Patent ID: 8259554

Claim:
An optical head device, comprising: a first light source which emits a first light beam; a second light source which emits a second light beam; a focusing optical system which focuses a light beam selectively emitted from said first light source and said second light source to an information recording medium having a track as convergent light; a diffractive element which diffracts a part of said light beam passing from said light source toward said information recording medium; a splitting instrument which splits said light beam reflected or diffracted by said information recording medium, in a direction different from a direction to said light source; and a photodetector which receives said light beam split by said splitting instrument, wherein said diffractive element is divided into at least four regions by dividing lines along a tangential direction of said track of said information recording medium, said four regions are arranged in numeric order as first, second, third, and fourth regions, in an orthogonal direction with respect to said tangential direction of said track, a relationship of arrangement of said second region and said third region corresponds to a relationship of arrangement of an optical axis of said first light beam and an optical axis of said second light beam on said diffractive element, said second region is located at a position crossing said optical axis of said first light beam, a phase difference between a phase of said first region and a phase of said third region is 180°, a phase difference between a phase of said third region and a phase of said fourth region is 180°, and said third region has a width which is not larger than amount of a position deviation of ±first-orderdiffracted light of said second light beam from zeroth-order diffracted light, said ±first-orderdiffracted light being generated by said information recording medium.