Patent ID: 8563991

Claim:
An optical semiconductor device comprising: a semiconductor substrate; an optical semiconductor region that is provided on the semiconductor substrate and has a width smaller than that of the semiconductor substrate; and a heater that is provided on the optical semiconductor region, the optical semiconductor region having a cladding region, an optical waveguide layer and a low thermal conductivity layer, the optical waveguide layer being provided in the cladding region and having a refractive index higher than that of the cladding region, the low thermal conductivity layer being provided between the optical waveguide layer and the semiconductor substrate and having a thermal conductivity lower than that of the cladding region, the low thermal conductivity layer being composed of a material having a refractive index lower than that of the optical waveguide layer, a difference between a refractive index of the low thermal conductivity layer and a refractive index of the cladding region being under 6%, the cladding region being composed of InP, the low thermal conductivity layer being composed of InAlGaAs or InAlAsP.