Patent ID: 8564882

Claim:
A diffractive optical element, comprising: first and second optical members which are stacked, wherein the first optical member includes a diffraction grating in which a plurality of raised portions each having a vertical surface and a surface inclined to the vertical surface are arranged, the second optical member includes a diffraction grating in which a plurality of recessed portions are arranged, to which the raised portions are fitted, the diffraction grating of the first optical member and the diffraction grating of the second optical member are in close contact with each other, a ridge portion formed by the vertical and inclined surfaces of the raised portion defines a curved surface, and the following expression (1) is satisfied when a curvature radius of the curved surface is R (μm) and a pitch of the raised portions is P (mm): R≦A 1 ×P 3 +B 1 ×P 2 +C 1 ×P (1) where Δn d is a refractive index difference between the first and second optical members at a d line, A 1 =−78479×Δn d 2 +20869×Δn d +36.356, B 1 =19085×Δn d 2 +5119.7×Δn d −370.98, and C 1 =2050×Δn d 2 −896.21×Δn d +143.61.