Method of fabricating a fused single-mode fiber bidirectional coupler

Abstract

An improvement for the coupling of optical energy in a single-mode fiber ures a stable bidirectional coupling. A method of fabricating a single-mode evanescent field coupler for optical data between two single-mode fibers calls for there first being an etching away of a substantial portion of the claddings about the cores of both single-mode fibers. This etching calls for the immersion of sections of the fibers in an etching solution for a predetermined time with the ends of the exposed sections being carefully masked to create smooth, unscored, tapered transitions. The etched sections of the single-mode fibers are placed in a side-by-side relationship, are longitudinally twisted about each other to assume a closely abutting helical disposition and are secured in place at their ends. A pair of tungsten electrodes are brought into the close proximity of the twisted etched fibers and an arc is initiated. Moving the electrodes along the length of the twisted etched fibers causes the arc to fuse them together to assure a stable configuration and, hence, stable evanescent field coupling. Once the fibers have been fused, a protective envelope encases the fused twisted fibers and it is filled with a liquid having the same index of refraction as the fibers' claddings. Thus, a stable coupler is created that is isolated from outside thermal and mechanical influences.