Patent ID: 7710650

Claim:
A diffractive optical element comprising: a plurality of diffraction gratings which includes two diffraction gratings made of materials having different dispersive powers, the plurality of diffraction gratings including, a first diffraction grating having a plurality of circular-zone grating portions which are provided on a first base surface, and a second diffraction grating having a plurality of circular-zone grating portions which are provided on a second base surface, the plurality of grating portions of the second diffraction grating including a grating portion of a central circular zone and a grating portion of a peripheral circular zone, wherein in the second diffraction grating, a height of the grating portion of the central circular zone from the second base surface is greater than a height of the grating portion of the peripheral circular zone from the second base surfaces, wherein a grating surface of the grating portion of the central circular zone of the second diffraction grating, and a grating surface of the grating portion of the central circular zone of the first diffraction grating are in a contact state, wherein an optical path difference function φ(r) of the diffractive optical element is determined as follows: φ( r )= C 1· r 2 +C 2 ·r 4 +C 3 ·r 6 + where r is a distance from the center of the diffractive optical element, and C 1 , C 2 , C 3 , . . . are constants, and wherein a curvature radius R of each of the grating surfaces of the grating portions of the central circular zones of the first and second diffraction gratings in the contact state satisfies the following condition: R= 0.5×( n 02 −n 01)/ C 1 where n 01 is a refractive index of the material of the first diffraction grating with a wavelength λ 0 , and n 02 is a refractive index of the material of the second diffraction grating with the wavelength λ 0 .