Patent ID: 7738523

Claim:
An optical semiconductor device comprising: a semiconductor substrate; a waveguide structure formed over said semiconductor substrate, having a width defined by sidewalls, and including a first wide width portion having a finite first length and a first width, a narrow width portion having a second length and a second width which is smaller than said first width, a second wide width portion having a finite third length and a third width which is wider than said second width a first intermediate width portion connecting said first wide width portion and said narrow width portion and having a width continuously changing from said first width to said second width, and a second intermediate width portion connecting said narrow width portion and said second wide width portion and having a width continuously changing form said second width to said third width, formed along a propagation direction; and a diffraction grating formed on the sidewalls of at least one of said first wide width portion, said narrow width portion, and said second wide width portion of said waveguide structure, said diffraction grating having vertical grooves periodically disposed along the propagation direction and defining a wavelength of propagation light.