1. Field of the Invention
The present invention relates to a method of producing an optical-fiber coupler to be used, for example, in optical communications or light measurements.
2. Description of Related Art
A fiber-type coupler is a device for branching/coupling light between a plurality of optical fibers. At present, a melt-drawing method is considered to be the most suitable for producing a single-mode fiber coupler ("The Latest Technique of Optical-Fiber Coupler", OPTORONICS, No.5, 1988, P.125). Such a melt-drawing method involves; partly removing the respective coatings of two optical fibers so as to form exposed portions; twisting the exposed portions so as to closely contact with each other or fixed so as to contact closely and parallel to each other; and, drawing this bundled portion while heating and fusing the bundled portion with a burner or the like.
At the same time, light which has been incident from one end of the optical fibers is measured at the other ends of the optical fibers so that the light branching ratio is detected. Drawing is stopped when a predetermined branching ratio is obtained. Finally, the optical fibers are fixed and made to adhere to a protection member to thereby produce a fiber-type coupler.
In the foregoing conventional method, optical fibers are drawn while only the branching ratio is being monitored. In such a method, however, the diameters of optical fibers decrease as the drawing advances, thereby increasing the drawing speed so that it is difficult to stop the drawing when the branching ratio has reached a desired, predetermined value. In the worst case, the optical fibers are broken.
Further, the refractive index of the heated glass differs from that of the finished product. This makes it impossible to measure the branching ratio of the heated product to create a finished, cooled product with a predetermined branching ratio. This makes it impossible to obtain a coupler having the branching ratio of a desired finished value.