US Patent 7898927 Optical pickup device

A pickup device includes a diffraction grating 12 for separating a light beam emitted from the light source into at least three light beams. The diffraction grating 12 is divided into four regions by straight lines extending in a direction parallel to a tangential direction of tracks of an optical information recording medium. A periodic structure of a second region 12B has a phase difference of approximately 180 degrees from a periodic structure of a third region 12C, and a periodic structure of a first region 12A has a phase difference of approximately 180 degrees from a periodic structure of a fourth region 12D.