Ferrule for optical fiber and optical connector using this

Abstract: A ferrule capable of preventing from injuring the fiber and forming tiny cracks inside when inserting the fiber in the ferrule, and small in manufacturing cost is presented. A ferrule 1 comprising a ceramic-made cylindrical main body 1b and a metal- or plastic-made back body 1a, wherein a penetration hole 21 formed in the axial direction of the back body 1b is a stepped hole having a larger hole 25 and a smaller hole 23, without step difference in the boundary of a taper hole 24 and smaller hole 23 at the back body 1a side, and the brim of the penetration hole formed in the rear end surface of the cylindrical main body 1b has a curved surface with a radius of curvature R of 0.2 to 0.5 mm.

Description: BACKGROUND OF THE INVENTION The present invention relates to a ferrule for an optical fiber as defined by the features of the preamble portion of claim 1, mounted in an optical connector for mutually connecting optical fibers used in optical communications. As shown in Figs. 8 and 9, an optical connector C for mutually connecting an optical fiber 2 and an other optical fiber installed at an adapter side has a ferrule 1 for holding the optical fiber 2 so that it may be exposed from the leading end, provided in a plastic frame 4 so as to thrust a spring 3 from the rear side, and as shown in Fig. 10, a pair of pawls 41, 41 of nearly rectangular section are disposed at confronting positions in a drilled wall of a flange 11 having the larger end of a rear body 1a of the ferrule 1 provided in the plastic frame 4, and rotation of the ferrule 1 is prevented by fitting into interrupt recesses 12 formed at confronting positions of the larger end of the rear body 1a, thereby minimizing the connection loss. Japanese Patent Publication 7-50217. discloses a conventional ferrule for optical fiber (hereinafter called ferrule) to be mounted on an optical fiber 2, composed of a main body and a rear body, in which the rear body is formed of a resin material that is fusible by ultrasonic waves, and the resin coat layer of the fiber and the ferrule are coupled together only by heating and fusing by ultrasonic waves, and the ferrule rear body has a through to hole as larger hole, and a taper hole is formed at the same side so as to be continuous with the smaller hole of the ferrule main body. Fig. 11 relates to a conventional ferrule 1, in which the cylindrical main body 1b of the ferrule is made of ceramics, and the rear body 1a is made of metal, and in this ferrule 1, a taper hole 24 and a smaller hole 22 are formed in the cylindrical main body 1b. These prior arts had, however, the following problems. In the conventional ferrules making use of ultrasonic fusion, there is a boundary of the taper hole and smaller hole on the junction end of the ferrule main body and rear body, and the taper hole and smaller hole are not perfectly matched in this area, but a large step difference was present. When inserting the fiber into the ferrule, the taper hole is used as the insertion guide, and the fiber is inserted into the smaller hole of the ferrule main body, and at this time the leading end of the fiber advancing obliquely to the smaller hole is caught by the step difference, and a tiny crack may be formed in the fiber by this shock. In the other conventional ferrule 1 shown in Fig. 11 in which the rear body 1a is made of metal, it takes time and labor in processing the ceramic cylindrical main body 1b in a nearly conical form in order to form the taper hole 24. Besides, there is an edge corner 32 in the boundary of the smaller hole 22 and taper hole 24 in the cylindrical main body 1b, and the fiber surface might be scratched by it. Fig. 12 is a magnified view of part A in Fig. 11, in which if the edge corner 32 can be formed in a round form by injection forming or the like (left side), the edge corner 32 becomes angular by processing the smaller hole 22 in a later process (right side). A ferrule for an optical fiber connector, on which the preamble portion of claim 1 is based, is described in US-A-4 634 214. A cylindrical elongate terminus piece is held rigidly in a connector body. The through hole provided axially through the terminus piece has a diameter which is considerably smaller than that of a smaller diameter section of a stepped through-hole provided axially in the connector body. JP-A-57 056811 describes an optical fiber connector with an connector end part provided with a stepped through-hole and a disk-shaped end part inserted into the smaller end portion of said through-hole. The disk-shaped end part made out of a diamond is provided with a through-hole for allowing insertion of the optical fiber and an end portion of this through-hole facing the interior'of the through-hole of the connector end part is expanded in a concave shape. DE-A-36 27 803 describes a different type of connector for an optical fiber which comprises an elongate outer cylindrical sleeve made from hard metal and plural support stones made from ruby or sapphire inserted therein and respectively provided with a through-hole for insertion of the optical fiber. The end portion of the through-hole of the support stones facing the adjacent support stone is slightly expanded, i.e. in the form of a curved rim in order to facilitate insertion of the optical fiber. It is the object of the present invention to provide a ferrule for an optical fiber which can be manufactured at low cost and which prevents damage of the optical fiber when the same is inserted. To solve this object the invention provides a ferrule for an optical fiber as defined in claim 1 and a connector using such ferrule, i.e. a connector for mutually connecting two different optical fibers for transmitting an optical signal between the two optical fibers or for connecting an optical fiber with an optical semi-conductor element for transmitting an optical signal between the optical fiber and the optical semi-conductor element. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a ferrule for optical fiber in an embodiment of the invention; Fig. 2 is a magnified sectional view of part B in Fig. 1; Fig. 3 is a magnified sectional view of part B in Fig. 1; Fig. 4 is a sectional view of a ferrule showing a taper angle of a taper hole of the ferrule for optical fiber in Fig. 2; Fig. 5 is a sectional view of a ferrule for optical fiber in another embodiment of the invention; Fig. 6 is a sectional view of a ferrule for optical fiber in another embodiment of the invention; Fig. 7 is a sectional view of a ferrule for optical fiber in another embodiment of the invention; Fig. 8 is a perspective view showing a structure of a conventional ferrule for optical fiber; Fig. 9 is a sectional view showing a structure of a conventional optical connector; Fig. 10 is a sectional view of line A-A in Fig. 9; Fig. 11 is a sectional view of another conventional ferrule for optical fiber and Fig. 12 is a magnified sectional view of part A in Fig. 11. EMBODIMENTS Some of the embodiments of the invention are described below while referring to the accompanying drawings. Fig. 1 shows an embodiment of a ferrule 1, and this ferrule 1 is formed by fitting a cylindrical main body 1b made of ceramics such as zirconia into a mounting hole 33 of a rear body 1a made of metal such as stainless steel or made of plastic, and both ends of the ferrule 1 are opened, and there is a through-hole 21 extending in the axial direction of the cylindrical main body 1b and of the rear body 1a. This through hole 21 is formed as a smaller hole 22 of a specific diameter in the cylindrical main body 1b, and in the rear body 1a it is formed as a smaller hole 23 having substantially the same diameter as the smaller hole 22 provided in the cylindrical main body 1b near the junction 30 with the cylindrical main body 1b, and behind it is formed as a taper hole 24 as a fiber insertion guide, and further behind a larger hole 25 is provided, and the larger hole 25 and taper hole 24 are filled with adhesive after inserting the fiber so that the fiber may be fixed. The ferrule 1 thus constituted in this embodiment is a ferrule 1 composed of a ceramic-made cylindrical main body 1b and a metal-made or plastic-made rear body 1a, and the through-hole 21 formed in the axial direction of the rear body 1a is a stepped hole comprising a larger hole 25 and a smaller hole 23, and the boundary of the taper hole 24 and smaller hole 23 is formed without step difference at the rear body 1a, and therefore when inserting the fiber into the ferrule 1, the fiber 2 is smoothly guided into the smaller hole 23, and when passing through the junction of the smaller hole 23 of the rear body 1a and the smaller hole 22 of the cylindrical main body 1b, the fiber passes through straightly, and hence if there is a slight step difference in the boundary of both smaller holes 22, 23, problems of injury of the fiber or formation of tiny cracks due to the fiber 2 being caught in the step difference may hardly occur. Fig. 2 is a magnified view of part B in Fig. 1, showing the process of forming the smaller hole 22 (left side) followed by curvature finishing. Therefore, there is no corner part such as edge corner 32 in Fig. 2, and the fiber surface will not be injured. Moreover, as shown in Fig. 3, when the fiber center axis SR is deviated from the center axis P of the smaller hole 23 of the rear body 1a at the time of insertion, the leading end of the fiber 2 slides on the curved surface, and problems of the fiber 2 being caught by rough edge may hardly occur. Incidentally, the distance E between the center axis Q of the smaller hole 22 of the cylindrical main-body 1b and the center axis P of the smaller hole 23 of the rear body 1a is preferred to be 20 µm or less. When this distance is not more than 20 µm, catching problems of fiber 2 rarely occur, but when larger than 20 µm, such possibility increases slightly. Fig. 3 shows the configuration of the parts, and the smaller hole 22 of the cylindrical main body 1b is usually 0.126 mm in diameter. The smaller hole 23 of the rear body 1a is preferred to be in the same diameter as the smaller hole 22 of the cylindrical main body 1b, but actually it may be set about 0.2 mm in diameter in consideration of the problem of processing precision due to mutual axial deviation. The rim of the rear end side of the smaller hole 22 provided in the cylindrical main body 1b is preferably enlarged with a radius of curvature R forming a ourved surface, but in consideration of the processing cost and improvement of insertion of fiber, the radius of curvature R is properly about 0.2 to 0.5 mm. When inserting the fiber 2, the fiber leading end is guided into the smaller hole 22 of the cylindrical main body 1b, and at this time it cannot be inserted smoothly unless the angle of the curved surface of the rim and the tangential line is 45 degrees or less. That is, the deviation of the fiber 2 is allowed up to R (1 - cos 45° ). Supposing the fiber is shifted transversally at maximum in the smaller hole 23 of the rear body 1a, since the diameter of the smaller hole 23 is 0.2mm and the outer diameter of the fiber 2 is 0.125 mm, the distance between the center axis SR of the fiber 2 and the center axis P of the smaller hole 23 of the rear body 1a is 0.0375 mm. Herein, supposing the distance between the center axis Q of the smaller hole 22 of the cylindrical main body 1b and the center axis P of the smaller hole 23 of the rear body 1a to be E, the relation E + 0.0375 < R (1 - cos 45° ) is established. That is, from E + 0.0375 < 0.2 (1 - 0.707), it follows that E < 0.0211, and it is known that the distance E between the center axis Q of the smaller hole 22 of the cylindrical main body 1b and the center axis P of the smaller hole 23 of the rear body 1a is preferred to be 20 µm or less. The taper angle of the taper hole 24 of the rear body 1a shown in Fig. 4 is preferred to be 10 to 90 degrees. That is, if less than 10 degrees, the taper portion is too long, and processing of the rear body 1a is difficult, or, to the contrary, if exceeding 90 degrees, the resistance is large when the fiber 2 slides along the taper, and the working efficiency becomes poor. Other embodiments of the invention are disclosed in Fig. 5 through Fig. 7, that is, the ferrule 1 shown in Fig. 5 is composed so that the larger hole 25 and smaller hole 23 of the rear body 1a may be continuous through a curved surface, the ferrule 1 in Fig. 6 has two stages of taper holes 24, 24 formed between the larger hole 25 and smaller hole 23, and the ferrule 1 in Fig. 7 has an intermediate hole 26 formed between the larger hole 25 and smaller hole 23, so that all holes may be continuous through the taper hole 24. These ferrules 1 are designed so that the fiber 2 may be smoothly guided and inserted into the ferrule 1. Examples In the ferrule 1 of the invention shown in Fig. 1, defining: smaller hole 22 of cylindrical main body: 0.126 mm smaller hole 23 of rear body 1a: 0.2 mm taper angle of taper hole 24 of rear body 1a: 60° while the distance E between the center axis P of the smaller hole 23 of the cylindrical main body 1b and the center axis Q of the rear body was varied as shown in Table 1, and the following experiment was conducted. Meanwhile, the distance E was determined by measuring the axial deviation of the mounting hole 33 and smaller hole 23 in the rear body 1a alone by means of roundness measuring instrument. The axial deviation of the smaller hole 22 of the ferrule 1 and outer diameter was 0.5 µm or less and was hence ignored. In these samples, fibers were actually inserted, and the smoothness of insertion was evaluated, of which results are summarized in Table 1. Distance E between axes (µm) Result 16 No problem 22 No problem 28 Slight catching, but possible to insert 34 Slight catching, but possible to insert 40 Impossible to insert As shown in Table 1, as far as the distance E between axes was less than 20 µm, there was no problem at all, and if less than 40 µm, it was possible to insert the fiber in spite of catching. By contrast, when the distance E between the axes was more than 40 µm, it was impossible to insert the fiber. As described herein, according to the invention, in the ferrule composed of a ceramic-made cylindrical main body and metal- or plastic-made rear body, by forming the through- hole provided in the axial direction of the rear body in a stepped hole comprising a larger hole and a smaller hole, and forming the taper hole at the rear body side, it is not necessary to process the rear end side of the ceramic-made cylindrical main body in a nearly conical form, and it does not need time and labor, so that it is very advantageous in manufacturing cost. Moreover, since the junction of the taper hole and smaller hole is formed without step difference at the rear body, when inserting the fiber into the ferrule, the fiber is smoothly guided into the smaller hole, and when passing through the boundary of the smaller hole of the rear body and the smaller hole of the cylindrical main body, the fiber passes straightly, and if there is a slight step difference in the junction between the two smaller holes, problems of the fiber being caught in the step difference to be damaged or formation of tiny cracks may hardly occur. Sill more, since the inlet of the smaller hole of the cylindrical main body is a curved surface, the fiber surface will not be injured by the edge angle. Thus, the invention brings about excellent effects.

Claims: A ferrule for an optical fiber,. comprising: an elongate cylindrical main body (1b) made of ceramic and provided with a through-hole (21,22) extending along the longitudinal axis of the main body (1b) for allowing insertion of an optical fiber; a rear body (1a) for fixing and holding the rear end portion of said main body (1b), said rear body (1a) being made of metal or plastic and being provided with a stepped through-hole (21) extending along the longitudinal axis of the rear body (1a) and comprising, successively, a smaller hole (23) which communicates by its front end portion with the rear end portion of the through-hole (21,22) provided in the main body (1b), a taper hole (24) continuous with said smaller hole (23) and a larger hole (25) continuous with said taper hole (24); characterized in that said smaller hole (23) of the stepped through-hole (21) provided in the rear body (1a) has a diameter that is substantially equal to, or slightly larger than, that of the through-hole (21,22) provided in the main body (1b), and the rear end portion of the through-hole (21,22) provided in the main body (1b) is convexly curved. The ferrule for an optical fiber of claim 1, wherein a deviation distance (E) between the center axis (Q) of the through-hole (21,22) provided in the main body (1b) and the center axis (P) of the smaller hole (23) of the stepped through-hole (21) provided in the rear body (1a) is 20µm or less. The ferrule for an optical fiber of claim 1 or 2, wherein the rear end portion of the through-hole (21,22) provided in the main body (1b) is convexly curved with a radius of 0.2 to 0.5mm. The ferrule for an optical fiber of claim 1, 2 or 3, wherein the taper hole (24) of the stepped through-hole (21) provided in the rear body (1a) has a convexly curved surface. The ferrule for an optical fiber of claim 1, 2 or 3, wherein the taper hole (24) of the stepped through-hole (21) provided in the rear body (1a) comprises two stages of taper holes (24) with an intermediate hole (26). An optical connector using a ferrule for an optical fiber as defined in any one of the claims 1 to 5.

Inventors: SAITOH YASUHIKO, SAITOH, YASUHIKO
Applicants: KYOCERA CORP, KYOCERA CORPORATION
Application year: 1996
Main classification: G02B6
IPC codes: G02B6