Patent ID: 7050236

Claim:
A diffractive optical element for use in an optical pickup apparatus reproducing and/or recording information including a first light source for emitting a first light flux with a wavelength λ 1 ; and a converging optical system having an objective lens for converging the first light flux on an information recording surface of a first optical information recording medium having a protective substrate with a thickness t 1 , the diffractive optical element comprising: a diffractive structure formed on at least one optical surface and on an area including a optical axis on the diffractive optical element, wherein the diffractive optical element satisfies following expressions: 20%≦E≦90% (1) 0.0016 ≦L/f≦ 0.032 (2) where E is a diffraction efficiency of a m-th order diffracted beam, m is a positive integer and a diffraction order of a diffracted beam used for information reproducing and/or recording on the first optical information medium among a plurality of diffracted beams with different diffraction orders generated when the first light flux is received a diffractive action by the diffractive structure, L [mm] is a distance between a paraxial converging position of a diffracted beam with higher diffraction efficiency among a (m+1)th order diffracted beam and a (m−1)th order diffracted beam and a paraxial converging position of a m-th order diffracted beam, f [mm] is a focal length of the diffractive optical element for the first light flux, and a 0 th order diffracted beam which is a (m−1)th order diffracted beam when m is 1, is a light flux passing through the diffractive structure without being diffracted because the diffractive structure doesn't provide an substantial optical path difference to the passing light flux.