Double refraction plate with refractive grating pattern around a centerally disposed transparent region to allow the passage of either an ordinary ray or an extraordinary ray

摘要

An optical element has a diffraction grating pattern formed on a double refracting thin plate around a centrally-disposed region of a desired shape. The diffraction grating pattern is composed of equally spaced ion exchange regions each having a predetermined width, a predetermined length and a predetermined thickness in the direction of optical axis of incident light thereon and dielectric films each formed all over the top surface of one of the ion exchange region. The thicknesses of each ion exchange region and each dielectric film are selected such that the diffraction grating pattern allows the passage therethrough of an ordinary ray and inhibit the passage therethrough of a zero-order diffracted wave of an extraordinary ray. The optical element permits implementation of an easy-to-handle optical pickup that facilitates miniaturization of the playback system.