Patent attributes
An optical pickup device comprising a plane-parallel plate beam splitter that reflects a laser beam emitted from a laser beam source, an objective lens that focuses the laser beam from the plane-parallel plate beam splitter so as to irradiate the focused beam into a signal recording medium, a photodetector that receives the laser beam reflected and returned from the signal recording medium through the beam splitter, the photodetector having light receiving regions each of which is divided at least into four parts by two dividing lines that intersect each other at right angle, a first plane-parallel plate disposed between the beam splitter and the photodetector, the first plane-parallel plate being tilted in the direction rotated π/2 radians around the optical axis with respect to the beam splitter, and a second plane-parallel plate disposed between the beam splitter and the photodetector, the second plane-parallel plate being tilted in the direction of rotation around the optic axis such that the direction of astigmatism projected on the light receiving regions of the photodetector has an angle of π/4 radians relative to each of the dividing lines, wherein astigmatism generated by the beam splitter is canceled by the first plane-parallel plate, and wherein astigmatism used for focus control is generated by the second plane-parallel plate.
