Patent ID: 8120852

Claim:
A diffractive optical element comprising: a first diffraction grating and a second diffraction grating, which are formed of materials different from each other, the first diffraction grating and the second diffraction grating are stacked so as not to provide any air layer therebetween, wherein the diffractive optical element satisfies the following conditions: nd1<nd2 νd1<νd2 1.65≦nd1, νd1≦20 1.73≦nd2, 15≦νd2≦60 0.9217≦( n 2(λ)− n 1(λ)) d/mλ≦ 1.0783 where nd1 and νd1 respectively represent a refractive index and an Abbe constant of the material of the first diffraction grating for a d-line, and nd2 and νd2 respectively represent a refractive index and an Abbe constant of the material of the second diffraction grating for the d-line, and d represents a grating height of each of the first diffraction grating and the second diffraction grating, and λ represents a use wavelength, n1(λ) and n2(λ) respectively represent refractive indexes of the first diffraction grating and the second diffraction grating at the use wavelength λ, and m represents a designed order.