Optical device used wavelength selective photocoupler

An optical device comprising:

• a substrate;
• a first guiding layer provided on said substrate and having a guided mode;
• a second guiding layer laminated with said first guiding layer on said substrate, said second guiding layer having a guided mode different from the guided mode of said first guiding layer;
• and
• a diffraction grating provided on an area in which said first and second guiding layers have their guided modes overlapping, said diffraction grating serving to couple the light in the specific range of wavelengths that propagate through said first guiding layer, with said second guiding layer,
• wherein said diffraction grating comprises a high refractive index region and a low refractive index region disposed periodically in the propagation direction of light, the ratio of the high refractive index region to the low refractive index region changing gradually in the propagation direction of light, and the pitch of said diffraction grating gradually changing in the propagation direction of light.