Patent ID: 8107346

Claim:
An optical head device comprising: a light source for emitting a light beam; a converging section for converging the light beam onto an optical storage medium having a track; a splitting section for splitting the light beam having been reflected from the optical storage medium; and a photodetection section for receiving the split light beams and outputting signals which are in accordance with received light amounts, wherein, the reflected light beam contains 0 th order diffracted light, +1 st order diffracted light and −1 st order diffracted light; the reflected light beam includes a first interfering portion in which the 0 th order diffracted light interferes with the +1 st order diffracted light and a second interfering portion in which the 0 th order diffracted light interferes with the −1 st order diffracted light; the splitting section includes a first main region transmitting the first interfering portion, a second main region transmitting the second interfering portion, and first and second sub-regions through which the first interfering portion is transmitted with a lower rate than in the first main region and through which the second interfering portion is transmitted with a lower rate than in the second main region; the first sub-region and the second sub-region are divided by a split line extending in a longitudinal direction of the first and second interfering portions in a cross section of the reflected light beam; the splitting section further includes a center dummy region through which a central portion of the cross section of the reflected light beam is transmitted; the center dummy region is surrounded by the first and second main regions and the first and second sub-regions; the splitting section splits the reflected light beam into a first main light beam transmitted through the first main region, a second main light beam transmitted through the second main region, a first sub-light beam transmitted through the first sub-region, a second sub-light beam transmitted through the second sub-region, and a dummy light beam transmitted through the center dummy region; the photodetection section includes a first photosensitive portion, a second photosensitive portion, a third photosensitive portion, and a fourth photosensitive portion; the first main light beam enters the first photosensitive portion; the second main light beam enters the second photosensitive portion; the first sub-light beam enters the third photosensitive portion; the second sub-light beam enters the fourth photosensitive portion; and the dummy light beam does not enter the first to fourth photosensitive portions.