The present invention relates to optical apparatus such as a photosensor, a semiconductor laser, an optical amplifier in which a wavelength selective photocoupler is used so as to couple two waveguides through a diffraction grating. A photosensor which is one of the optical apparatus according to the present invention comprises a substrate, a first waveguide layer formed on the substrate, a second waveguide layer formed on the first waveguide layer to be stacked in a direction of thickness and which has a guided mode difference from that of the first waveguide layer, a diffraction grating formed on an overlapping region of the guided modes of the first and second waveguide layers and which couples light components of a specific wavelength range of light propagating through the first waveguide layer to the second waveguide layer, a light absorption layer for absorbing at least some light components of the light components coupled to the second waveguide layer, and an electrode for converting the light components absorbed by the light absorption layer into an electrical signal and outputting the electrical signal.