Wide-band optical fiber coupler and its manufacturing method

Abstract: Two optical fibers (11, 12) are reduced in diameter over a predetermined length to form reduced-diameter portions (13) whose center portions have different diameters. The first and the second optical fibers are intersected at a small angle to each other substantially centrally of their reduced-diameter portions and then are fused together at the intersecting portions and the fused portions of the first and second optical fibers are heated and stretched to provide an optical coupler having a desired coupling ratio.

Description: BACKGROUND OF THE INVENTION The present invention relates to a wide-band optical fiber coupler having a reduced wavelength dependence of its splitting ratio and a method of making such an optical fiber coupler. A conventional optical fiber coupler is produced by twisting together a prestretched optical fiber and an unstretched one, or by fusing together such optical fibers held in parallel juxtaposition with each other and then stretching their coupling region, as set forth in PCT Publication WO87/00934 or JP-A-108311/88. In the manufacture of an optical fiber coupler having a reduced wavelength dependence of its splitting ratio, the ratio between the diameters of the two optical fibers in the region where they are fused together determines the optical characteristics of a finished optical fiber coupler. It is therefore an important problem to hold the diameter ratio of the two optical fibers at the fused portions thereof substantially constant. However, since the diameter of a reduced-diameter portion 13 of a prestretched optical fiber 11 varies along its longitudinal direction as shown in Fig. 1, the diameter ratio between the diameters of the two optical fibers changes with the position where they are fused together. The conventional manufacturing method disclosed in the abovementioned PCT publication does not ever take into account a solution to such problem, and hence is very low in the yield rate of product. In the above-mentioned JP-A-108311/88, it is proposed as a solution to the problem to reduce the diameter of a portion of an optical fiber by stretching the fiber or chemically etching the portion thereof so that the diameter of the reduced diameter portion cannot vary along the longitudinal direction of the fiber. It is very difficult, however, to draw or stretch the optical fiber while holding its diameter uniform lengthwise thereof. Moreover, even if the diameter of each optical fiber can be reduced uniformly over a certain length, it is also very hard to maintain the two optical fibers in parallel juxtaposition with each other. As shown in Fig. 2, the optical fibers each bend largely at both ends of their contact portions 13 disposed in parallel to each other, and this bending increases the transmission loss of the optical fiber coupler. As mentioned above, the manufacture of an optical fiber coupler having a reduced wavelength dependence of its splitting ratio calls for stabilizing the ratio between the diameters of the two optical fibers in the region where they are fused together. To this end, the prior art stretches each optical fiber or decreases its diameter by chemical etching so that the diameter is uniform in its longitudinal direction. However, much difficulty is encountered in reducing the diameter of the optical fiber without changing the diameter lengthwise thereof, besides the reduced-diameter portion is smaller in diameter than its both end portions, and consequently, it is very difficult to maintain the two optical fibers in parallel juxtaposition with each other. The document US-A-4,291,940 discloses an optical fiber coupler including two multi-mode optical fibers, each having a biconical taper section. At the biconical taper sections the fibers are twisted about each other and fused together to provide optical coupling between the fibers. The document mentions that if it is desired to have a preferential coupling, two fibers of different diameters may be used. In this case the diameter at the center of the reduced-diameter portion of one optical fiber will have to be assumed to be different from that of the other optical fiber. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical fiber coupler which has a reduced wavelength dependence of its splitting ratio even if the diameter of a reduced-diameter portion of each of the optical fibers varies lengthwise thereof, and in which their reduced-diameter portions need not be disposed in parallel juxtaposition with each other. Another object of the present invention is to provide a method of producing such an optical fiber coupler. These objects are achieved with an optical fiber coupler and a method of producing it, respectively, as claimed. As shown in Fig. 1, the diameter of an optical fiber normally stretched while being heated is usually minimum at its center portion and the diameter increases toward both ends thereof. The portion where the diameter changing ratio is the smallest is the center portion. Also in the case where the diameter of an optical fiber is reduced by chemical etching, there is the tendency of the central portion becoming thinner than any other portions, though not so conspicuous as in the above case. According to the present invention, since the optical fibers are fused together at their central intersecting portions where the diameter changing ratio is the smallest, the diameter ratio between the optical fibers in the coupling region is substantially constant and stable, thus permitting the fabrication of optical fiber couplers of uniform performance. Furthermore, since the present invention does not involve difficult work of stretching each optical fiber so that the diameter of the reduced diameter portion becomes constant in its longitudinal direction and of maintaining the optical fibers in parallel juxtaposition with each other as in the prior art, the optical fiber coupler of the present invention is easy of manufacture. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a reduced-diameter portion 13 of an optical fiber for use in a conventional optical fiber coupler manufacturing method; Fig. 2 is a diagram for explaining a conventional manufacturing method; Fig. 3A is a diagram for explaining a step involved in the manufacture of the optical fiber coupler according to the present invention; Fig. 3B is a diagram for explaining a manufacturing step immediately subsequent to the step depicted in Fig. 3A; and Fig. 4 is a diagram for explaining an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Figs. 3A and 3B are diagrams for explaining the optical fiber coupler manufacturing method according to the present invention. As shown in Fig. 3A, optical fibers 11 and 12 are each previously heated and stretched or chemically etched over a desired length thereof to form a reduced-diameter portion 13. Then, the optical fibers 11 and 12 are disposed so that they intersect each other substantially, centrally of their reduced-diameter portions 13 as depicted in Fig. 3B. The intersecting portions 14 are fused together and the fused portion or coupling region and the neighboring regions are stretched until a desired coupling ratio is obtained. The optical fibers 11 and 12 each having a clad outer diameter of 125 µm, for example, were prestretched by about 10 mm longitudinally thereof in the diameter reducing step shown in Fig. 3A and reduced-diameter portions 13 about 10 mm in length were formed with the smallest diameters at the centers of the reduced diameter portions being about 86 µm and 79 µm, respectively. Next, the optical fibers 11 and 12 were disposed to intersect each other substantially at the centers of their reduced-diameter portions 13 and fused together as depicted in Fig. 3B. In this instance, as shown in Fig. 4, the optical fibers 11 and 12 were held in substantially parallel and adjacent relation to each other by means of jigs 21 and 22 outside their reduced-diameter portions 13, and the plane of arrangement of the optical fibers 11 and 12 in the jig 21 was flush with the plane of their arrangement in the jig 22. In other words, four segments 11a, 11b and 12a, 12b of the optical fibers 11 and 12 positioned at the outside of the reduced-diameter portions 13 are flush with one another, besides the segments lla and 12a of the optical fibers 11 and 12 at one side are exchanged in position with respect to the segments llb and 12b at the other side. Letting the length of the reduced-diameter portion 13 and the diameter of each optical fiber be represented by L and D, respectively, the angle of intersection between the optical fibers 11 and 12 is less than 2D/L rad at the largest. The intersecting portions 14 of the optical fibers 11 and 12 disposed as mentioned above and held by the jigs 21 and 22 were fused together by heating them with a burner 23, and thereafter were being stretched. The excess loss of the optical fiber coupler produced by the above-described method was measured. The excess loss of the optical fiber coupler of the prior art example (shown in Fig. 2) was about 1 dB, whereas the excess loss of the optical fiber coupler according to this embodiment was less than 0.5 dB -- this is a substantial improvement of its performance. As described above, according to the present invention, two optical fibers are previously reduced in their diameter over a predetermined length, respectively, and are disposed to intersect each other at a small angle substantially at the centers of their reduced-diameter portions where the changing ratios of their reduced diameters are minimum, and then are fused together. Consequently, the ratio between the diameters of the two fused portions of the two optical fibers remains substantially constant, permitting the fabrication of optical fiber couplers of uniform performance, in particular, whose excess loss is stable at a small value. Moreover, the reduced-diameter portions are not maintained in parallel juxtaposition with each other, but intersect each other, so that the present invention allows ease in producing the optical fiber coupler and improves productivity accordingly. It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention as claimed.

Claims: A wide-band optical fiber coupler comprising: a first and a second optical fiber (11, 12) each having a longitudinally extending reduced-diameter portion (13) with the diameter at the center of the reduced-diameter portion of the first optical fiber being different from that of the second optical fiber, the two optical fibers intersecting each other at a small angle substantially at the centers of their respective reduced-diameter portions where the diameters are substantially the smallest, the optical fibers being fused together, only at the portion (14) where said centers of the reduced-diameter portions intersect, and stretched, and said optical fibers having further portions (11a, 11b, 12a, 12b) at opposing ends of the coupler which are disposed adjacent and in parallel to each other and in the same plane on either side of said reduced-diameter portions, the respective positions of said further portions (11a, 12a) at one end of the coupler being exchanged relative to the respective positions of said further portions (11b, 12b) at the other end of the coupler. A method of producing a wide-band optical fiber coupler, comprising the steps of: heating and stretching or chemically etching a portion of each of a first and a second optical fiber (11, 12) to reduce their diameters over predetermined lengths thereof so as to form reduced-diameter portions (13) with the diameters at the center of the reduced-diameter portions being different from each other; disposing the two optical fibers such that they are in contact at the reduced-diameter portions thereof and intersect each other at a small angle substantially at the centers of their reduced-diameter portions where the diameters are substantially the smallest and holding further portions (11a, 11b, 12a, 12b) of the optical fibers in parallel and adjacent relation to each other in the same plane at either side of said reduced-diameter portions, the respective positions of said further portions (11a, 12a) at one end of the coupler being exchanged relative to the respective positions of said further portions (11b, 12b) at the other end of the coupler; and fusing and stretching together the intersecting portions (14) of the optical fibers such that the optical fibers are fused only at the portion (14) where they intersect.

Inventors: HANAFUSA HIROAKI, NODA JUICHI, SASAKI HIROYUKI C O JAPAN AVIA, SHIMAMURA MASATO C O JAPAN AVI, TAKEUCHI YOSHIAKI, HANAFUSA, HIROAKI, NODA, JUICHI, SASAKI, HIROYUKI, C/O JAPAN AVIATION ELECTRONICS, SHIMAMURA, MASATO, C/O JAPAN AVIATION ELECTRONICS, TAKEUCHI, YOSHIAKI
Applicants: JAPAN AVIATION ELECTRON, NIPPON TELEGRAPH & TELEPHONE, JAPAN AVIATION ELECTRONICS INDUSTRY LIMITED, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1990
Main classification: G02B6
IPC codes: G02B6