Transmissive diffraction grating, and spectral separation element and spectroscope using the same

Abstract

A transmissive diffraction grating includes a substrate and a plurality of ridges provided in a mutually parallel manner at constant periodicity p on the substrate. The ridges include a first layer, a second layer (refractive index n: 2.0-2.5), 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. For a single ridge, the 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 0.75pkθ/(n−1)<S<1.20pkθ/(n−1), were, θ is the angle of incidence onto the diffraction grating face, expressed in radian units, and the constant kis 1.1. The thickness dof the second layer is in the range of 0.70pkθn/(n−1)<d<1.30pkθn/(n−1), where, the constant kis 0.69.