Patent ID: 6888681

Claim:
An optical demultiplexer comprising: a light input unit for inputting incident light rays having a plurality of optical signals different in wavelength as a multiplex signal; a light collimating element for converting said incident light rays input by said light input unit into parallel light rays; a diffraction grating for demultiplexing said parallel light rays collimated by said light collimating element in accordance with wavelengths; and a plurality of photo acceptance units provided in different positions where a plurality of light rays at different wavelengths demultiplexed by said diffraction grating are converged and image-formed in accordance with the wavelengths by said collimating element; wherein distances h 1 (Î» 0 ), h 1 (Î» 1 ), . . . , h 1 (Î» n ) between an optical axis of said collimating element and optical axes of said plurality of photo acceptance units in accordance with the demultiplexed wavelengths satisfy an expression: h 1 (Î» j )= f tan(sin âˆ’1 ( NmÎ» j âˆ’A )âˆ’Î¸ g ) in which j is an integer of 0 to n, A is equal to sin(Î¸g+tan âˆ’1 (h 0 /f)), f is a focal length of said light collimating element, Î¸g is an angle between the optical axis of said light collimating element and a line normal to a diffraction surface of said diffraction grating, h 0 , is a distance between said light input unit and the optical axis of said light collimating element, N is a number of grooves in said diffraction grating, m is order of diffraction, and Î» 0 , Î» 1 , . . . , Î» n are demultiplexed wavelengths.