The present invention relates a method for manufacturing an optical coupler/splitter.
A Y-shaped waveguide core is formed by focusing a laser beam into a cladding layer
that is formed from a glass material. At least one portion of this Y-shaped waveguide
core other than the input port and the output ports is treated as a refractive
index adjustment area. After the portions of the Y-shaped waveguide core other
than the refractive index adjustment area have been formed by focusing a laser
beam, signal light is inputted into the input port of the Y-shaped waveguide core
and the signal light outputted from the output ports of the Y-shaped waveguide
core are monitored. The refractive index of the refractive index adjustment area
is determined based on the result of the monitoring and is adjusted by focusing
a laser beam repeatedly.