Ferrule for optical connector

Abstract: A ferrule 1 for optical connectors which is of funnel type comprising a ferrule body 2, containing a small-diameter hole 3 and a large-diameter hole 4 having a larger diameter than the small-diameter hole 3, the small-diameter hole 3 and the large-diameter hole 4 being formed so as to allow an optical fibre to pass therethrough and communicate with each other. The ferrule 1 further comprising a buffering hole 5 having a diameter which is larger than that of the small-diameter hole 3 but smaller than that of the large-diameter hole 4, the buffering hole 5 being formed between the small-diameter hole 3 and the large-diameter hole 4 and containing grooves 6 of concave and convex pattern on the internal surface of the buffering hole 5. The ferrule body 2 further contains three lines of protrusions 51A, 51B, 51C extending along the axis thereof, which are formed on the internal surface of the large-diameter hole 4.

Description: This invention relates to a ferrule for optical connectors. Generally, the ferrule for optical connectors is used to connect optical fibres directly with each other. As shown in FIG. 7, a conventional ferrule contains an optical fibre inserting hole 101 formed through a ferrule body 100 in its axial direction and an optical fibre inserted into the optical fibre inserting hole 101 is fixed therein by an adhesive. An excessive portion of the optical fibre protruding from each end of the ferrule body 100 is cut off and the cut portion thereof is finished to a mirror state so as to be used as a connecting end. By bringing this connecting end into contact with that of other ferrule body constructed in the same manner, the optical fibres are directly connected with each other. Because the optical fibres are directly connected with each other through the ferrule as described above, the optical fibre must be firmly fixed to the interior of the optical fibre inserting hole 101 in the ferrule with an adhesive so as to have a predetermined fixing strength. Thus, this fixing strength is closely related to a length L of the optical fibre inserting hole 101 and the longer the length, the larger the fixing strength of the optical fibre in the optical fibre inserting hole 101 is. Usually, that length L is often selected to be about 10mm. However, if the length of the optical fibre inserting hole 101 is prolonged like this, the following difficulties may occur in processing of the optical fibre. That is, the linearity of the optical fibre inserting hole 101 cannot be easily obtained. Further, hole processing for making the optical fibre inserting hole 101 becomes difficult and it takes very long to process the internal surface of the optical fibre inserting hole 101. So that, a funnel type optical connector ferrule as shown in FIG. 8 has been proposed. This optical fibre inserting hole is constructed so as to have two steps, a large-diameter hole 102 and a small-diameter hole 103. By holding an optical buffered fibre by the large-diameter hole 102, the length LN at the small-diameter hole 103 for fixing the optical fibre is shortened. Although the tunnel type ferrule can solve the problem of a difficulty in processing by shortening the length LN of the small-diameter hole 103 in which the optical fibre is to be fixed, a sufficient fibre fixing strength cannot be obtained easily and further a deviation in axis between the large-diameter hole 102 and the small-diameter hole 103 may occur. This often reduces the connecting performance of the optical fibre. That is, this type of ferrule is processed by centring in which the external diameter thereof is finished to a predetermined size with a small-diameter hole of the ferrule as a centre of this processing. In this step, both ends of the funnel type ferrule are supported by two conical-shaped centres and then the ferrule is rotated to grind the outer peripheral surface thereof. Thus, if there is a difference in shape accuracy (e.g., circularity) between the small-diameter hole and the large-diameter hole, support points of both the centres cannot be set in the centre of each of them, so that a deviation in axis occurs between the large-diameter hole and the small-diameter hole, thereby deteriorating connecting performance of the ferrule. Therefore, an object of the present invention is to provide a ferrule for optical connectors which has been improved to solve the above described problem of the funnel type optical connector ferrule. To solve the above described problem, the invention provides a ferrule for optical connectors as defined in claim 1. The diameter of the large-diameter hole may be of an appropriate size to allow the optical buffered fibre to pass therethrough. When the optical fibre is inserted into the small-diameter hole and fixed thereto with an adhesive, the adhesive goes in between the optical fibre and the small-diameter hole; and at the same time, the adhesive goes in between the optical fibre and the buffering hole. In this case, because grooves of concave and convex pattern (serrations) are formed on the internal wall of the buffering hole, a great amount of adhesive goes in between the internal wall of the buffering hole and the optical fibre. As a result, the optical fibre is fixed in the ferrule firmly by the adhesive. Because a required fixing strength between the optical fibre and the ferrule mostly depends on this bonding, the length of the small-diameter hole may be short. Further, because the internal surface of the small-diameter hole determines the accuracy of an optical fibre fixing position in the ferrule and so it is sufficient to finish only the internal surface of the small-diameter hole which may be relatively short, to a predetermined size, this leads to a reduction of processing time for the ferrule. Furthermore, there is provided a ferrule for optical connectors wherein the internal surface of the large-diameter hole has three lines of protrusions extending along the axial direction thereof. These lines of protrusions are preferred to be provided at intervals of 120°. The height of the protrusions should be determined to be an appropriate one in which a circle passing through respective vertexes of three protrusions at an external end of the large-diameter hole does not interfere with the internal surface of the large-diameter hole. This structure enables an accurate centring by a conical-shaped centre applied thereto for the following reason when the outer diameter of the ferrule is ground with respect to the centre of the diameter hole to a predetermined size. The inner peripheral surface of the small-diameter hole is substantially circular by polishing. On the other hand, a conical-shaped centre applied to the outer side end of the large-diameter hole is supported by three protrusions provided thereon such that it is stably supported by three points. Thus, even if the large-diameter hole is itself not of absolute circular shape, accurate centring can be conducted. As a result, it is possible to process by both centres with the axes of the small-diameter hole and the large-diameter hole coincident with each other. Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic figures, in which: FIG. 1 is a longitudinal sectional view showing a first embodiment of an optical connector ferrule according to the present invention. FIG. 2A is a sectional view taken along the line X-X of the ferrule shown in FIG. 1, indicating its left portion. FIG. 2B and 2C are enlarged views of a portion around the buffering hole 5 in respective first and second embodiments. FIG. 3 is a longitudinal sectional view showing a third embodiment of the optical connector ferrule according to the present invention. FIG. 4 is a sectional view taken along the line Y-Y of the ferrule shown in Fig. 3 indicating its left portion. FIG. 5 is a sectional view showing a fourth embodiment of an optical connector ferrule. FIG. 6 is a sectional view showing a fifth embodiment of an optical connector ferrule. FIG. 7 is a longitudinal sectional view of a conventional ferrule. FIG. 8 is a longitudinal sectional view of a conventional funnel type ferrule. Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a longitudinal sectional view of a first embodiment of the optical connector ferrule according to the present invention. A ferrule 1 for optical connectors has a cylindrical ferrule body 2 having a through-hole for fixing an optical fibre. The through-hole comprises a small-diameter hole 3 and a large-diameter hole 4 having a larger diameter than the small-diameter hole 3. A buffering hole 5 is provided between the small-diameter hole 3 and the large-diameter hole 4. The diameter of the small-diameter hole 3 is determined to be slightly larger than the outer diameter of an optical fibre so that the optical fibre is inserted into it and positioned thereby. The small-diameter hole 3 is formed so as to have the same cross section throughout the entire length L1, as shown in Fig. 1. The internal surface of the small-diameter hole 3 is polished so as to have a predetermined level of surface roughness. The large-diameter hole 4 is formed so as to have a slightly larger diameter than the external diameter of the optical buffered fibre allowing the optical buttered fibre to pass through and have a length of L3, as shown in Fig. 1. It has substantially the same cross section throughout the entire length thereof. A buffering hole 5 is formed so as to have a diameter which is larger than the internal diameter of the small-diameter hole 3 and smaller than the internal diameter of the large-diameter hole 4. The buffering hole 5 has substantially the same cross section throughout the entire length of L2, as shown in Fig. 1. As shown in FIG. 2A, grooves 6 of concave and convex pattern are formed on the internal wall of the buttering hole 5 so as to form space for holding an adhesive between the optical fibre to be inserted into the buffering hole 5 and the walls of the buffering hole 5. This space may be formed appropriately so as to define a volume sufficient for holding a sufficient amount of adhesive for fixing the optical fibre to the ferrule 2 with a predetermined fixing strength. The ferrule body 2 may be made of any known material such as ceramic, plastic, glass and metal. FIG. 2B shows an enlarged view of the buffering hole 5 in FIG. 2A in the first embodiment, and FIG. 2C shows an enlarged view of a portion in the vicinity of the buttering hole 5 according to a second embodiment. According to this structure, when the optical fibre is inserted into the small-diameter hole 3 and fixed therein with an adhesive, the adhesive goes in between the optical fibre and the small-diameter hole 3 and also between the optical fibre and the buffering hole 5. In this case, because grooves of concave and convex pattern are formed on the internal wall of the buffering hole 5, adhesive of an amount sufficient for fixing the optical fibre on the internal wall of the buffering hole 5 goes in therebetween. This portion provides most of a fixing strength required between the optical fibre and the ferrule 2 and therefore the length L1 of the small-diameter hole 3 may be short. Here, the internal surface of the small-diameter hole 3 determines an accuracy of the optical fibre fixing position in the ferrule 2. Therefore, this can be done by only grinding the internal surface of the small-diameter hole 3 along the length L1 which is relatively short. Thus, it is possible to reduce a processing time for the formation of the ferrule 2. Meanwhile, because the buffering hole 5 does not function to directly determine the position of the optical fibre, finishing processing accuracy (hole diameter, shape) thereof may be relaxed as compared to the small-diameter hole. FIG. 3 is a longitudinal sectional view of a third embodiment of the optical connector ferrule according to an invention claimed in claim 2. FIG. 4 is a sectional view taken along the line Y-Y indicating a portion to the left therefrom in FIG. 3. Because a ferrule 11 for optical connector shown in Fig. 3 is basically the same as ferrule 1 shown in FIG. 1, the same reference numerals are attached to parts corresponding to each part of FIG. 1 and a description thereof is omitted. The ferrule 11 shown in FIG. 3 is different from the ferrule 1 shown in FIG. 1 in that three lines of protrusions 4A, 4B, 4C are formed axially on the internal wall of the large-diameter hole 4. According to the embodiment shown in this Figure, the protrusions 4A, 4B, 4C are formed with rectangular sections and at each interval of 120°C on the internal wall of the large-diameter hole 4. The ferrule 11 shown in FIG. 3 provides the following effect in addition to an effect provided by the ferrule 1 shown in FIG. 1. That is, when the external surface of the ferrule 2 is ground with respect to the centre of the small-diameter hole 3 to a predetermined size, because the internal surface of the small-diameter hole 3 is in a substantially circular state due to grinding, accurate centring can be conducted by a conical centre (not shown) applied thereto. On the other hand, a conical centre (not shown) applied to an external end 41 of the large-diameter hole 4 is supported at three protrusions 4A to 4C provided thereon such that it is stably supported by three points. Thus, even it the large-diameter hole 4 is itself not of absolute circular shape, accurate centring can be conducted. As a result, processing by both centres in which the axes of the small-diameter hole 3 and the large-diameter hole 4 easily coincide with each other is enabled. FIG. 5 is a longitudinal sectional view of a fourth embodiment of the optical connector ferrule. With respect to parts shown in FIG. 5, the same reference numerals are used to designate parts corresponding to FIG. 1 and a description thereof is omitted. A ferrule 20 shown in FIG. 5 has three protruded sections 51A, 51B, 51C each having a rectangular cross section and a small protrusion group 60 formed in configuration of a plurality of concave and convex portions. The three protruded sections 51A, 51B, 51C and the small protrusions 60 are formed on the internal surface of the large-diameter hole 4 and the buffering hole 5 in the axial direction. The three protruded sections 51A, 51B, 51C are preferred to be provided at each interval of 120° like the third embodiment. In this structure, the small protrusion group 60 connecting the three protruded sections 51A, 51B, 51C allows a large amount of adhesive to go in between the optical buffered fibre and the large-diameter hole 4, thereby producing an anchor effect to increase the fixing strength. By providing the internal wall of the buffering hole 5 with the same structure, the fixing strength can be increased as in the above described embodiment. Further, in this case, the shape of the large-diameter hole 4 is continuous with that of the buffering hole 5 so that processing thereof is made easy. FIG. 6 shows a fifth embodiment in which respective parts are the same as those of FIG. 5 except that the small protrusion group 61 each have a rectangular cross section, which is smaller than that of the protruded sections 51A, 51B, 51C. Because grooves of concave and convex pattern(serration) are formed on the internal wall of the buffering hole 5 in the axial direction, a great amount of adhesive goes in between the internal wall of the buffering hole 5 and the optical fibre. As a result, the fixing strength between the optical fibre and the ferrule can be remarkably increased and the length of the small-diameter hole 3 may be short. This leads to a reduction of processing time for the ferrule. When the external surface of the ferrule is ground with respect to the centre of the small-diameter hole 3 to a predetermined size, because the internal surface of the small-diameter hole 3 is in a substantially circular state produced by grinding, accurate centring can be conducted by a conical centre applied thereto. On the other hand, a conical centre applied to an external end of the large-diameter hole 4 is supported by three protrusions 51A, 51B, 51C provided thereon such that it is stably supported at three points. Thus, even if the large-diameter hole 4 is itself not of absolute circular shape, accurate centring can be conducted. As a result, processing by both centres in which the axes of the small-diameter hole 3 and the large-diameter hole 4 easily coincide with each other is enabled. Meanwhile, although in the embodiments described above the small-diameter hole and the buffering hole are subjected to final finishing processing such as grinding, this invention may also be applied to ferrules in which a target dimension, shape and accuracy may be obtained by only molding processing. The aforegoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention.

Claims: A ferrule (1) for optical connectors which comprises a ferrule body (2) provided with an internal passageway having a small-diameter portion (3) and a large-diameter portion (4), said small-diameter portion and said large-diameter portion being formed so as to allow an optical fibre to pass therethrough and communicate with each other, said ferrule further comprising a buffering portion (5) of substantially constant cross section whose diameter is larger than that of said small-diameter portion but smaller than that of said large-diameter portion, said buffering portion being formed between said small-diameter portion and said large-diameter portion characterised by the internal surface of the buffering portion containing grooves (6) thereby forming a concave and convex pattern in the internal surface said grooves extending along the buffering portion but not beyond. A ferrule for optical connectors as claimed in claim 1, wherein the internal surface of the large-diameter portion is grooved to form three lines of protrusions (4A,4B,4C; 51A,51B,51C) extending parallel to the axis thereof. A ferrule for optical connectors as claimed in claim 2, wherein the internal surface of the large-diameter portion is further grooved to form a small protrusion group (60; 61) between said three lines of protrusions (51A,51B,51C). A ferrule for optical connectors as claimed in claim 3, wherein the protrusions of said three lines of protrusions and said small protrusion group each have a rectangular cross section. A ferrule as claimed in any one of claims 2 to 4, wherein said grooves forming said concave and convex pattern in the internal surface of said buffering portion and the protrusions in the internal surface of said large-diameter portion have the same cross section and angular position.

Inventors: KOSHIGOE TATSUO, MINAMI KOUJI, NAKAJIMA MASAHIRO, SUZUKI NOBUO, TAIRA JUNJI, TOKITA HIROYUKI, KOSHIGOE, TATSUO, MINAMI, KOUJI, NAKAJIMA, MASAHIRO, SUZUKI, NOBUO, TAIRA, JUNJI, TOKITA, HIROYUKI
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1997
Main classification: G02B6
IPC codes: G02B6