Patent ID: 8295145

Claim:
An optical pickup device comprising: a semiconductor laser element; an objective lens which collects optical beams emitted from said semiconductor laser element so as to irradiate the collected optical beam onto an optical disc; an actuator which moves said objective lens in order that said collected optical beam is irradiated onto a predetermined position on said optical disc; a diffraction grating which diffracts said optical beam diffracted from said optical disc; and a photodetector which receives said optical beam diffracted by said diffraction grating; wherein: said diffraction grating includes five regions comprising a first region, a second region, a third region, a fourth region, and a fifth region; the first region and the third region are located in a line symmetrical manner with respect to a center axis of said diffraction grating; the second region and the fourth region are located in a line symmetrical manner with respect to the center axis of said diffraction grating; even when the second region and the fourth region are separated from said center axis of the diffraction grating, widths of both said second region and said fourth region are equal to each other, or said widths thereof gradually become narrower in accordance with separation distances from the center axis of the diffraction grating; wherein among the diffracted light diffracted from the track on said optical disc, zeroth-order diffracted light enters to the first region and the third region; the zeroth-order diffracted light, + first-order diffracted light, and − first-order diffracted light enter to said second region and said fourth region; and wherein: a focusing error signal acquired from said optical disc in accordance with an astigmatic detection method is produced based upon signals obtained by detecting said optical beams diffracted from or transmitted through said first region and said third region of said diffraction grating by using said photodetector, and said diffraction grating comprises a polarized diffraction grating configured so that linear polarization of the optical beam transmitted through said polarized diffraction grating intersects substantially perpendicular to linear polarization of the optical beam diffracted from said polarized diffraction grating.