Planar optical waveguide device, DP-QPSK modulator, coherent receiver, and polarization diversity

A planar optical waveguide device comprising:

• a substrate;
• a core that forms a first waveguide and a second waveguide that are arranged in parallel on the substrate;
• and
• a cladding that covers the core and has a refractive index smaller than that of the core, wherein
• the core comprises a first rib portion, a second rib portion, and a slab portion, the first rib portion forming the first waveguide, the second rib portion forming the second waveguide, and the slab portion being provided only on one side of the first rib portion and the second rib portion in a width direction to have a thickness smaller than the thicknesses of the first rib portion and the second rib portion, the slab portion being shared between the first rib portion and the second rib portion,
• the first waveguide and the second waveguide form a mode converting section that is configured to convert a mode of light input to an input side thereof into a mode different from the former mode of the light between the input side and an output side thereof,
• the mode converting section comprises a waveguide structure in which mode coupling is generated between the first waveguide and the second waveguide,
• an effective refractive index of a first waveguide mode in the first waveguide and an effective refractive index of a second waveguide mode in the second waveguide match each other on at least a cross section perpendicular to a light transmission direction in the waveguide structure,
• a magnitude relationship between the effective refractive index of the first waveguide mode and the effective refractive index of the second waveguide mode is switched between the input side and the output side with the one cross section being interposed therebetween, and
• the mode converting section comprises a narrow width portion in which the width of the slab portion becomes smallest, a width-decreasing portion in which the width of the slab portion continuously decreases toward the narrow width portion from the input side, and a width-increasing portion in which the width of the slab portion continuously increases toward the output side from the narrow width portion.