SEMICONDUCTOR OPTICAL DEVICE

PURPOSE:To obtain a semiconductor optical device having a narrow selection wavelength band width even for an incident light having a wide wavelength band by providing a transmission waveguide by which a reflected light from a first diffraction grating enters a second diffraction grating and outputting a light selected in its wavelength from the second diffraction grating. CONSTITUTION:An incident light 1 enters a diffraction grating 2 for diffracting a light having a predetermined wavelength band with Bragg's wavelength lambda0 as a center through an incident waveguide 5. The diffracted light diffracted (reflected) by the diffraction grating 2 enters a lambda/4 shift type diffraction grating 3 through a transmission waveguide 7. The lambda/4 shift type diffraction grating 3 has a light transmission band of a narrow wavelength band width in the reflecting wavelength band of the diffraction grating 2, and a wavelength-selected light 4 is output from the diffraction grating 3. The diffraction gratings 2, 3 each have wavelength dependant transmittance characteristics as shown. Thus, if the wavelength band of the incident light of the diffraction grating 3 is narrowed as compared with the blocking band of the diffraction grating 3 by the diffraction grating 2 of the wavelength band of the incident light of the diffraction grating 3, when the wavelength is selected by the diffraction grating 3, a wavelength light which is not desired is not leaked.