Patent ID: 6946684

Claim:
An optical semiconductor device comprising: an optical semiconductor element having an equivalent refractive index nc and an end face for receiving and emitting light; and a coating film layer structure which includes a first coating film disposed on the end face of said optical semiconductor element, having a refractive index n 1 and a thickness a 0 *d 1 , where a 0 is a positive real number, and a second coating film disposed on said first coating film and having a refractive index n 2 and a thickness a 0 *d 2 , wherein n 0 and λ 0 respectively denote a refractive index of free space at a surface of said coating film layer structure and a wavelength of light propagating through said optical semiconductor element, both a real part and an imaginary part of an amplitude reflectance of the coating film layer structure, which is determined by the wavelength λ 0 , the refractive indexes n 1 and n 2 , and the film thickness a 0 *d 1 and a 0 *d 2 , are zero, and only one of the refractive indexes n 1 and n 2 is smaller than a square root of a product of the refractive indexes nc and n 0 .