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

摘要

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 n₂: 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 0.75p²k₁θ²/(n₂−1)S1.20p²k₁θ²/(n₂−1), were, θ is the angle of incidence onto the diffraction grating face, expressed in radian units, and the constant k₁ is 1.1. The thickness d₂ of the second layer is in the range of 0.70pk₂θn₂/(n₂−1)d₂1.30pk₂θn₂/(n₂−1), where, the constant k₂ is 0.69.