PURPOSE: To widen a dynamic range with a simple constitution by separating undeflected light which is not deflected with a surface acoustic wave and deflected light which is a signal component spatially in a plane optical waveguide and removing the undeflected light from the plane waveguide.

CONSTITUTION: Light emitted by a light source 12 is guided to a three- dimensional waveguide 11 provided on the surface of a substrate and then propagated in the waveguide and the plane waveguide 13. The light 23 propagated in the waveguide 13 has an angle of deflection determined almost by the wavelength of the light source, the beam width of light propagated in the waveguide 1, and the refractive index of the waveguide 13, and the light 23 is considered almost as collimated light in an area where the angle of deflection is small. This light 23 is deflected with the surface acoustic wave 19 generated by an electrode 18 for surface acoustic wave generation and separated into the deflected light 16 and undeflected light 17 spatially in the waveguide 13. Then, the undeflected light 17 is removed from the waveguide 13 by a reflecting or absorbing material and only the deflected light 14 is converged by a plane lens 14 and detected by an array detector 20.