PURPOSE: To enable an accurate frequency analysis, by providing a grating coupler in an area through which a non-diffracted light of a light waveguide passes to detect a diffracted light with a high S/N ratio while the position of condensing of the non-diffracted light is measured accurately.

CONSTITUTION: A grating coupler 14 is set at an area of a light waveguide 2 through which non-diffracted light 11 passes to a photo detector 8 from a second diodesic lens 5. The coupler 14 is made up of a relief type grating 18 and converts a waveguide light 15 into a first radiation light 16 being radiated into a piezo-electric substrate 1 or a second radiation light 17 being radiated into the air. As a result, a condensing spot can be vanished at a point A of the non-diffracted light 11. This enables the removal of the non-diffracted light 11 overlapped with a condensing spot of a diffracted light 12 as noise when the condensing spot thereof 12 at a point B is detected with the photo detector 8. The intensity of the condensing spot of the non-diffracted light 11 at the point A can be adjusted by regulating the overall length L of the coupler.