Optical semiconductor device and method of manufacturing optical semiconductor device

A method of manufacturing an optical semiconductor device, comprising:

• a step of forming a stacked structure by stacking a cladding layer of a first conductivity type, an active layer, and a first cladding layer of a second conductivity type as an opposite conductivity type to the first conductivity type, in this order, on a surface of a substrate of the first conductivity type;
• a step of etching both side portions of the stacked structure up to a position nearer to the substrate of the first conductivity type than to the active layer, to thereby form a ridge stripe;
• a step of burying, using a burying material doped with a semi-insulative material, respective both sides of the ridge stripe while leaving a top part of the ridge stripe exposed, to thereby form first burying layers;
• a step of forming a second burying layer of the first conductivity type while covering, with at least one mask, a middle portion of the top part of the ridge stripe except for both ends thereof so that the second burying layer has a current narrowing window where respective protruding portions of the second burying layer that are protruding from the first burying layers onto the top part of the ridge stripe are opposed to each other with an interval therebetween narrower than a width of the top part;
• and
• a step of forming, using a material of the second conductivity type, a second cladding layer of the second conductivity type under which the second burying layer of the first conductivity type is buried together with the current narrowing window;
• wherein, in the step of forming the stacked structure, a diffraction grating in which a λ/4 phase shifter is placed at an intermediate portion in a light traveling direction, is formed in the cladding layer of the first conductivity type or the first cladding layer of the second conductivity type;
• and
• wherein, in the step of forming the second burying layer of the first conductivity type, at least one of a width and a use number of said at least one mask is changed depending on a position in the light traveling direction so that, at a region in the light traveling direction where the λ/4 phase shifter is placed, at least one of following conditions is satisfied:
• an opening width of the current narrowing window is maximum;
• and a thickness of the protruding portions of the second burying layer of the first conductivity type is minimum.