Patent ID: 7136344

Claim:
An optical head device comprising: a light source; a diffractive optical element for diffracting light emitted from said light source to split the light into a main beam and sub-beams, said diffractive optical element is divided into first, second, third, and fourth regions by a first straight line that crosses an optical axis of the light and extends in an extending direction of a grating of said diffractive optical element and a second straight line that crosses the optical axis and is perpendicular to said first line; an objective lens for focusing said main beam and said sub-beams onto an optical recording medium; an astigmatism generation unit for providing astigmatism to light reflected from said optical recording medium; and a photodetector for receiving light transmitted through said astigmatism generation unit to receive the main beam and the sub-beams, wherein a phase of the grating in said first region and said second region located diagonally with respect to said first region is different from a phase of the grating in said third region and said fourth region that are adjacent to said first and second regions and are located diagonally by substantially π, and the main beam and the sub-beams are focused in a region containing the same track on said optical recording medium.