Reinforcing method for optical-fiber coupler

Abstract: A reinforcing apparatus for an optical-fiber coupler is formed by placing fused optical fibers (15) into a grooved U-shaped base structure (12). A cover (13) which has a different longitudinal length than the base structure is centered on top of the U-shaped base structure. There exists an exposed groove section (14) of the U-shaped base structure which lies on either side of the cover. An adhesive is inserted into these exposed groove sections so as to fix the reinforcing casing and the optical-fiber coupler to each other.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for reinforcing a fiber optic coupler according to the preamble of claim 1. An optical-fiber coupler is an important fiber optic element for communication systems and sensor systems which use light waves. In an optical fiber coupler, light is propagated through a fiber core and is radiated into a cladding or distribution-coupler so as to distribute at a predetermined degree of coupling to thereby enclose the light in the core again. One way to make an optical-fiber coupler is to arrange two optical fibers in parallel with an unsheathed section of each fiber touching. A heat source, such as a gas flame, is applied to the center of the fibers until the fibers fuse together. While the fibers are still malleable, they are drawn out in the longitudinal direction to make a thin tapered portion of fiber. This tapered portion has an average outer diameter of about 30 µm, which can be easily broken. An apparatus is needed to reinforce and protect the thin tapered portion. Japanese Utility Model Unexamined Publication No. Hei-2-29008 and in Japanese Patent Unexamined Publication No. Sho-63-271208 describe reinforcing apparatuses. An example of a conventional reinforcing apparatus is shown in Figure 11 (prior art). After the fibers have been drawn out, an adhesive (2) is applied on both sides of the thin tapered portion. Then the optical-fiber coupler is sandwiched between two flat plates (3) to form a conventional reinforcing apparatus. This simple design has a weak structure because the plates do not form a solid case. Therefore, the plates can vibrate and allow the fibers to twist. Also, the thin tapered portion is exposed to the atmosphere where dust, dirt, smoke, etc., can attach to the optical-fiber coupler to cause leakage of light. Another reinforcing apparatus which is slightly better is shown in Figure 12 (prior art). A glass tube (4) is used to reinforce the optical-fiber coupler by applying the adhesive (2) to the optical fibers (1) and securing the coupler to the glass. This is a stronger apparatus than the design shown in Figure 11 (prior art), but it is difficult to apply the adhesive to the optical coupler while it is in the glass tube (4). This structure also allows the thin tapered portion to be in contact with the atmosphere. One solution is to make an apparatus shown in Figure 13 (prior art). The optical-fiber coupler is housed in groove portion of the U-shaped base (5). An adhesive (2) is applied on each side of the thin tapered portion of the optical-fiber coupler and the cover (6) is placed on over the groove portion of the U-shaped base (5). The adhesive (2) holds the cover in place. The advantage of this reinforcing apparatus is that the thin tapered portion of the coupler is completely sealed within the reinforcing apparatus keeping all atmospheric dust away from the exposed fused fibers. The disadvantage is that assembling the apparatus can be troublesome. EP-A-0 357 429 shows a reinforcing structure for a fiberoptic coupler comprising a U-shaped base member and a flat rectangular cover having the same lateral dimensions as the base member. The reinforcing structure is assembled by applying an adhesive bonding simultaneously the cover to the base and the fiberoptic coupler to the reinforcing structure. EP-A-0 151 065 discloses an optical fiber coupler which is placed in a groove of a support member which is subsequently filled with a liquid index matching medium and on top of which a glass plate is placed. Through the glass plate, ultraviolet radiation is illuminated to cure the liquid index matching medium, which then adheres to the glass plate when the glass plate is peeled off the support. JP-A-1 063 907 (patent abstract of Japan, volume 13, no. 271) discloses a reinforcing method for an optical coupler comprising an integral sheathing which has partially cut end parts. The fiberoptic coupler is secured in the sheath by an adhesive. SUMMARY OF THE INVENTION In view of that disadvantage, it is an object of the present invention to provide a method for reinforcing a fiber optic-coupler which is simple easy and inexpensive. This object is solved according to the invention with a method having the steps as set out in claim 1. As specifically described with respect to the embodiments, the present invention, according to the presently preferred embodiment, is to have a U-shaped base member which forms a groove. The cover is shorter than the channel length which leaves, when viewed from the top, exposed groove sections on either side of the cover. Adhesive is applied at these exposed groove sections to bond together the optical fibers, the U-shaped base and the cover. This reinforcing apparatus is easy to manufacture and the cost is low. The casing is previously manufactured so that the molding work at the opposite-end opening portions is extremely easy in manufacturing the optical-fiber coupler, resulting in an improvement of the whole work efficiency. Accordingly, when the mold casing is set in manufacturing the optical-fiber coupler, the cover and the groove-formed portion can be set at the same time unlike the conventional case, and therefore automatization can be realized more preferably than the conventional case, so that the cost of the optical-fiber coupler can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a), 1(b) and 1(c) are a schematic view of the reinforcing structure according to an embodiment of the present invention. Fig. 2 is a view showing the mold work state of the reinforcing structure. Fig. 3 is a schematic view of a reinforcing casing. Fig. 4 is a schematic view of a reinforcing casing according to another embodiment of the present invention. Fig. 5 is a schematic view of a reinforcing casing according to a comparative embodiment. Fig. 6 is a schematic view of a reinforcing casing according to another comparative embodiment. Fig. 7 is a schematic view of a reinforcing casing according to yet another comparative embodiment. Fig. 8 is a schematic view of a reinforcing casing according to yet another comparative embodiment. Fig. 9 is a schematic view of a reinforcing casing according to yet another comparative embodiment. Fig. 10 is a perspective view showing a manufacturing example of the optical-fiber coupler using the reinforcing casing according to the present invention. Fig. 11 is a perspective view showing a reinforcing structure according to the prior art. Fig. 12 is a perspective view showing a reinforcing structure according to the prior art. Fig. 13 is a perspective view showing a reinforcing structure according to the prior art. DETAILED DESCRIPTION OF THE INVENTION The reinforcing apparatus for an optical-fiber coupler according to the preferred embodiment of the present invention will be described with reference to the accompanying drawings. As shown in Figure 1, the reinforcing apparatus (10) is made of two parts: a top cover (13) and a U-shaped bottom piece (12). The U-shaped bottom piece forms a groove (11) running from one axial end to the other axial end which can hold the thin tapered optical-fiber coupler. The cover has the same width as the U-shaped bottom piece, but the cover's length is shorter. When the cover is centered on the U-shaped bottom piece, there remains an exposed portion of the groove in the U-shaped bottom piece on both sides of the cover. This exposed portion (14) starts at the edge of the cover and goes to axial edge of the U-shaped bottom piece. The reinforcing apparatus is manufactured so that, as shown in Figure 3, a square bar having the groove (11) is used as the U-shaped bottom piece (12). A top plate which is used as the cover (13) which has a length shorter than the length of the U-shaped bottom piece. The cover is centered on the U-shaped bottom piece and bonded with an adhesive. There are exposed portions of the groove which now exist. To form the reinforcing apparatus according to the present invention, an optical-fiber coupler (15) is inserted in the groove (11) of the U-shaped bottom piece (12). As can be seen in Figure 1(b), the optical-fiber coupler (15) is made of three sections. The first section (15a) is the thin tapered portion of fused optical fibers. The second section (15b) is the optical fiber which has the coating removed. A second section (15b) lies on each sides of the fused optical fibers (15a). The third sections (15c) are coated optical fibers which are attached to the section (15b). The U-shaped bottom piece must be long enough to contain all three sections of the optical fibers. The cover is long enough to enclose all of the fused optical fibers (15a) and a portion of the uncoated optical fibers (15b) on each side of the fused optical fibers. What remains exposed in the groove (11) after the cover is in place is a portion of the uncoated optical fibers (15b) and the coated optical fibers (15c) which extend beyond the axial edges of the U-shaped bottom piece. As shown in Figure 2, an adhesive (17) is poured into the exposed portions (14) of the groove. The adhesive is directed into the groove by using a dispenser (16) or something similar. Once the adhesive is in place, the reinforcing structure (10) and the optical-fiber coupler (15) are bonded to each other. The exposed portions (14) are completely filled with adhesive. In this embodiment, the method of inserting the adhesive is easily performed due to the exposed portion (14) of the groove. Once the exposed portions (14) have been completely filled, the optical-fiber coupler is sealed from the environment. Therefore, dust and light which can interfere and affect the performance of the coupler have been eliminated. Of course, the configuration of such a reinforcing casing is not limited only to the groove-formed square bar and the flat cover. It is possible to consider, for example, a circular bar worked to have a plane surface portion, a triangular bar, a pentagonal bar, a hexagonal bar, etc., and those bars having grooves therein. A shape as shown in Figure 4 can be provided by way of example as other shape of the reinforcing casing. A reinforcing casing 20A shown in Figure 4 is such that a top plate 23A is bonded with an adhesive to a member which is prepared by working a U-shaped circular bar 22A so as to have a semicircular groove 21A. A reinforcing casing 20B of a comparative embodiment shown in Figure 5 is such that groove-formed square bars 22B and 23B respectively having substantially channel-like sections so as to form grooves 21B and respectively having lengths different from each other are bonded to each other with an adhesive. A reinforcing casing 20C of a further comparative embodiment shown in Figure 6 is such that round bars 22C and 23C respectively having substantially U-shaped sections so as to form grooves 21B and respectively having lengths different from each other are bonded to each other with an adhesive. A reinforcing casing 20D of yet another comparative embodiment shown in Figure 7 is such that a casing member 23D which has a channel-like section, which is shorter than another elongated plate-like casing member 22D and which has a groove 21D is put so as to cover the upper surface of the other elongated plate-like casing member 22D and bonded to the latter with an adhesive. A reinforcing casing 20E of yet another comparative embodiment shown in Figure 8 is such that a casing member 23E which has a substantially U-shaped section, which is shorter than another elongated plate-like casing member 22E and which has a groove 21E is put so as to cover the upper surface of the other elongated plate-like casing member 22E and bonded to the latter with an adhesive. Further, it is unnecessary that the two members constituting each of the reinforcing casings are solid as described above. As shown in the comparative embodiment of Figure 9, members 22F and 23F molded so as to have hollow portions, that is, pipe-like members, may be bonded to each other with an adhesive so as to form a reinforcing casing 20F. With respect to the reinforcing casings 20D and 20E shown in Figures 7 and 8, the adhesive used must have a viscosity which allows bonding to be performed after the viscosity of the adhesive has been adjusted. Further, the quality of the material of the hollow casing is not specifically limited so long as the material has a certain extent of strength. Glass, plastics, ceramics, etc. may be used as the material. In consideration of thermal expansion, it is preferable to use a material having a linear expansion coefficient equal to the linear expansion coefficient of an optical fibers. For example, materials such as quartz are the most suitable. When cost of the reinforcing apparatus is a consideration, materials such as plastics or ceramics are advantageous. Further, the adhesive according to the present invention is not specifically limited. There are various kinds of adhesives on the market which may be used, such as an ultraviolet-ray curable type, or a hot-melt type, or a thermal curable type, etc. Figure 10 shows a more specific example of how to use the reinforcing apparatus in manufacturing a completed optical-fiber coupler. As shown in the drawings, the coatings are removed from a middle portion of two fiber optical fibers (30) so that the glass portion (31) is exposed. After the two exposed glass portions (31) are passed through the reinforcing apparatus (10), the two fiber optical fibers are secured to extension stages (33A and 33B) by using optical fiber clamps (32A and 32B). Next, the exposed glass portion (31) are positioned so that they are touching by using glass portion clamps (34A and 34B). The central portions of the glass portions are fused together by using a flame (36) of a burner (35). After fusion of the glass portions is complete, the glass portion clamps (34A and 34B) are removed by moving glass portion clamp stages (38A and 38B). The central portion of the fused glass portions (37) is heated by the flame (36) again and simultaneously the optical fiber clamps (32A and 32B) are moved outwardly by extension stages (33A and 33B). The flame (36) is removed and a thin tapered portion of fused optical fibers is the optical-fiber coupler. The reinforcing apparatus (10) is mounted on a bracket (40) arranged so that it may move in the axial direction of the fiber by using a straight-moving stage (39). The stage is moved so that the central portion of the thin tapered fused glass portions is centered in the groove section of the reinforcing apparatus (10). Then, as shown in Figure 2, the exposed groove portions (14) of the reinforcing apparatus (1) are molded with an adhesive (17) to thereby complete the reinforcing structure. As described above, the reinforcing structure according to the present invention can be formed through extremely simple work. It should be understood that the foregoing relates to only preferred embodiments of the present invention, and that it is intended to cover all changes and modifications of the embodiment of the invention herein used for the purposes of disclosure, which do not institute departures from the scope of the invention.

Claims: A method of reinforcing a fiberoptic coupler, comprising the steps of: providing a U-shaped base member (12; 22A) and a cover of flat rectangular shape(13; 23A) said base member having two axial open ends and a longitudinal groove (11; 21A) therebetween and the cover and the base member having substantially the same width; and arranging the fiberoptic coupler in said groove (11, 21A); said cover being approximately centered on top of said base member; characterized in that said cover is selected to be shorter in length than said base member, so that portions (14) of the groove, each of which starts at the edge of said cover and extends to an axial edge of said base member, remain exposed, that said cover is bonded by an adhesive to said base member prior to bonding the fiberoptic coupler to said base member and said cover, and that said exposed portions (14) are completely filled with an adhesive (17) for bonding said fiberoptic coupler so as to seal the coupler from the environment. Method according to claim 1, wherein the base member (12, 22A) and the cover (13, 23A) are selected to have a linear expansion coefficient equal to that of said fiberoptic coupler. Method according to claim 2, wherein the base member (12, 22A) and the cover (13, 23A) are made of quartz. Method according to claim 1, wherein said U-shaped base member (12) is rectangular. Method according to claim 1, wherein the U-shaped member (22A) is semi-circular. Method according to claim 1, wherein the adhesive (17) is of an ultraviolet curable type. Method according to claim 1, wherein the adhesive (17) is of a thermal curable type. Method according to claim 1, wherein the adhesive (17) is of a hot-melt type.

Inventors: ARIMOTO KOZUHIKO, HATTORI TOMOYUKI, SUGANUMA HIROSHI, TAKIMOTO HIROAKI, YOSHIKAWA JUNICHI, ARIMOTO, KAZUHIKI, HATTORI, TOMOYUKI, SUGANUMA, HIROSHI, TAKIMOTO, HIROAKI, YOSHIKAWA, JUNICHI, Arimoto, Kazuhiki, c/o Sumiden Opcom, Ltd., HATTORI, TOMOYUKI, C/O YOKOHAMA WORKS, SUGANUMA, HIROSHI, C/O YOKOHAMA WORKS, TAKIMOTO, HIROAKI, C/O YOKOHAMA WORKS, Yoshikawa, Junichi, c/o Yokohama Works
Applicants: SUMIDEN OPCOM LTD, SUMITOMO ELECTRIC INDUSTRIES, SUMIDEN OPCOM, LTD., SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: G02B6
IPC codes: G02B6