Optical-fiber coupler and manufacturing process therefor

In an optical-fiber coupler formed by disposing two single-mode optical fibers 1 and 2 in parallel, and fusing and elongating the optical fibers at one position in the longitudinal extent of the optical fibers 1 and 2, variation of insertion loss dependent on the polarization can be eliminated in its manufacturing by imparting a twist to the optical coupling part 3' of the fused-elongated portion 3 so that with respect to the direction of a line on a plane orthogonally intersecting the longitudinal direction of the optical fibers 1 and 2, the line linking the centers of the two optical fibers 1 and 2, the direction of the line on a plane at one end of the optical coupling part 3' of the fused-elongated portion 3 makes an angle of 90 degrees with the direction of the line on a plane at the other end of the optical coupling part 3'.