In an aspect of the present invention, a thermo-optic phase shifter
includes a substrate, a heater, a clad layer formed directly or
indirectly on a substrate, an optical waveguide clad layer formed apart
from the substrate and the clad layer in a portion corresponding to the
heater, and a core layer provided inside said bridge section clad layer.
The optical waveguide clad layer is connected with the clad layer in a
portion of the phase shifter other than the heater corresponding portion.
The optical waveguide clad layer and the core layer form an optical
waveguide in the heater corresponding portion. The heater is provided
inside or outside the optical waveguide apart from the core layer in the
heater corresponding portion, and generates heat to change a phase of a
light signal propagated in the optical waveguide.