An optical semiconductor, includes a semiconductor substrate having a
(100) principal surface, a waveguide mesa stripe formed on a first region
of the semiconductor substrate, the waveguide mesa stripe guiding a light
therethrough; a plurality of dummy mesa patterns formed on the
semiconductor substrate in a second region at a forward side of the first
region, and a semi-insulating buried semiconductor layer formed on the
semiconductor substrate so as to cover the first and second regions
continuously, the semi-insulating buried semiconductor layer filling a
right side and a left side of the waveguide mesa stripe in the first
region and a gap between the plurality of dummy mesa patterns in the
second region.