Diffraction element, objective lens unit, optical pickup, optical disc apparatus and design method for diffraction element

摘要

The first and second materials of the diffraction element are selected such that the first material's refraction indexes n1(lambda1), n1(lambda2) and n1(lambda3) for the first, second and third wavelengths lambda1, lambda2 and lambda3 and the second material's refraction indexes n2(lambda1), n2(lambda2) and n2(lambda3) for the first, second and third wavelengths lambda1, lambda2 and lambda3 satisfy one of the conditions (1) or (2).