Diffraction element and optical disk device

An optical disk device for use with an optical disk comprising:

• a diffraction element comprising:
• a plurality of groove parts;
• and
• a plurality of protruded parts which are alternately arranged with a plurality of the groove parts;
• wherein a depth dimension between upper faces of the protruded parts on both sides of the groove part and a bottom part of the groove part varies according to position;
• a laser light source;
• a photo-detector;
• and
• an optical system for structuring a forward path that guides a laser beam emitted from the laser light source to an optical disk and a return path that guides a return light beam reflected by the optical disk to the photo-detector;
• wherein the optical system includes the diffraction element which is disposed at a middle position of the forward path as a three-beam generating element that generates a main beam comprised of a 0-order light beam and two sub-beams comprised of diffracted light beams from the laser beam emitted from the laser light source.