Modal evolution optical coupler and method for manufacturing the coupler

Abstract

A modal evolution coupler having a low-loss or non-loss effect when a transmitted light passes through an optical fiber and is output via an expanding element, includes: a first optical fiber which gradually, thermally expands a core to evolute the mode of a transmitted light, a second optical fiber which gradually, thermally expands a core to evolute the mode of a transmitted light, an expanding element which is placed between the first and second optical fibers, and is formed of an ultraviolet curing resin, and has ends whose section areas are equal to the respective ends of the first and second expanded optical fibers when an ultraviolet ray is incident thereon and cured, and includes expanded portions which are each expanded over a predetermined expansion distance starting from a connecting portion to the first or second optical fiber and a middle portion keeping its size constant for a predetermined length between the expanded portions of the first and second optical fibers, and does not move in the expansion distance, a channel transmission path which has two ends to which the first and second optical fibers are respectively connected and allows the transmitted light to be output after the expanding element is filled between the optical fibers placed on the ends thereof, and a coverlet positioned on the channel transmission path to enable a filling of the channel transmission path with an ultraviolet curing resin while aligning the first and second optical fibers and to prevent the resin from flowing out of the channel transmission path. Accordingly, the expansion distance of a core is long, so that an expanding element formed by ultraviolet ray irradiation is prevented from moving. Also, a loss of the transmitted light due to an influence of a resin existing between the section of an optical fiber and the wall of the channel transmission path is remarkably reduced. Furthermore, the use of a U- or V-shaped groove makes it possible for the optical fibers to be self-aligned and an emission loss is reduced.