having been inserted and secured in the center hole of the ferrule, into contact with a polishing member having a polishing layer, which comprises a binder and silica particles dispersed in the binder,

[0028] ii) subjecting the end face of the optical fiber connector to sliding movement on the polishing member, while the end face of the optical fiber connector is being in contact with the polishing member, and

[0029] iii) supplying a silica slurry, which acts as a polishing liquid, to the sliding contact area, at which the end face of the optical fiber connector is in sliding contact with the polishing member, the silica slurry containing fine-particle silica, which has a mean particle size falling within the range of 1 nm, inclusive, to a value smaller than 20 nm, and water,

[0030] whereby the end face of the optical fiber connector is finish-polished into a convex spherical surface shape.

[0031] The present invention further provides a second method of polishing an end face of an optical fiber, the method comprising the steps of:

[0032] i) bringing an end face of an optical fiber connector, which comprises a ferrule having a center hole and an optical fiber having been inserted and secured in the center hole of the ferrule, into contact with a polishing member having a polishing layer, which comprises a binder and silica particles dispersed in the binder,

[0033] ii) subjecting the end face of the optical fiber connector to sliding movement on the polishing member, while the end face of the optical fiber connector is being in contact with the polishing member, and

[0034] iii) supplying a silica slurry, which acts as a polishing liquid, to the sliding contact area, at which the end face of the optical fiber connector is in sliding contact with the polishing member, the silica slurry containing coarse-particle silica, which has a mean particle size falling within the range of 20 nm, inclusive, to 500 nm, inclusive, and water, and having a pH value falling within the range of 8 to 12,

[0035] whereby the end face of the optical fiber connector is finish-polished into a convex spherical surface shape.

[0036] In the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, as the polishing particles contained in the polishing layer of the polishing member, the silica particles maybe utilized alone. Alternatively, the silica particles may be utilized in combination with at least one different kind of polishing particles. In cases where the silica particles are utilized in combination with at least one different kind of polishing particles, the at least one different kind of polishing particles should preferably be selected from the group consisting of cerium oxide, alumina, chromium oxide, silicon carbide, and diamond, and the silica particles should preferably constitute at least 50% of the polishing particles contained in the polishing layer. The silica particles should preferably have a mean particle size falling within the range of 0.001 μm to 1.00 μm.

[0037] Also, in the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, the binder contained in the polishing layer should preferably be at least one kind of resin selected from the group consisting of a thermoplastic resin, a thermosetting resin, and an ultraviolet-curing resin.

[0038] Further, in the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, the surface of the polishing layer of the polishing member should preferably be provided with a fine concavity-convexity pattern, such as a worm-shaped wrinkle pattern, a block-shaped pattern, an orange peel-shaped pattern, an arrayed protrusion pattern, or a random protrusion pattern.

[0039] Furthermore, in the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, the surface of the polishing layer of the polishing member should preferably have a surface roughness Ra, expressed in terms of arithmetical mean roughness, falling within the range of 0.001 μm to 0.1 μm.

[0040] In the first method of polishing an end face of an optical fiber in accordance with the present invention, the fine-particle silica contained in the silica slurry acting as the polishing liquid has a mean particle size falling within the range of 1 nm to a value smaller than 20 nm. Also, in the second method of polishing an end face of an optical fiber in accordance with the present invention, the coarse-particle silica contained in the silica slurry acting as the polishing liquid has a mean particle size falling within the range of 20 nm to 500 nm. In the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, water should preferably be deionized water and should preferably have an electric conductivity of at most 0.500 μS/cm at 25° C.

[0041] With the polishing member for polishing an end face of an optical fiber in accordance with the present invention, the polishing layer contains the first particles, which act as the polishing particles and are constituted of the silica particles having a mean particle diameter falling within the range of 5 nm to 50 nm. The polishing layer also contains the second particles, which are constituted of the silica particles having a mean particle diameter 50 to 2,500 times as large as the mean particle diameter of the first particles. Therefore, polishing debris, such as polishing particles released during the polishing operation and scraped fiber fragments, is capable of being discharged quickly from the polished area by virtue of the adsorption effects of the second particles. As a result, polishing flaws do not occur, and the end face of the optical fiber connector is capable of being polished accurately. Also, the mirror-smooth characteristics and the optical characteristics of the polished end face of the optical fiber are capable of being enhanced. Specifically, the return loss characteristics are capable of being kept to be better than −65 dB. Further, the polishing operation is capable of being performed such that little difference in level occurs between the polished end faces of the optical fiber and the ferrule. Furthermore, the optical fiber region of the end face of the optical fiber connector is capable of being polished into the desired convex spherical surface shape.

[0042] Also, with the polishing member for polishing an end face of an optical fiber in accordance with the present invention, the coating film strength of the polishing layer is enhanced without the proportion of the binder with respect to polishing particles being set to be high. Therefore, the polishing operation is capable of being performed quickly with a high efficiency. Also, the problems do not occur in that the polishing layer peels off from the substrate due to breakage of the coating film during the polishing operation. Accordingly, the polishing member in accordance with the present invention has a high durability, and the number of changeovers of the polishing member during the polishing process is capable of being reduced. Further, with the polishing member in accordance with the present invention, the polishing particles are capable of being bound to one another with a small amount of binder, and the accurate polishing operation is capable of being performed efficiently.

[0043] With each of the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, the end face of the optical fiber connector, which comprises the ferrule having the center hole and the optical fiber having been inserted and secured in the center hole of the ferrule, is brought into contact with the polishing member having the polishing layer, which comprises the binder and the silica particles dispersed in the binder. The end face of the optical fiber connector is subjected to the sliding movement on the polishing member, while the end face of the optical fiber connector is being in contact with the polishing member. Also, the silica slurry, which acts as the polishing liquid and contains silica and water, is supplied to the sliding contact area, at which the end face of the optical fiber connector is in sliding contact with the polishing member. In this manner, the end face of the optical fiber connector is finish-polished into the convex spherical surface shape. Therefore, the end face of the optical fiber connector is capable of being accurately polished into the desired convex spherical surface shape. Also, the mirror-smooth characteristics and the optical characteristics of the polished end face of the optical fiber are capable of being enhanced. Specifically, the return loss characteristics are capable of being kept to be better than −57 dB, e.g. to be approximately −60 dB or −63 dB. Under optimum conditions, the return loss characteristics are capable of being kept to be better than −65 dB. Further, the polishing operation is capable of being performed such that little difference in level occurs between the polished end faces of the optical fiber and the ferrule, and such that scratch flaws do not occur on the polished end faces of the optical fiber and the ferrule. Furthermore, the optical fiber region of the end face of the optical fiber connector is capable of being polished into the desired convex spherical surface shape.

[0044] In the first method of polishing an end face of an optical fiber in accordance with the present invention, wherein the silica contained in the silica slurry is the fine-particle silica, which has a mean particle size falling within the range of 1 nm to a value smaller than 20 nm, e.g. a mean particle size of approximately 5 nm, the silica slurry should preferably also contain an alcohol besides the fine-particle silica and water. In such cases, the advantages are capable of being obtained in that, after storage for a long period (e.g., at least 60 days), agglomeration and gelation of the fine-particle silica do not occur.

[0045] In the second method of polishing an end face of an optical fiber in accordance with the present invention, wherein the silica contained in the silica slurry is the coarse-particle silica, which has a mean particle size falling within the range of 20 nm to 500 nm, e.g. a mean particle size of approximately 30 nm, the pH value of the silica slurry is adjusted at a value falling within the range of 8 to 12. Therefore, in such cases, the polishing operation is capable of being performed such that little difference in level occurs between the polished end faces of the optical fiber and the ferrule.

[0046] In each of the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, the polishing member, which primarily contains silica as the polishing particles, is capable of being formed at a comparatively low cost. Also, the polishing member, which primarily contains silica as the polishing particles, has markedly enhanced polishing characteristics and causes no scratch flaws due to the polishing operation to occur on the end face of the optical fiber. Further, since the silica slurry, which acts as the polishing liquid, is supplied to the sliding contact area, at which the end face of the optical fiber connector is in sliding contact with the polishing member, soiling substances do not cling to the polished end face of the optical fiber connector. In particular, soiling substances do not cling to the polished end face of the region of the ferrule. Furthermore, the polishing of the end face of the optical fiber connector into the convex spherical surface shape, particularly the polishing of the end face of the optical fiber region into the convex spherical surface shape, is capable of being performed easily. Also, little difference in level occurs between the polished end faces of the optical fiber and the ferrule, which are constituted of different materials.

[0047] Further, in the first and second methods of polishing an end face of an optical fiber in accordance with the present invention, the surface of the polishing layer of the polishing member may be provided with the fine concavity-convexity pattern, such as the worm-shaped wrinkle pattern, the block-shaped pattern, the orange peel-shaped pattern, the arrayed protrusion pattern, or the random protrusion pattern. In such cases, by virtue of the fine concavity-convexity pattern, the polishing efficiency is capable of being enhanced, the end face of the optical fiber connector is capable of being polished more accurately, and the optical characteristics are capable of being enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] FIG. 1 is a conceptual front view showing how an end face of an optical fiber connector is polished with an embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention,

[0049] FIG. 2 is an enlarged explanatory view showing an example of a surface of a polishing layer of a polishing member employed in an embodiment of the method of polishing an end face of an optical fiber in accordance with the present invention, and

[0050] FIG. 3 is an enlarged explanatory view showing a different example of a surface of a polishing layer of a polishing member employed in the embodiment of the method of polishing an end face of an optical fiber in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] The present invention will hereinbelow be described in further detail with reference to the accompanying drawings.

[0052] FIG. 1 is a conceptual front view showing how an end face of an optical fiber connector is polished with an embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention.

[0053] With reference to FIG. 1, a polishing member 1 for polishing an end face of an optical fiber comprises a substrate 2, which is formed from polyester film, or the like, and which has a thickness falling within the range of 25 μm to 100 μm, and a polishing layer 3, which is overlaid on the substrate 2. The polishing layer 3 contains a binder containing a polyurethane resin, first particles acting as the polishing particles, and second particles, which first and second particles are dispersed in the binder. The first particles acting as the polishing particles are constituted of silica particles having a mean particle diameter (D50) falling within the range of 5 nm to 50 nm (e.g., a mean particle diameter falling within the range of 5 nm to 30 nm). The second particles are constituted of silica particles having a mean particle diameter 50 to 2,500 times as large as the mean particle diameter of the first particles. In the polishing layer 3, the first particles, the second particles, and the binder are mixed together, such that the ratio of the first particles and the second particles to the binder takes a value between 1:0.2 and 1:2. The polishing layer 3 has a thickness falling within the range of, for example, 1 μm to 15 μm. The polishing member 1 is formed as, for example, a polishing sheet having a predetermined shape, such as a disk-like shape.

[0054] As illustrated in FIG. 1, the optical fiber connector 5 is polished by the polishing member 1. The optical fiber connector 5 comprises the ferrule 6, which is formed from a ceramic material, such as zirconia, and which has a center hole, and the optical fiber 7, which is formed from a glass material, such as a quartz glass, and which is inserted and secured to the center hole of the ferrule 6. The polishing of the end face of the optical fiber connector 5 is performed by attaching the polishing member 1 to the elastic member 11, which is located on the support base (the rotating base) 10 and which is formed from rubber, or the like, and pushing the end face of the optical fiber connector 5 against the polishing member 1 with a predetermined pressure so as to cause the end face of the optical fiber connector 5 to contact with the polishing member 1. (Ordinarily, a plurality of optical fiber connectors 5, 5, . . . are simultaneously pushed against the polishing member 1.) Also, for example, the support base 10 is rotated at a predetermined rotation speed, and the support base 10 or the optical fiber connector 5 is caused to undergo planetary movement. Further, during the polishing operation, a polishing liquid 16, which may be constituted of distilled water or a slurry, such as a silica slurry, is supplied from the supply nozzle 15 to the polishing area. In this manner, a wet polishing operation is performed.

[0055] In the process for producing the polishing member 1, the coating composition for the formation of the polishing layer is prepared in the manner described below. Specifically, for example, the solvent, the binder, the first particles, the second particles, and the like, are introduced into a mechanical type of kneader and kneaded together. The viscosity of the kneaded mixture is then adjusted. Thereafter, the kneaded mixture is introduced into a dispersing machine. In the dispersing machine, a solvent, and the like, are added to the kneaded mixture, and the resulting mixture is subjected to the dispersing process. In this manner, the coating composition for the formation of the polishing layer, in which the solid concentration has been adjusted at a value falling within the range of 5% by weight to 50% by weight, is prepared. The thus prepared coating composition for the formation of the polishing layer is applied to a predetermined thickness onto the substrate 2, which is being moved at a predetermined speed, in a coating apparatus employing the micro gravure coating technique, the comb coating technique, the Meyer bar coating technique, or the like. The applied coating composition for the formation of the polishing layer is then dried in an atmosphere at a temperature of 110° C. to 130° C. In this manner, the polishing layer 3 is formed on the substrate 2. Thereafter, punching, slitting, and the like, are performed in order to form the polishing member 1 having a predetermined shape.

[0056] The polishing particles (i.e., the first particles) contained in the polishing layer 3 are silica (silicon dioxide). The first particles have the characteristics such that the mean particle diameter of the primary particles falls within the range of 5 nm to 50 nm. In cases where the mean particle diameter of the polishing particles is small, good optical characteristics are obtained from the polishing operation. However, in such cases, the dispersibility of the polishing particles becomes low, and therefore it becomes necessary for a comparatively large amount of binder to be used. The amounts of the polishing particles and the binder used may be set such that the proportion of the binder falls within the range of 20 to 200 parts by weight per 100 parts by weight of the polishing particles (i.e., such that the ratio of the polishing particles to the binder takes a value between 1:0.2 and 1:2). The amounts of the polishing particles and the binder used should preferably be set such that the proportion of the binder falls within the range of 25 to 150 parts by weight per 100 parts by weight of the polishing particles.

[0057] Examples of the silica particles include synthetic spherical silica (supplied by Tokuyama Co., Ltd.), Aerosil (supplied by Nippon Aerosil Corp.), Nipsil (supplied by Nippon Silica Industrial Co., Ltd.), Silicia (supplied by Fuji Silicia Chemical Co., Ltd.), and Mizucasil (supplied by MIZUSAWA INDUSTRIAL CHEMICALS, Ltd.)

[0058] The second particles contained in the polishing layer 3 are silica. The second particles should preferably exhibit oil absorption falling within the range of 100 ml/100 g to 400 ml/100 g, and should more preferably exhibit oil absorption falling within the range of 200 ml/100 g to 350 ml/100 g. In cases where the oil absorption takes a large value, a large volume of voids are contained in the particles, and scraped fragments are capable of being confined in the voids. Also, in such cases, the particles become comparatively brittle and are not apt to scratch the fiber surface. However, if the oil absorption takes a markedly large value, the particles will be crushed during the process for forming a coating composition containing the particles, and the effects of confining the scraped fragments in the voids will become small. The second particles have a mean particle diameter 50 to 2,500 times as large as the mean particle diameter of the first particles, and should preferably have a mean particle diameter 100 to 500 times as large as the mean particle diameter of the first particles. If the difference in particle diameter between the second particles and the first particles is small, the scraped fragments cannot be discharged quickly. If the difference in particle diameter between the second particles and the first particles is markedly large, the polishing characteristics with the first particles will become bad. The second particles should preferably have a spherical shape.

[0059] Also, in the polishing member for polishing an end face of an optical fiber in accordance with the present invention, the binder contained in the polishing layer 3 should preferably contain a polyurethane resin. Further, thermoplastic resins, thermosetting resins, reactive resins, electron beam-curing resins, ultraviolet-curing resins, visible light-curing resins, or mixtures of two or more of these resins may be employed as the binder.

[0060] Such that the difference in level between the polished end faces of the optical fiber and the ferrule may be kept small, the polyurethane resin should preferably have the characteristics, such that the glass transition temperature Tg is at most 40° C. The polyurethane resin should more preferably have the characteristics, such that the glass transition temperature Tg is at most 0° C. Also, the polyurethane resin should preferably have the characteristics, such that the elongation falls within the range of 200% to 800%. The polyurethane resin should more preferably have the characteristics, such that the elongation falls within the range of 400% to 600%. Further, the polyurethane resin should preferably have an average molecular weight (MW) falling within the range of approximately 10,000 to approximately 100,000, and should more preferably have an average molecular weight falling within the range of approximately 15,000 to approximately 50,000.

[0061] As resins other than the polyurethane resin, the resins described below may be utilized. As the thermosetting resins or the reactive resins, which may be used as the binder in the polishing layer 3, there should preferably be employed the resins, which have a molecular weight of 200,000 or less when the resins take on the form of coating compositions, and which exhibit an infinite increase in the molecular weight through the condensation reactions, the addition reactions, or the like, when the coating compositions are heated and humidified after being applied onto substrates and dried. Among these resins, the resins, which do not soften or melt before they decompose thermally, should more preferably be employed. Specifically, examples of the thermosetting resins or the reactive resins include a phenol resin, a phenoxy resin, an epoxy resin, a polyester resin, a polyurethane polycarbonate resin, a urea resin, a melamine resin, an alkyd resin, a silicone resin, an acrylic reactive resin (an electron beam-curing resin), an epoxy-polyamide resin, a nitrocellulose melamine resin, a mixture of a high-molecular weight polyester resin with an isocyanate prepolymer, a mixture of a methacrylate copolymer with a diisocyanate prepolymer, a mixture of a polyester polyol with a polyisocyanate, a urea-formaldehyde resin, a mixture of a low-molecular weight glycol, a high-molecular weight diol and a triphenylmethane triisocyanate, a polyamine resin, a polyimine resin, and a mixture of two or more of these compounds.

[0062] In general, the thermoplastic resins, the thermosetting resins, and the reactive resins described above respectively have their major functional groups, and one to six kinds of other functional groups. Each of the other functional groups should preferably be contained in proportions within the range of 1×10−6 equivalent to 1×10−2 equivalent per gram of the resin. Examples of the other functional groups include acid groups, such as a carboxylic acid group (COOM), a sulfinic acid group, a sulfenic acid group, a sulfonic acid group (SO3M), a phosphoric acid group [PO(OM)(OM)], a phosphonic acid group, a sulfuric acid group (OSO3M), and ester groups with these acids, wherein M represents H, an alkali metal, an alkaline earth metal, or a hydrocarbon group; groups of amphoteric compounds, such as a group of an amino acid, a group of an aminosulfonic acid, a group of a sulfuric ester of amino-alcohol, a group of a phosphoric ester of amino-alcohol, and an alkyl betaine form group; basic groups, such as an amino group, an imino group, an imido group, and an amido group; a hydroxyl group; an alkoxyl group; a thiol group; an alkylthio group; halogen groups, such as F, Cl, Br, and I; a silyl group; a siloxane group; an epoxy group; an isocyanato group; a cyano group; a nitrile group; an oxo group; an acryl group; and a phosphine group.

[0063] When necessary, the polishing layer 3 may contain various additives. Examples of the additives include a lubricating agent, a dispersing agent and a dispersion assisting auxiliary for the polishing particles, a mildew-proofing agent, an antistatic agent, an antioxidant, and a coupling agent. Examples of the lubricating agents include fine particles of inorganic materials, fine particles of resins, organic compounds, organic acids, organic acid ester compounds, fatty acids, fatty acid amides, fatty acid alkyl amides, aliphatic alcohols, and antioxidants. These lubricating agents are added in proportions falling within the range of 0.01 to 30 parts by weight per 100 parts by weight of the binder. Examples of the dispersing agents and the dispersion assisting auxiliaries for the polishing particles include fatty acids having 2 to 40 carbon atoms, higher alcohols having 4 to 40 carbon atoms, metallic soaps, fatty acid amides, and sulfuric esters of these compounds. Examples of the mildew-proofing agents include 2-(4-thiazolyl)-benzimidazole, N-(fluorodichloromethylthio)-phthalimide, 10,10′-oxybisphenoxarsine, 2,4,5,6-tetrachloroisophthalonitrile, p-tolyldiiodomethylsulfone, triiodoallyl alcohol, dihydroacetonic acid, mercury phenyloleate, bis(tributyltin)oxide, and salicylanilide. Examples of the antistatic agents include conductive particles, such as carbon black and titanium oxide-tin oxide-antimony oxide, natural surface active agents such as saponin, nonionic surface active agents, cationic surface active agents, anionic surface active agents, and amphoteric surface active agents.

[0064] During the dispersing, kneading, and coating processes for the coating composition for the formation of the polishing layer 3, organic solvents may be used in any proportion. Examples of such organic solvents include ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; alcohols, such as methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, isobutyl alcohol, isopropyl alcohol (IPA), and methylcyclohexanol; esters, such as methyl acetate, ethyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, and 2-ethoxyethyl acetate; ethers, such as diethyl ether, tetrahydrofuran, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, and dioxane; aromatic hydrocarbons, such as benzene, toluene, xylene, cresol, chlorobenzene, and styrene; chlorinated hydrocarbons, such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, and dichlorobenzene; N,N-dimethylformamide, and hexane. Ordinarily, two or more of the above-enumerated organic solvents are used in combination in arbitrary proportions. The organic solvents may contain small amounts of impurities (e.g., polymerization products of the organic solvents, moisture, and raw material constituents of the organic solvents) in proportions of not larger than 1% by weight.

[0065] No limitation is imposed on how the dispersing and kneading processes for the coating composition for the formation of the polishing layer are carried out. The order, in which the constituents (the resins, the particles, the lubricants, the solvents, and the like) are added, the timing, with which the constituents are added during the dispersion and kneading processes, the temperature at which the dispersion process is carried out (and which will ordinarily fall within the range of 0° C. to 80° C.), and the like, may be selected appropriately. In order to prepare the coating composition for the formation of the polishing layer, one of various types of kneading machines may be used. For example, it is possible to use a twin roll mill, a triple roll mill, a ball mill, a pebble mill, a trom mill, a paint shaker, a sand grinder, a Szegvari attritor, a high-speed impeller dispersing machine, a high-speed stone mill, a high-speed impact mill, a disperser, a kneader, a high-speed mixer, a ribbon blender, a co-kneader, an intensive mixer, a tumbler, a blender, a homogenizer, a single-screw extruder, a twin-screw extruder, or an ultrasonic dispersing machine. As auxiliary means for the dispersing and kneading techniques, steel balls, steel beads, ceramic beads, glass beads, and organic polymer beads, which have sizes equivalent to sphere diameters of 0.05 mm to 10 cm, maybe used in order to carry out the dispersing and kneading processes efficiently. The shapes of these materials are not limited to spherical shapes.

[0066] The substrate 2 may be made from one of various materials, which have smooth surfaces and onto which the coating composition for the formation of the polishing layer is capable of being applied. From the view point of the cost and easiness of processing, a polyethylene terephthalate (PET) film should preferably be utilized as the substrate 2. The substrate 2 may have a thickness falling within the range of 25 μm to 100 μm, and should preferably have a thickness of 75 μm. Before the coating composition for the formation of the polishing layer is applied onto the substrate 2, the substrate 2 may be subjected to pre-treatment for enhancing the adhesion with the polishing layer 3, such as corona discharge treatment, plasma treatment, electron beam (EB) treatment, adhesive layer coating treatment, and/or heat treatment.

[0067] After the polishing layer 3 has been formed on the substrate 2, the polishing layer 3 is dried at a temperature falling within the range of 11° C. to 130° C. and cooled. Thereafter, the thus formed polishing member web is wound up, slitted, and then processed into polishing members having a desired shape.

[0068] The slurry utilized as the polishing liquid 16 contains water and free polishing particles, which have been dispersed in water. The free polishing particles contained in the slurry have a mean particle size falling within the range of 5 nm to 50 nm, and should preferably have a mean particle size falling within the range of 5 nm to 30 nm. The slurry may also contain an alcohol, such as methanol. As the free polishing particles, particles of silica, cerium oxide, titanium oxide, aluminum oxide, or the like, may be utilized. One kind of the free polishing particles may be used alone. Alternatively, two or more kinds of the free polishing particles maybe used in combination. The solid concentration of the slurry may fall within the range of 1 to 50 parts by weight, and should preferably fall within the range of 5 to 30 parts by weight.

EXAMPLES

[0069] The present invention will further be illustrated by the following nonlimitative examples. In these examples, the term “parts” means parts by weight.

[0070] In Examples 1 to 18 and Comparative Examples 1, 2, and 3, colloidal silica (supplied by CATALYSTS & CHEMICALS INDUSTRIES Co., Ltd.) was employed as the silica particles. The mean particle diameter X (in units of nm) of the first particles, the mean particle diameter Y (in units of μm) of the second particles, and the magnification ratio Y/X of the mean particle diameter of the second particles to the mean particle diameter of the first particles was set as listed in Table 2 shown later. Also, as the binders, polyurethane resins A to F (supplied by NIPPON POLYURETHANE INDUSTRY Co., Ltd.) were employed. The polyurethane resins A to F had the characteristics (the glass transition temperature Tg, the elongation, and the molecular weight) shown in Table 1.

TABLE 1
Resin	Tg (° C.)	Elongation (%)	Molecular Weight (Mw)
Resin A	−3	500	50000
Resin B	20	420	50000
Resin C	40	270	50000
Resin D	63	170	30000
Resin E	85	10 or less	30000
Resin F	106	10 or less	30000

Example 1

[0071] Constituents for a coating composition for the formation of a polishing layer, which were shown below, were introduced into a mechanical type of kneader and kneaded together by rotation of blades. Thereafter, the kneaded mixture was thus diluted. The resulting mixture was then introduced into a dispersing machine and subjected to a dispersing process. Thereafter, additives, and the like, were added to the mixture, and the dispersing process was performed even further. In this manner, a coating composition for the formation of a polishing layer was prepared.

[0072] The thus prepared coating composition for the formation of a polishing layer was applied onto a 75 μm-thick polyethylene terephthalate (PET) substrate and at a rate such that the thickness of the coating layer of the coating composition after being dried might become equal to 5 μm. The coating layer of the coating composition was then dried, and a polishing layer was thus formed on the PET substrate. In this manner, a sample of a polishing sheet was obtained. The drying process was performed in an oven at a temperature of 115° C. for 60 seconds.

[0073] In this example, the mean particle diameter X of the first particles was 12 nm. The mean particle diameter Y of the second particles was 2.7 μm. Therefore, the magnification ratio Y/X of the mean particle diameter of the second particles to the mean particle diameter of the first particles was equal to 225. The oil absorption of the second particles was 300 ml/100 g. Also, the second particles were used in a proportion of 1 part by weight per 100 parts by weight of the first particles.

[0074] The thus obtained polishing sheet was processed into a circular disk having a diameter of 110 mm, and fitted on a polishing machine (supplied by Seiko Instruments Inc.). Also, 3 ml of distilled water was used as the polishing liquid, and end faces of 12 optical fiber connectors having a diameter of 2.5 mm was polished simultaneously for 90 seconds. Thereafter, as for the polished optical fibers, the return loss characteristics, the difference in level between the polished end faces of the optical fiber and the ferrule, and the occurrence of flaws were investigated.

[0075] [Constitution of Coating Composition for the Formation of Polishing Layer]

	Polishing particles:
	First particles (silica, mean	10	parts
	particle diameter: 12 nm)
	Second particles (silica, mean particle	0.1	part
	diameter: 2.7 μm)
	Binder (resin A)	10	parts
	Solvent (IPA)	79	parts
	(Total)	(99.1	parts)

Example 2

[0076] A polishing sheet was formed in the same manner as that in Example 1, except that, in lieu of the resin A, the resin B was used as the binder. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 3

[0077] A polishing sheet was formed in the same manner as that in Example 1, except that, in lieu of the resin A, the resin C was used as the binder. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 4

[0078] A polishing sheet was formed in the same manner as that in Example 1, except that silica particles having a mean particle diameter of 5 nm were utilized as the first particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 5

[0079] A polishing sheet was formed in the same manner as that in Example 1, except that silica particles having a mean particle diameter of 30 nm were utilized as the first particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 6

[0080] A polishing sheet was formed in the same manner as that in Example 1, except that silica particles having a mean particle diameter of 3.6 μm (300 times as large as the mean particle diameter of the first particles) and exhibiting oil absorption of 290 ml/100 g were utilized as the second particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 7

[0081] A polishing sheet was formed in the same manner as that in Example 1, except that silica particles having a mean particle diameter of 9.6 μm (800 times as large as the mean particle diameter of the first particles) and exhibiting oil absorption of 250 ml/100 g were utilized as the second particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 8

[0082] A polishing operation was performed in the same manner as that in Example 1, except that, in lieu of distilled water, a silica slurry containing silica particles having a mean particle diameter of 10 nm was utilized as the polishing liquid.

Example 9

[0083] A polishing sheet was formed in the same manner as that in Example 6. Also, a polishing operation was performed in the same manner as that in Example 6, except that the same silica slurry as the silica slurry in Example 8 was utilized as the polishing liquid.

Example 10

[0084] A polishing sheet was formed in the same manner as that in Example 6. Also, a polishing operation was performed in the same manner as that in Example 6, except that a silica slurry containing silica particles having a mean particle diameter of 30 nm was utilized as the polishing liquid.

Example 11

[0085] A polishing sheet was formed in the same manner as that in Example 1, except that the proportion of the second particles was altered from 1 part by weight per 100 parts by weight of the first particles to 5 parts by weight per 100 parts by weight of the first particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 12

[0086] A polishing sheet was formed in the same manner as that in Example 7, except that the proportion of the second particles was altered from 1 part by weight per 100 parts by weight of the first particles to 5 parts by weight per 100 parts by weight of the first particles. Also, a polishing operation was performed in the same manner as that in Example 7 by using the thus formed polishing sheet.

Example 13

[0087] A polishing operation was performed in the same manner as that in Example 8, except that, in lieu of the silica slurry containing the silica particles, a slurry containing cerium oxide (ceria) having a mean particle diameter of 30 nm was utilized as the polishing liquid.

Example 14

[0088] A polishing sheet was formed in the same manner as that in Example 1, except that, in lieu of the resin A, the resin D was used as the binder. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 15

[0089] A polishing sheet was formed in the same manner as that in Example 1, except that, in lieu of the resin A, the resin E was used as the binder. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 16

[0090] A polishing sheet was formed in the same manner as that in Example 1, except that, in lieu of the resin A, the resin F was used as the binder. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 17

[0091] A polishing sheet was formed in the same manner as that in Example 1, except that silica particles exhibiting oil absorption of 95 ml/100 g (and having a mean particle diameter of 2.7 μm) were utilized as the second particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Example 18

[0092] A polishing sheet was formed in the same manner as that in Example 6, except that silica particles exhibiting oil absorption of 95 ml/100 g (and having a mean particle diameter of 2.7 μm) were utilized as the second particles. Also, a polishing operation was performed in the same manner as that in Example 6 by using the thus formed polishing sheet.

Comparative Example 1

[0093] A polishing sheet was formed in the same manner as that in Example 1, except that silica particles having a mean particle diameter of 1 μm were utilized as the first particles acting as the polishing particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Comparative Example 2

[0094] A polishing sheet was formed in the same manner as that in Example 1, except that silica particles having a mean particle diameter of 60 μm (5000 times as large as the mean particle diameter of the first particles) were utilized as the second particles. Also, a polishing operation was performed in the same manner as that in Example 1 by using the thus formed polishing sheet.

Comparative Example 3

[0095] A polishing operation was performed in the same manner as that in Example 1, except that a PET film having no polishing layer was utilized as the polishing member, and a silica slurry containing silica particles having a mean particle diameter of lm was utilized as the polishing liquid.

[0096] A polishing test was conducted by using the sample of the polishing sheet in each of Examples 1 to 18 and Comparative Examples 1, 2, and 3. Also, polishing characteristics were evaluated in the manner described below. The results shown in Table 2 below were obtained. In Table 2, various conditions are also shown.

	TABLE 2
	Silica particles		Return loss	Difference		Overall
	Binder resin	X nm	Y μm	Y/X	Polishing liquid	dB	in level nm	Flaws	judgement
Example 1	Resin A	12	2.7	225	Distilled water	−63.6	−5.5	⊚	◯
Example 2	Resin B	12	2.7	225	Distilled water	−62.2	−10.2	◯	◯
Example 3	Resin C	12	2.7	225	Distilled water	−61.8	−15.1	◯	◯
Example 4	Resin A	5	2.7	540	Distilled water	−62.1	−1.2	⊚	◯
Example 5	Resin A	30	2.7	90	Distilled water	−60.6	−4.3	⊚	◯
Example 6	Resin A	12	3.6	300	Distilled water	−60.3	−8.2	⊚	◯
Example 7	Resin A	12	9.6	800	Distilled water	−60.0	−4.5	⊚	◯
Example 8	Resin A	12	2.7	225	Silica Slurry (10 nm)	−66.3	+12.5	⊚	◯
Example 9	Resin A	12	3.6	300	Silica Slurry (10 nm)	−67.7	+26.1	⊚	◯
Example	Resin A	12	3.6	300	Silica Slurry (30 nm)	−65.2	+14.2	◯	◯
10
Example	Resin A	12	2.7	225	Distilled water	−64.8	−10.5	◯	◯
11
Example	Resin A	12	9.6	800	Distilled water	−64.8	−16.2	◯	◯
12
Example	Resin A	12	2.7	225	Ceria slurry (30 nm)	−63.2	−25.2	⊚	◯
13
Example	Resin D	12	2.7	225	Distilled water	−59.6	−36.2	◯	◯
14
Example	Resin E	12	2.7	225	Distilled water	−58.2	−30.0	◯	◯
15
Example	Resin F	12	2.7	225	Distilled water	−58.3	−40.3	◯	◯
16
Example	Resin A	12	2.7	225	Distilled water	−58.1	−10.3	◯	◯
17
Example	Resin A	12	2.7	300	Distilled water	−57.7	−12.4	◯	◯
18
Comp.	Resin A	1000	2.7	2.7	Distilled water	−47.1	−80.2	X	X
Ex. 1
Comp.	Resin A	12	60	5000	Distilled water	−48.3	−30.2	X	X
Ex. 2
Comp.	—	—	—	—	Silica Slurry (1 μm)	−49.8	−80.2	X	X
Ex. 3

[0097] <Evaluation Method>

[0098] The sample of the polishing sheet was attached to a polishing machine, and a polishing operation was performed in the manner described below. Specifically, 3 ml of the polishing liquid was supplied onto the sample of the polishing sheet, and the end faces of the 12 optical fiber connectors having a diameter of 2.5 mm were polished simultaneously for 90 seconds. After the polishing operation was completed, the optical fiber connectors were washed with water, and the end faces of the optical fiber connectors were cleaned. Thereafter, evaluation was made in the manner described below. As the polishing machine, OFL-12 (supplied by Seiko Instruments Inc.) was used.

[0099] Evaluation of Return Loss Characteristics:

[0100] The return loss characteristics (in units of dB) of the optical fiber were measured at λ of 1,310 nm by use of a testing machine (RM3750B, supplied by JDS Uniphase Corporation). As the return loss characteristics, the quantity of loss of the transferred light quantity due to reflection from the polished face of the optical fiber was measured. The return loss characteristics are represented by 10 log(P1/P2), in which P2 represents the quantity of input light, and P1 represents the quantity of output light. In accordance with the return loss characteristics, the surface smoothness, or the like, is capable of being rated. A small dB value, i.e. a dB value which takes a large value to the minus side, indicates a good transfer state with low reflection, little transfer loss, and a high transfer efficiency.

[0101] Evaluation of Difference in Level Between Polished End Faces of Optical Fiber and Ferrule:

[0102] The difference in level (in units of nm) between the polished end faces of the optical fiber and the ferrule was measured with a measuring machine (ZX-1 MINI, supplied by Direct Optical Research Company). Specifically, the difference between the height of a center point of a virtual curved surface of the ferrule end face and the height of a center point of the optical fiber end face was measured as the difference in level between the polished end faces of the optical fiber and the ferrule. A “+” value represents the protruding direction. A value close to ±0 nm is optimum as the aforesaid difference in level. An allowable range of the aforesaid difference in level is ±50 nm.

[0103] Evaluation of Flaws on Fiber End Face:

[0104] The polished end face was observed with a microscope of 400-power magnification, and the number of the optical fibers having flaws among the 12 optical fibers was counted. The mark “⊚” represents that the number of the optical fibers having flaws was at most one. The mark “◯” represents that the number of the optical fibers having flaws was two or three. The mark “Δ” represents that the number of the optical fibers having flaws was four or five. The mark “x” represents that the number of the optical fibers having flaws was at least six. Overall judgment: The mark “◯” represents that good results were obtained with respect to all of the three evaluation items described above. The mark “Δ” represents that bad results were obtained with respect to at least one of the three evaluation items described above. The mark “x” represents that bad results were obtained with respect to all of the three evaluation items described above.

[0105] <Results of Evaluation>

[0106] As described above, in each of Examples 1 to 18, the polishing layer contained the first particles, which acted as the polishing particles and were constituted of silica particles having a mean particle diameter falling within the range of 5 nm to 30 nm, and the second particles, which were constituted of silica particles having a mean particle diameter 90 to 800 times as large as the mean particle diameter of the first particles. Therefore, as clear from Table 2, the polishing operation could be performed, such that flaws due to scraped fragments did not occur on the polished end face of the optical fiber, and the polished end face of the optical fiber had good characteristics (i.e., good return loss characteristics and little difference in level between the polished end faces of the optical fiber and the ferrule).

[0107] In each of Examples 8, 9, 10, and 13, the slurry containing free polishing particles (silica or cerium oxide) was utilized as the polishing liquid. Therefore, more enhanced results could be obtained. Also, in each of Examples 11 and 12, wherein the amount of the second particles added was increased, good results could be obtained. Further, in each of Examples 14, 15, and 16, wherein the resin (polyurethane resin) having a high glass transition temperature Tg and low elongation was utilized as the binder, the return loss characteristics of the polished fiber end face, which were better than the return loss characteristics obtained in Examples 1, 2, and 3, could be obtained. Furthermore, in each of Examples 17 and 18, wherein the silica particles exhibiting low oil absorption were utilized as the second particles, the results of the polishing operation were not as good as the results obtained in Examples 1, 2, and 3.

[0108] In Comparative Example 1, the mean particle diameter of the first particles was large, and the difference in particle diameter between the first particles and the second particles was small. Therefore, the scraped fiber fragments could not be discharged sufficiently, and flaws occurred on the end face of the optical fiber. Also, the difference in level between the polished end faces of the optical fiber and the ferrule was large, and the return loss characteristics were bad.

[0109] In Comparative Example 2, the mean particle diameter of the second particles was large, and the difference in particle diameter between the first particles and the second particles was very large. Therefore, flaws occurred on the end face of the optical fiber. Also, the return loss characteristics were bad.

[0110] In Comparative Example 3, the polishing operation was performed only with the free polishing particles contained in the silica slurry. Therefore, flaws occurred on the end face of the optical fiber. Also, the difference in level between the polished end faces of the optical fiber and the ferrule was large, and the return loss characteristics were bad.

[0111] An embodiment of the method of polishing an end face of an optical fiber in accordance with the present invention will be described hereinbelow. The embodiment of the method of polishing an end face of an optical fiber in accordance with the present invention may be illustrated by the same conceptual front view as that of FIG. 1. Therefore, the embodiment of the method of polishing an end face of an optical fiber in accordance with the present invention will be described hereinbelow with reference to FIG. 1.

[0112] With reference to FIG. 1, a polishing sheet (a polishing member) 1 utilized in the embodiment of the method of polishing an end face of an optical fiber in accordance with the present invention comprises a substrate 2, which is formed from polyester film, or the like, and which has a thickness falling within the range of 25 μm to 100 μm, and a polishing layer 3, which is overlaid on the substrate 2. The polishing layer 3 comprises a binder and silica particles, which act as polishing particles and which are dispersed in the binder. The binder is constituted of at least one resin selected from the group consisting of a thermoplastic resin, a thermosetting resin, and an ultraviolet-curing resin. In the polishing layer 3, the binder is contained in a proportion falling within the range of 10 to 200 parts by weight per 100 parts by weight of the polishing particles. The polishing layer 3 has a thickness falling within the range of, for example, 1 μm to 15 μm, and should preferably have a thickness falling within the range of 5 μm to 10 μm. The polishing sheet 1 is formed as, for example, a polishing sheet having a predetermined shape, such as a disk-like shape. In the embodiment of the method of polishing an end face of an optical fiber in accordance with the present invention, as the polishing sheet 1, the polishing member 1, which is the aforesaid embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention, may also be employed.

[0113] As the polishing particles contained in the polishing layer 3, the silica particles may be utilized alone. Alternatively, the silica particles may be utilized in combination with at least one different kind of polishing particles. The silica particles have a mean particle size (D50) falling within the range of 0.001 μm to 1 μm (e.g., a mean particle size of 10 nm). In cases where the silica particles are utilized in combination with at least one different kind of polishing particles, the at least one different kind of polishing particles should preferably be selected from the group consisting of cerium oxide, alumina, chromium oxide, silicon carbide, and diamond, and the silica particles should preferably constitute at least 50% of the polishing particles contained in the polishing layer 3. The polishing particles should preferably have a mean particle size falling within the range of 0.001 μm to 1 μm. In cases where the mean particle size of the polishing particles is small, good optical characteristics are obtained from the polishing operation. However, in such cases, the dispersibility of the polishing particles becomes low, and therefore it becomes necessary for a comparatively large amount of binder to be used.

[0114] An end face of an optical fiber connector 5 is polished by the polishing sheet 1 basically in the same manner as that in the aforesaid embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention. During the polishing operation, a polishing liquid 16, which is constituted of a silica slurry, is supplied from a supply nozzle 15 to the polishing area. In this manner, a wet polishing operation is performed. In this case, the polishing liquid 16 is the silica slurry containing water and silica particles, which act as free polishing particles and have been dispersed in water. The silica particles contained in the silica slurry has a mean particle size falling within the range of 1 nm to 500 nm. Water contained in the silica slurry is deionized water and has an electric conductivity of at most 0.500 μS/cm at 25° C. The proportion of the silica particles (i.e., the solid concentration) falls within the range of 1 part by weight to 70 parts by weight (e.g., the proportion of the silica particles is 10 parts by weight).

[0115] In cases where the silica particles contained in the silica slurry acting as the polishing liquid 16 is fine-particle silica having a mean particle size falling within the range of 1 nm to a value smaller than 20 nm (e.g., a mean particle size of approximately 5 nm), an alcohol should preferably be added to the silica slurry, such that the fine-particle silica may not agglomerate during a long period of storage. In such cases, the pH value of the silica slurry should preferably fall within the range of 3 to a value smaller than 8. As the alcohol, ethanol, methanol, ethylene glycol, isopropanol, or the like may be utilized. The alcohol is added to the silica slurry in a proportion falling within the range of 3 to 50 parts by weight.

[0116] In cases where the silica particles contained in the silica slurry acting as the polishing liquid 16 is coarse-particle silica having a mean particle size falling within the range of 20 nm to 500 nm (e.g., a mean particle size of approximately 30 nm), the pH value of the silica slurry is adjusted at a value falling within the range of 8 to 12 by the addition of a base, such that no difference in level may occur between polished faces of the optical fiber 7 and the ferrule 6. In such cases, the alcohol described above may be added to the silica slurry.

[0117] The polishing liquid 16 may be prepared with a process wherein, while water, an alcohol, and the like, are being stirred quickly, the silica particles are introduced little by little into the liquid which is being stirred, and the resulting mixture is stirred quickly. When necessary, a mildew-proofing agent, a dispersing agent, and other additives may be added to the polishing liquid 16.

[0118] The surface of the polishing layer 3 of the polishing sheet 1 should preferably be provided with a fine concavity-convexity pattern, such as a worm-shaped wrinkle pattern, a block-shaped pattern, an orange peel-shaped pattern, an arrayed protrusion pattern, or a random protrusion pattern.

[0119] FIG. 2 is an enlarged explanatory view showing an example of a surface of a polishing layer. By way of example, as illustrated in FIG. 2, the fine concavity-convexity pattern may be the worm-shaped wrinkle pattern. Specifically, in the example illustrated in FIG. 2, the surface of the polishing layer 3 has a flat area 31, which constitutes a major part of the surface of the polishing layer 3, and at which the concentration of the polishing particles takes an ordinary value. Also, the surface of the polishing layer 3 is provided with discontinuous worm-shaped wrinkle areas 32, 32, . . . , at which the concentration of the polishing particles is low. Each of the worm-shaped wrinkle areas 32, 32, . . . has a length falling within the range of 0.1 mm to 30 mm and a width falling within the range of 25 μm to 200 μm. The worm-shaped wrinkle areas 32, 32, . . . are located at a density falling within the range of 10 pieces per 100 mm2 to 200 pieces per 100 mm2. The surface of the flat area 31 is smooth (e.g., has a surface roughness Ra, expressed in terms of arithmetical mean roughness, falling within the range of 0.001 μm to 0.1 m). The worm-shaped wrinkle areas 32, 32, . . . are formed as grooves.

[0120] By way of example, the worm-shaped wrinkle areas 32, 32, . . . may be formed in the manner described below. Specifically, a coating composition for the formation of the polishing layer 3 is prepared by mixing the silica particles, which act as the polishing particles, and a binder, which contains, for example, a polyurethane resin having the characteristics, such that the glass transition temperature Tg is at most 40° C., and the elongation is at least 200%. The proportion of the binder is set to fall within the range of 20 to 200 parts by weight per 100 parts by weight of the polishing particles. Also, the solid concentration in the coating composition for the formation of the polishing layer 3 is adjusted at a value falling within the range of 5% by weight to 30% by weight. The thus prepared coating composition is applied onto the substrate 2 and dried at a temperature of, for example, 115° C. The worm-shaped wrinkle areas 32, 32, . . . may be formed due to shrinkage of the coating composition when the coating composition is thus dried. The worm-shaped wrinkle areas 32, 32, . . . are formed so as to extend in the vertical direction in FIG. 2, which direction corresponds to the movement direction of the substrate (film) 2. The width of each of the worm-shaped wrinkle areas 32, . . . 32, is small at both ends and increases toward the middle part of the worm-shaped wrinkle area. Also, each of the worm-shaped wrinkle areas 32, 32, . . . extends approximately in a straight line or in a zigzag form. The region within each of the worm-shaped wrinkle areas 32, 32, . . . tears as if the region were pulled toward the both ends of the worm-shaped wrinkle area. The region within each of the worm-shaped wrinkle areas 32, . . . 32, is thus formed as a groove. The bottom of the groove has a concavity-convexity pattern extending in the width direction of the worm-shaped wrinkle area. The worm-shaped wrinkle areas 32, 32, . . . have worm-like plane shapes. The worm-shaped wrinkle areas 32, 32, . . . , at which the concentration of the polishing particles is low, do not extend in the direction normal to the movement direction of the substrate 2 and are formed such that the worm-shaped wrinkle areas 32, 32 adjacent to each other with respect to the horizontal direction in FIG. 2 are discontinuous from each other.

[0121] FIG. 3 is an enlarged explanatory view showing a different example of a surface of a polishing layer. In this example, as illustrated in FIG. 3, the fine concavity-convexity pattern is constituted as the block-shaped pattern. Specifically, in the example illustrated in FIG. 3, the block-shaped pattern, which is constituted of an aggregation of approximately rectangular block areas 33, 33, . . . arrayed continuously in a cell membrane form of a plant, is formed on the surface of the polishing layer 3 of the polishing sheet 1. Each of the block areas 33, 33, . . . has a size, such that the vertical length falls within the range of 50 μm to 100 μm, and the horizontal length falls within the range of 75 μm to 150 μm. The block areas 33, 33, . . . are located at a density falling within the range of 50 pieces per 1 mm2 to 500 pieces per 1 mm2. The surface of the polishing layer 3 is smooth (e.g., has a surface roughness Ra, expressed in terms of arithmetical mean roughness, falling within the range of 0.001 μm to 0.1 μm). Peripheral partition wall regions of each of the block areas 33, 33, . . . have a thickness smaller than the thickness of the middle region of the block area.

[0122] By way of example, the block areas 33, 33, . . . may be formed in the manner described below. Specifically, a coating composition for the formation of the polishing layer 3 is prepared by mixing the silica particles, which act as the polishing particles, and a binder, which contains, for example, a polyurethane resin having the characteristics described above. The proportion of the binder with respect to the polishing particles is set to fall within the range described above. Also, the solid concentration in the coating composition for the formation of the polishing layer 3 is adjusted at a value falling within the range described above. The thus prepared coating composition is applied onto the substrate 2 and dried at a temperature of, for example, 110° C. The block areas 33, 33, . . . may be formed due to shrinkage of the coating composition when the coating composition is thus dried. The block areas 33, 33, . . . are formed so as to be horizontally long with respect to the vertical direction in FIG. 3, which direction corresponds to the movement direction of the substrate (film) 2. The horizontally extending partition walls of each of the block areas 33, 33, . . . are approximately parallel with each other. The vertically extending partition walls of each of the block areas 33, 33, . . . are pulled by the horizontally extending partition wall of the adjacent block area 33. In this manner, each of the block areas 33, 33, . . . is formed so as to have a polygonal shape.

[0123] As another alternative, though not shown, the fine concavity-convexity pattern may be constituted as the orange peel-shaped pattern. The orange peel-shaped pattern is constituted of orange peel-shaped fine concavities, which are formed on the surface of the polishing layer 3 by a Bénard convection occurring due to, for example, a difference in temperature of the coating composition for the formation of the polishing layer when the coating composition has been applied onto the substrate 2. The Bénard convection, which causes the orange peel-shaped fine concavities to be formed, occurs in the manner described below. Specifically, in cases where a vertical temperature gradient is given to a horizontal liquid layer, and the temperature gradient becomes larger than a certain critical value, the liquid layer is divided into cell-shaped swirl regions (i.e., Bénard cells) having a shape ranging from a tetragonal shape to a hexagonal shape. An upward liquid flow occurs at the center area of each of the Bénard cells, and a downward liquid flow occurs at the peripheral areas of each of the Bénard cells. For the formation of the orange peel-shaped fine concavities, the Bénard convection is caused to arise when the coating composition for the formation of the polishing layer has been applied onto the substrate 2 and dried. On the surface of the polishing layer after being dried, a notch-shaped concavity is formed at the peripheral boundary regions of each of the Bénard cells, and a point-shaped concavity is formed at the center region of each of the Bénard cells. In this manner, the surface of each of the Bénard cells as a whole forms the orange peel-shaped fine concavity. The form (the size, the depth, the width, and the like, of the orange peel-shaped fine concavity) varies in accordance with the constitution of the coating composition for the formation of the polishing layer, the drying conditions, and the like. Also, as for the worm-shaped wrinkle pattern and the block-shaped pattern described above, the form of the pattern varies in accordance with the constitution of the coating composition for the formation of the polishing layer, the drying conditions, and the like.

[0124] In the process for producing the polishing sheet 1, the coating composition for the formation of the polishing layer is prepared in the manner described below. Specifically, for example, the solvent, the binder, the polishing particles, and the like, are introduced into a mechanical type of kneader and kneaded together. Thereafter, a lubricating agent, a solvent (a diluting agent), and the like, are added to the kneaded mixture, and the viscosity of the kneaded mixture is adjusted. Thereafter, the kneaded mixture is introduced into a dispersing machine. In the dispersing machine, a curing agent (a crosslinking agent), a solvent (an adjusting solvent), and the like, are added to the kneaded mixture, and the resulting mixture is subjected to the dispersing process. In this manner, the coating composition for the formation of the polishing layer, in which the solid concentration has been adjusted at a value falling within the range of 5% by weight to 50% by weight, is prepared. The thus prepared coating composition for the formation of the polishing layer is applied to a predetermined thickness onto the substrate 2, which is being moved at a predetermined speed, in a coating apparatus employing a micro gravure coating technique, a comb coating technique, a Meyer bar coating technique, or the like. The applied coating composition for the formation of the polishing layer is then dried in an atmosphere at a temperature of 25° C. to 130° C. In this manner, the polishing layer 3 is formed on the substrate 2. During the drying operation, the fine concavity-convexity pattern composed of the worm-shaped wrinkle areas 32, 32, . . . , the block areas 33, 33, . . . , or the like, is formed under the aforesaid conditions on the surface of the polishing layer 3. Alternatively, the constitution of the coating composition for the formation of the polishing layer, the processing conditions, and the like, may be set such that the polishing layer 3 may have a flat surface. Thereafter, punching, slitting, and the like, are performed to form the polishing sheet 1 having a predetermined shape.

[0125] The thermoplastic resins, which may be used as the binder in the polishing layer 3, have a softening point of 200° C. or lower, an average molecular weight falling within the range of approximately 10,000 to approximately 300,000, and a polymerization degree falling within the range of approximately 50 to approximately 2,000. The polymerization degrees of the thermoplastic resins should preferably fall within the range of approximately 200 to approximately 800. Specifically, as the thermoplastic resin, it is possible to use, for example, a vinyl chloride-vinyl acetate copolymer, a vinyl chloride copolymer, a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a vinyl chloride-vinyl alcohol copolymer, a vinyl chloride-vinylidene chloride copolymer, a vinyl chloride-acrylonitrile copolymer, an acrylic ester-acrylonitrile copolymer, an acrylic ester-vinylidene chloride copolymer, an acrylic ester-styrene copolymer, a methacrylic ester-acrylonitrile copolymer, a methacrylic ester-vinylidene chloride copolymer, a methacrylic ester-styrene copolymer, a urethane elastomer, a nylon-silicone resin, a nitrocellulose-polyamide resin, polyvinyl fluoride resin, a vinylidene chloride-acrylonitrile copolymer, a butadiene-acrylonitrile copolymer, a polyamide resin, a polyvinyl butyral resin, a cellulose derivative (such as cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, ethyl cellulose, methyl cellulose, propyl cellulose, methyl ethyl cellulose, carboxymethyl cellulose, or acetyl cellulose), a styrene-butadiene copolymer, a polyester resin, a polycarbonate resin, a chlorovinyl ether-acrylic ester copolymer, an amino resin, a polyamide resin, a synthetic rubber type thermoplastic resin, or a mixture of two or more of the above-enumerated compounds.

[0126] As the thermosetting resins or the reactive resins, which may be used as the binder in the polishing layer 3, the polyurethane resins and other resins, which are described above with reference to the embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention may be employed.

[0127] In general, the thermoplastic resins, the thermosetting resins, and the reactive resins described above respectively have their major functional groups, and one to six kinds of other functional groups, which are described above with reference to the embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention.

[0128] In the polishing layer 3, polyisocyanates may be contained as a curing agent (a crosslinking agent). As the polyisocyanates, it is possible to use, for example, isocyanates, such as tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, and triphenylmethane triisocyanate. As the polyisocyanates, it is also possible to use products of reactions between the above-enumerated isocyanates and polyalcohols, and dimer to decamer polyisocyanates produced from condensation of isocyanates, and products which are obtained from reactions between polyisocyanates and polyurethanes and which have isocyanate groups as terminal functional groups. The polyisocyanates enumerated above should preferably have an average molecular weight falling within the range of 100 to 20,000. The polyisocyanates enumerated above may be used alone, or a mixture of two or more of them may be used by the utilization of differences in curing reaction properties.

[0129] Also, in order to promote the curing reaction, compounds having a hydroxyl group (such as butanediol, hexanediol, polyurethane having a molecular weight within the range of 1,000 to 10,000, and water), compounds having an amino group (such as monomethylamine, dimethylamine, and trimethylamine), catalysts, such as metal oxides and iron acetylacetonate, may be used together with the polyisocyanates. The compounds having a hydroxyl group or an amino group should preferably be polyfunctional. The proportions of the polyisocyanate used should preferably fall within the range of 2 to 70 parts by weight per 100 parts by weight of the total of the binder and the polyisocyanate, and should more preferably fall within the range of 5 to 50 parts by weight per 100 parts by weight of the total of the binder and the polyisocyanate.

[0130] As in the aforesaid embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention, when necessary, additives having various functions, such as a dispersing agent, a lubricating agent, an antistatic agent, an antioxidant, a mildew-proofing agent, and a solvent, may be added to the polishing layer 3. Also, the dispersing and kneading processes for the coating composition for the formation of the polishing layer may be performed in the same manner as that in the aforesaid embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention. Further, the substrate 2 may be constituted in the same manner as that in the aforesaid embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention.

[0131] After the polishing layer 3 has been formed on the substrate 2, the polishing layer 3 is dried at a temperature falling within the range of 25° C. to 130° C. and cooled. Thereafter, the thus formed polishing sheet web is wound up, slitted, and then processed into polishing sheets having a desired shape.

Examples

[0132] The method of polishing an end face of an optical fiber in accordance with the present invention will further be illustrated by the following nonlimitative examples. In these examples, the term “parts” means parts by weight.

Example 19

[0133] Of the constitution of a coating composition for the formation of a polishing layer, constituents of a group A were introduced into a mechanical type of kneader and kneaded together by rotation of blades. Thereafter, constituents of a group B were added to the thus kneaded mixture, and the kneaded mixture was thus diluted. The resulting mixture was then introduced into a dispersing machine and subjected to a dispersing process. Thereafter, constituents of a group C were added to the mixture, and the dispersing process was performed even further. In this manner, a coating composition for the formation of a polishing layer was prepared.

[0134] The thus prepared coating composition for the formation of a polishing layer was applied onto a 75 μm-thick polyethylene terephthalate (PET) substrate and at a rate such that the thickness of the coating layer of the coating composition after being dried might become equal to 5 μm. The coating layer of the coating composition was then dried, and a polishing layer was thus formed on the PET substrate. In this manner, a sample of a polishing sheet was obtained. The drying process was performed in an oven at a temperature of 115° C. for 60 seconds, and a worm-shaped wrinkle pattern was thereby formed on the surface of the polishing layer. The thus obtained polishing sheet was processed into a circular disk having a diameter of 100 mm, and fitted on a polishing machine (supplied by Seiko Instruments Inc.).

[0135] [Constitution of Coating Composition for the Formation of Polishing Layer]

	- Group A -
	Polishing particles (silica particles,	100	parts
	mean particle size: 10 nm)
	Binder (polyester resin)	6	parts
	Solvent (MEK)	20	parts
	- Group B -
	Binder (polyurethane resin,	5	parts
	content of sulfonic acid sodium salt:
	2 × 10−3 equivalents per g of resin,
	Mw: 70,000)
	Lubricating agent (stearic acid/oleyl oleate)	0.5	part
	Diluting agent	150	parts
	(methyl ethyl ketone/cyclohexanone = 2/1)
	- Group C -
	Curing agent (polyisocyanate,	5	parts
	a reaction product of 3 mols of
	tolylene diisocyanate with 1 mol
	of trimethylolpropane)
	Solvent	200	parts
	(methyl ethyl ketone/cyclohexanone = 2/1)

[0136] Also, as a polishing liquid, a silica slurry having a constitution shown below and containing fine-particle silica and an alcohol was prepared.

[0137] With the polishing machine described above, an end face of an optical fiber connector having a diameter of 2.5 mm was polished for 90 seconds by using the polishing sheet described above and the silica slurry acting as the polishing liquid. Thereafter, the return loss characteristics (i.e., the light quantity transfer loss characteristics) of the optical fiber was measured.

	[Slurry constitution]
	Silica (mean particle size: 5 nm)	10	parts
	Distilled water	85	parts
	Ethanol	5	parts
	EDTA	0.01	part
	2-Methyl tetraazaindene	0.01	part

Examples 20 and 21

[0138] In each of Examples 20 and 21, as a polishing sheet, the polishing sheet formed in Example 19, in which the silica particles were employed as the polishing particles, was used. In Example 20, as a polishing liquid, a silica slurry was prepared in the same manner as that in Example 19, except that the mean particle size of the silica contained in the silica slurry was altered to 10 nm. Also, in Example 21, as a polishing liquid, a silica slurry was prepared in the same manner as that in Example 19, except that the mean particle size of the silica contained in the silica slurry was altered to 15 nm. In each of Examples 20 and 21, the polishing operation was performed in the same manner as that in Example 19 by using the polishing sheet described above and the thus prepared silica slurry acting as the polishing liquid.

Comparative Example 4

[0139] A sample of a polishing sheet was formed in the same manner as that in Example 19, except that, in lieu of the silica particles, cerium oxide particles having a mean particle size of 0.5 μm were employed as the polishing particles contained in the polishing sheet. As the polishing liquid, the same silica slurry as that in Example 19 was used. The polishing operation was performed in the same manner as that in Example 19 by using the thus formed polishing sheet and the silica slurry acting as the polishing liquid.

Comparative Example 5

[0140] A sample of a polishing sheet was formed in the same manner as that in Example 19, except that, in lieu of the silica particles, iron oxide particles having a mean particle size of 0.1 μm were employed as the polishing particles contained in the polishing sheet. As the polishing liquid, the same silica slurry as that in Example 19 was used. The polishing operation was performed in the same manner as that in Example 19 by using the thus formed polishing sheet and the silica slurry acting as the polishing liquid.

Comparative Example 6

[0141] A sample of a polishing sheet was formed in the same manner as that in Example 19, except that, in lieu of the silica particles, chromium oxide particles having a mean particle size of 0.1 μm were employed as the polishing particles contained in the polishing sheet. As the polishing liquid, the same silica slurry as that in Example 19 was used. The polishing operation was performed in the same manner as that in Example 19 by using the thus formed polishing sheet and the silica slurry acting as the polishing liquid.

Comparative Example 7

[0142] As a polishing sheet, the polishing sheet formed in Example 19, in which the silica particles were employed as the polishing particles, was used. Also, as the polishing liquid, distilled water was used. The polishing operation was performed in the same manner as that in Example 19 by using the polishing sheet described above and distilled water acting as the polishing liquid.

Comparative Example 8

[0143] As a polishing sheet, the polishing sheet formed in Comparative Example 4, in which the cerium oxide particles having a mean particle size of 0.5 μm were employed as the polishing particles contained in the polishing sheet, was used. Also, as the polishing liquid, distilled water was used. The polishing operation was performed in the same manner as that in Example 19 by using the polishing sheet described above and distilled water acting as the polishing liquid.

Comparative Example 9

[0144] As a polishing sheet, the polishing sheet formed in Comparative Example 5, in which the iron oxide particles having a mean particle size of 0.1 μm were employed as the polishing particles contained in the polishing sheet, was used. Also, as the polishing liquid, distilled water was used. The polishing operation was performed in the same manner as that in Example 19 by using the polishing sheet described above and distilled water acting as the polishing liquid.

Comparative Example 10

[0145] As a polishing sheet, the polishing sheet formed in Comparative Example 6, in which the chromium oxide particles having a mean particle size of 0.1 μm were employed as the polishing particles contained in the polishing sheet, was used. Also, as the polishing liquid, distilled water was used. The polishing operation was performed in the same manner as that in Example 19 by using the polishing sheet described above and distilled water acting as the polishing liquid.

Comparative Examples 11 and 12

[0146] As a polishing sheet, the polishing sheet formed in Example 19, in which the silica particles were employed as the polishing particles, was used. In Comparative Example 11, as a polishing liquid, a silica slurry was prepared in the same manner as that in Example 19, except that the mean particle size of the silica contained in the silica slurry was altered to 20 nm. Also, in Comparative Example 12, as a polishing liquid, a silica slurry was prepared in the same manner as that in Example 19, except that the mean particle size of the silica contained in the silica slurry was altered to 30 nm. In each of Comparative Examples 11 and 12, the polishing operation was performed in the same manner as that in Example 19 by using the polishing sheet described above and the thus prepared silica slurry acting as the polishing liquid.

[0147] A polishing test was conducted by using the sample of the polishing sheet and the polishing liquid in each of Examples 19, 20, and 21 and Comparative Examples 4 to 12. Also, polishing characteristics (the return loss characteristics and the difference in level between polished end faces of the optical fiber and the ferrule) were evaluated in the manner described below. The results shown in Table 3 below were obtained.

	TABLE 3
	Polishiing sheet			Difference
	(polishing	Polushing liquid	Return	in level
	particles)	(particle dia.)	loss dB	nm
Example 19	Silica	Silica slurry (5 nm)	−67	+0
Example 20	Silica	Silica slurry	−67	+0
		(10 nm)
Example 21	Silica	Silica slurry	−67	+0
		(15 nm)
Comp. Ex. 4	Cerium oxide	Silica slurry (5 nm)	−52	−30
Comp. Ex. 5	Iron oxide	Silica slurry (5 nm)	−48	−60
Comp. Ex. 6	Chromium	Silica slurry (5 nm)	−53	−20
	oxide
Comp. Ex. 7	Silica	Distilled water	−55	−20
Comp. Ex. 8	Cerium oxide	Distilled water	−46	−10
Comp. Ex. 9	Iron oxide	Distilled water	−44	−40
Comp. Ex.	Chromium	Distilled water	−45	−30
10	oxide
Comp. Ex.	Silica	Silica slurry	−65	−80
11
Comp. Ex.	Silica	Silica slurry	−64	−100
12

[0148] <Evaluation Method>

[0149] The sample of the polishing sheet was attached to a polishing machine (OFL-12, supplied by Seiko Instruments Inc.), and a finish polishing operation was performed in the manner described below. Specifically, 2 ml of the polishing liquid (i.e., the silica slurry or distilled water) was supplied onto the sample of the polishing sheet, and the end face of the optical fiber connector having a diameter of 2.5 mm was polished for 60 seconds. In the finish polishing operation for polishing the end face of the optical fiber connector, the rotation speed was set at 30 rpm. After the finish polishing operation was completed, the optical fiber connector was washed with water, and the end face of the optical fiber connector was cleaned. Thereafter, the return loss characteristics and the difference in level between the polished end faces of the optical fiber and the ferrule were evaluated in the same manner as that in the aforesaid embodiment of the polishing member for polishing an end face of an optical fiber in accordance with the present invention. Before the finish polishing operation was performed in the manner described above, a pre-polishing operation with a diamond polishing sheet (D4000) was performed for 60 seconds.

[0150] <Results of Evaluation>

[0151] As described above, in each of Examples 19, 20, and 21, the silica particles were employed as the polishing particles contained in the polishing sheet, and the silica slurry containing the fine-particle silica having a mean particle size falling within the range of 5 nm to 15 nm was employed as the polishing liquid. Therefore, as clear from Table 3, with the polishing operation in each of Examples 19, 20, and 21, by virtue of the synergistic effects of the silica particles acting as the polishing particles contained in the polishing sheet and the silica slurry acting as the polishing liquid, the end face of the optical fiber connector was capable of being polished uniformly and accurately, and the return loss characteristics as good as −67 dB were capable of being obtained. In each of Examples 19, 20, and 21, the end face polishing operation was thus capable of being performed such that markedly good optical characteristics were obtained. Also, in each of Examples 19, 20, and 21, the difference in level between the polished end faces of the optical fiber and the ferrule was as small as 0 nm. Further, the polished end face of the optical fiber was free from any polishing flaw and any damage.

[0152] In each of Comparative Examples 4, 5, and 6, the silica slurry was employed as the polishing liquid. However, in lieu of the silica particles, the polishing layer of the polishing sheet contained other polishing particles. Therefore, the aforesaid synergistic effects of the silica particles acting as the polishing particles contained in the polishing sheet and the silica slurry acting as the polishing liquid could not be obtained. As a result, the return loss characteristics were −48 dB to −53 dB and were thus worse than the return loss characteristics obtained in Example 19. Also, the difference in level between the polished end faces of the optical fiber and the ferrule was as large as −20 nm to −60 nm.

[0153] In Comparative Example 7, the silica particles were employed as the polishing particles contained in the polishing sheet. However, in lieu of the silica slurry, distilled water was employed as the polishing liquid. Therefore, the return loss characteristics were as bad as −55 dB. Also, the difference in level between the polished end faces of the optical fiber and the ferrule was as large as −20 nm.

[0154] In each of Comparative Examples 8, 9, and 10, in lieu of the silica particles, the polishing layer of the polishing sheet contained other polishing particles. Also, distilled water was employed as the polishing liquid. Therefore, the return loss characteristics were −44 dB to −46 dB and were thus worse than the return loss characteristics obtained in each of Comparative Examples 4, 5, and 6. Also, the difference in level between the polished end faces of the optical fiber and the ferrule was as large as −10 nm to −40 nm.

[0155] In each of Comparative Examples 11 and 12, the silica slurry containing silica having a mean particle size of 20 nm or 30 nm was employed as the polishing liquid. Therefore, the return loss characteristics were −65 dB and −64 dB, but the difference in level between the polished end faces of the optical fiber and the ferrule was as large as −80 nm to −100 nm.

Example 22

[0156] Constituents shown below were kneaded and subjected to a dispersing process in accordance with the same procedure as that employed in Example 19. In this manner, a coating composition for the formation of a polishing layer was prepared. The thus prepared coating composition for the formation of a polishing layer was applied onto a 75 μm-thick polyethylene terephthalate (PET) substrate. In this manner, a sample of a polishing sheet was obtained.

[0157] [Constitution of Coating Composition for the Formation of Polishing Layer]

	- Group A -
	Polishing particles (silica particles,	100	parts
	mean particle size: 10 nm)
	Binder (polyester resin)	20	parts
	Solvent (MEK)	80	parts
	- Group B -
	Lubricating agent (oleic acid/oleyl oleate)	0.5	part
	Diluting agent	150	parts
	(methyl ethyl ketone/cyclohexanone = 2/1)
	- Group C -
	Curing agent (polyisocyanate,	7	parts
	a reaction product of 3 mols of
	tolylene diisocyanate with 1 mol
	of trimethylolpropane)
	Solvent	200	parts
	(methyl ethyl ketone/cyclohexanone = 2/1)

[0158] Also, as a polishing liquid, a silica slurry having a constitution shown below and containing coarse-particle silica was prepared. The pH value of the silica slurry was adjusted by the addition of a base. In this example, the mean particle size (X nm) of the coarse-particle silica contained in the silica slurry was 30 nm, and the pH value (Y) of the silica slurry was equal to 8.

	[Slurry constitution]
	Silica (mean particle size: X nm)	10	parts
	Distilled water	90	parts
	EDTA	0.01	part
	pH value (adjusted by the addition of	Y
	sodium hydroxide)

Example 23

[0159] As a polishing sheet, the polishing sheet formed in Example 22, in which the silica particles were employed as the polishing particles, was used. In this example, the mean particle size (X nm) of the coarse-particle silica contained in the silica slurry was 30 nm, and the pH value (Y) of the silica slurry was altered to 10.

Example 24

[0160] As a polishing sheet, the polishing sheet formed in Example 22, in which the silica particles were employed as the polishing particles, was used. In this example, the mean particle size (X nm) of the coarse-particle silica contained in the silica slurry was altered to 100 nm, and the pH value (Y) of the silica slurry was altered to 12.

Comparative Example 13

[0161] As a polishing sheet, the polishing sheet formed in Example 22, in which the silica particles were employed as the polishing particles, was used. In this example, the mean particle size (X nm) of the coarse-particle silica contained in the silica slurry was 30 nm, and the pH value (Y) of the silica slurry was altered to 7.

Comparative Example 14

[0162] As a polishing sheet, the polishing sheet formed in Example 22, in which the silica particles were employed as the polishing particles, was used. In this example, the mean particle size (X nm) of the coarse-particle silica contained in the silica slurry was 30 nm, and the pH value (Y) of the silica slurry was altered to 5. In order for the pH value (Y) of the silica slurry to be adjusted at 5, in lieu of the base (sodium hydroxide), an acid (phosphoric acid) was used.

[0163] A polishing test was conducted by using the sample of the polishing sheet and the polishing liquid in each of Examples 22, 23, and 24 and Comparative Examples 13 and 14. Also, polishing characteristics (the return loss characteristics and the difference in level between polished end faces of the optical fiber and the ferrule) were evaluated in the manner described below. The results shown in Table 4 below were obtained.

	TABLE 4
	Polishiing sheet	Sulica slurry		Difference
	(polishing	Particle		Return	in level
	particles)	dia.: X nm	pH:Y	loss dB	nm
Example 22	Silica	30	8	−65	+0
Example 23	Silica	30	10	−65	+10
Example 24	Silica	100	12	−66	−10
Comp. Ex. 13	Silica	30	7	−53	−50
Comp. Ex. 14	Silica	30	5	−55	−70

[0164] <Evaluation Method>

[0165] The sample of the polishing sheet was attached to a polishing machine (OFL-12, supplied by Seiko Instruments Inc.), and a finish polishing operation was performed in the manner described below. Specifically, 2 ml of the polishing liquid (i.e., the silica slurry) was supplied onto the sample of the polishing sheet, and the end face of the optical fiber connector having a diameter of 2.5 mm was polished for 60 seconds. In the finish polishing operation for polishing the end face of the optical fiber connector, the rotation speed was set at 30 rpm. After the finish polishing operation was completed, the optical fiber connector was washed with water, and the end face of the optical fiber connector was cleaned. Thereafter, the return loss characteristics and the difference in level between the polished end faces of the optical fiber and the ferrule were evaluated in the same manner as that described above. Before the finish polishing operation was performed in the manner described above, a pre-polishing operation with a diamond polishing sheet (D8000) was performed for 60 seconds.

[0166] <Results of Evaluation>

[0167] As described above, in each of Examples 22, 23, and 24, the silica particles were employed as the polishing particles contained in the polishing sheet. Also, the silica slurry containing the coarse-particle silica and having a pH value adjusted at a value falling within the range of 8 to 12, was employed as the polishing liquid. Therefore, as clear from Table 4, with the polishing operation in each of Examples 22, 23, and 24, the difference in level between the polished end faces of the optical fiber and the ferrule was as small as 0 nm to 10 nm. Also, by virtue of the synergistic effects of the silica particles acting as the polishing particles contained in the polishing sheet and the silica slurry acting as the polishing liquid, the end face of the optical fiber connector was capable of being polished uniformly and accurately, and the return loss characteristics as good as −65 dB to −66 dB were capable of being obtained. In each of Examples 22, 23, and 24, the end face polishing operation was thus capable of being performed such that markedly good optical characteristics were obtained.

[0168] In each of Comparative Examples 13 and 14, the silica slurry containing the coarse-particle silica having a mean particle size of 30 nm was employed as the polishing liquid. However, the pH value of the silica slurry was 7 or 5. Therefore, the difference in level between the polished end faces of the optical fiber and the ferrule was as large as −50 nm or −70 nm. Also, the return loss characteristics were as bad as −53 dB or −55 dB.

Claims

1. A polishing member for polishing an end face of an optical fiber, the polishing member comprising: i) a substrate, and ii) a polishing layer overlaid on the substrate, the polishing layer having been formed with application of a coating composition, which contains a binder and polishing particles as principal constituents, onto the substrate, wherein the polishing layer contains: a) first particles, which act as the polishing particles and are constituted of silica particles having a mean particle diameter falling within the range of 5 nm to 50 nm, and b) second particles, which are constituted of silica particles having a mean particle diameter 50 to 2,500 times as large as the mean particle diameter of the first particles.

2. A polishing member as defined in claim 1 wherein the second particles are used in a proportion falling within the range of 1 part by weight to 5 parts by weight per 100 parts by weight of the first particles.

3. A polishing member as defined in claim 1 wherein the second particles exhibit oil absorption falling within the range of 10 ml/100 g to 400 ml/100 g.

4. A polishing member as defined in claim 1 wherein the second particles have a spherical shape.

5. A polishing member as defined in claim 1 wherein the binder contained in the polishing layer contains a polyurethane resin.

6. A polishing member as defined in claim 5 wherein the polyurethane resin has the characteristics, such that a glass transition temperature Tg is at most 40° C., and/or an elongation is at least 200%.

7. A polishing member as defined in claim 1 wherein the ratio of the first particles and the second particles to the binder takes a value between 1:0.2 and 1:2.

8. A method of polishing an end face of an optical fiber, the method comprising the steps of: i) bringing an end face of an optical fiber connector, which comprises a ferrule having a center hole and an optical fiber having been inserted and secured in the center hole of the ferrule, into contact with a polishing member having a polishing layer, which comprises a binder and silica particles dispersed in the binder, ii) subjecting the end face of the optical fiber connector to sliding movement on the polishing member, while the end face of the optical fiber connector is being in contact with the polishing member, and iii) supplying a silica slurry, which acts as a polishing liquid, to the sliding contact area, at which the end face of the optical fiber connector is in sliding contact with the polishing member, the silica slurry containing fine-particle silica, which has a mean particle size falling within the range of 1 nm, inclusive, to a value smaller than 20 nm, and water, whereby the end face of the optical fiber connector is finish-polished into a convex spherical surface shape.

9. A method as defined in claim 8 wherein the silica slurry acting as the polishing liquid further contains an alcohol.

10. A method as defined in claim 8 wherein water contained in the silica slurry is deionized water and has an electric conductivity of at most 0.500 μS/cm at 25° C.

11. A method as defined in claim 8 wherein, as the polishing particles contained in the polishing layer of the polishing member, the silica particles are utilized alone or in combination with at least one different kind of polishing particles selected from the group consisting of cerium oxide, alumina, chromium oxide, silicon carbide, and diamond, and the silica particles constitute at least 50% of the polishing particles contained in the polishing layer of the polishing member.

12. A method as defined in claim 8 wherein the surface of the polishing layer of the polishing member is provided with a fine concavity-convexity pattern, such as a worm-shaped wrinkle pattern, a block-shaped pattern, an orange peel-shaped pattern, an arrayed protrusion pattern, or a random protrusion pattern.

13. A method of polishing an end face of an optical fiber, the method comprising the steps of: i) bringing an end face of an optical fiber connector, which comprises a ferrule having a center hole and an optical fiber having been inserted and secured in the center hole of the ferrule, into contact with a polishing member having a polishing layer, which comprises a binder and silica particles dispersed in the binder, ii) subjecting the end face of the optical fiber connector to sliding movement on the polishing member, while the end face of the optical fiber connector is being in contact with the polishing member, and iii) supplying a silica slurry, which acts as a polishing liquid, to the sliding contact area, at which the end face of the optical fiber connector is in sliding contact with the polishing member, the silica slurry containing coarse-particle silica, which has a mean particle size falling within the range of 20 nm, inclusive, to 500 nm, inclusive, and water, and having a pH value falling within the range of 8 to 12, whereby the end face of the optical fiber connector is finish-polished into a convex spherical surface shape.

14. A method as defined in claim 13 wherein water contained in the silica slurry is deionized water and has an electric conductivity of at most 0.500 μS/cm at 25° C.

15. A method as defined in claim 13 wherein, as the polishing particles contained in the polishing layer of the polishing member, the silica particles are utilized alone or in combination with at least one different kind of polishing particles selected from the group consisting of cerium oxide, alumina, chromium oxide, silicon carbide, and diamond, and the silica particles constitute at least 50% of the polishing particles contained in the polishing layer of the polishing member.

16. A method as defined in claim 13 wherein the surface of the polishing layer of the polishing member is provided with a fine concavity-convexity pattern, such as a worm-shaped wrinkle pattern, a block-shaped pattern, an orange peel-shaped pattern, an arrayed protrusion pattern, or a random protrusion pattern.