PURPOSE: To easily and accurately correct the refractive index of a certain area of an optical device by providing adjacent areas consisting of transparent materials and making at least one area include chlorine enough to change optical capacities of parts.

CONSTITUTION: With respect to a fiber optical coupler provided with at least first and second optical fibers having cores and clads, fibers are welded along a part in the lengthwise direction to form a coupling area where at least a part of the optical signal transmitted in the first fiber is coupled to the second fiber. The clad of the second fiber includes chlorine enough to make its refractive index n2' higher than a refractive index n2 of the first fiber. It doesn't matter whether adjacent areas of a first passage may include chlorine or not. Thus, the refractive index of the area is easily and accurately corrected.