Optical demultiplexer

Incident light rays ( 11 ) given from an optical fiber ( 10 ) are converted into parallel light rays ( 12 ) by a collimator lens ( 2 ). The parallel light rays ( 12 ) are demultiplexed in accordance with wavelengths by a diffraction grating ( 3 ). After the demultiplexed light rays arrive at the collimator lens ( 2 ) again, the light rays are converted into convergent light rays ( 14 ) and converged into a photo acceptance element array. Because the converging/image-forming positions of respective channels are arranged at irregular pitches in accordance with wavelengths, signals of the channels can be taken out accurately and efficiently from photo acceptance elements respectively when the photo acceptance elements are provided in the converging/image-forming positions respectively.