Patent ID: 7688512

Claim:
A transmissive diffraction grating comprising a substrate and a plurality of ridges provided parallel to one another at constant periodicity p on the substrate, wherein the ridges include a first layer, a second layer, and a third layer with non-continuous refractive indices arranged in that order from the substrate outward, the first layer adjacent the substrate, in terms of its refractive index, exhibits a difference of 0.1 or less relative to the substrate, the second layer has a higher refractive index than the first layer and third layer and satisfies the following conditions (A) to (C): (A) the refractive index n 2 of the second layer is 2.0 to 2.5; (B) for a single ridge, a cross-sectional area S of a cross-section of the second layer perpendicular to the longitudinal direction of said ridge is in the range of 0.75p 2 k 1 θ 2 /(n 2 −1)<S<1.20p 2 k 1 θ 2 /(n 2 −1), where, θ is the angle of incidence onto the diffraction grating face, expressed in radian units, and the constant k 1 is 1.1; and (C) a thickness d 2 of the second layer is in the range of 0.70pk 2 θn 2 /(n 2 −1)<d 2 <1.30pk 2 θn 2 /(n 2 −1), where, the constant k 2 is 0.69.