Patent ID: 6839173

Claim:
In a diffraction grating having a groove period which is 0.1 to 10 times as large as a wavelength of incident light λ, a reflection type diffraction grating comprising (1) a laminate of a metal film as a first layer with reflectance not lower than 30%; wherein said metal film has a refractive index not higher than 2.0 and an extinction coefficient not smaller than 6.0 with respect to the wavelength of the incident light; and (2) a transparent dielectric film as a second layer, wherein the transparent dielectric film has an optical film thickness in a range of 0.20λ to 0.80λ, said first and second layers being laminated in order on a surface side of said diffraction grating, whereby incident light is reflected by said first layer through said second layer; wherein said dielectric film has a refractive index in a range of from 1.30 to 1.65, both inclusively, with respect to the wavelength of the incident light.