Optical pickup device and diffractive optical element

摘要

When a wavelength having the same value as a reference wavelength lambda 1 of a first laser light is set to a designed wavelength lambda, a diffractive optical element has: an inner circular side irregular diffraction pattern portion, in which a plurality of irregular portions in which a height of a convex portion is set to approximately 1lambda-fold of the designed wavelength lambda with respect to a concave portion are repeated, being formed in an annular shape in an inner circular area having a predetermined diameter for the correction of a spherical aberration generated due to a difference in substrate thickness between first and second optical recording mediums centering on a central point through which an optical axis runs while gradually changing a pitch of the irregular portions in a radial direction toward the outer circular side; and an outer circular side tiered diffraction pattern portion which is intended to improve a chromatic aberration with respect to the first laser light by forming in an annular shape in an outer circular area outside the inner circular side irregular diffraction pattern portion a plurality of tiers by setting a height of each tier to approximately mlambda-fold (where m is a natural number which does not include 0 ) of the designed wavelength lambda or by setting the same while changing a value of m for each tier.