Diffraction element, optical head device including the diffraction element, optical information apparatus including the optical head device, system including the optical information apparatus and method of designing diffraction grating of the diffraction element

摘要

A diffraction element includes a light-transmittable member and a diffraction grating formed on at least one face of the light-transmittable member. When a first laser beam having a first wavelength lambda1 and a second laser beam having a second wavelength lambda2 are transmitted through the diffraction element at first and second diffraction efficiencies, respectively, the diffraction element functions to equalize the first and second diffraction efficiencies to each other by only the one face of the diffraction element. The diffraction grating has a first phase modulation amount ø1 for the first laser beam and a second phase modulation amount ø2 for the second laser beam and the first and second phase modulation amounts ø1 and ø2 are, respectively, approximate to (2piN1±Deltaø) and (2piN2±Deltaø) in which "N1" and "N2" are natural numbers and "Deltaø" is a phase variation amount.