Patent ID: 7706235

Claim:
An optical pickup device, comprising: a light source for emitting a beam having a predetermined wavelength; an objective lens for focusing the beam emitted from the light source on an optical storage medium; a beam splitter for passing or reflecting the beam emitted from the light source or beams reflected from the optical storage medium; a first diffractive element for generating a second beam having no component corresponding to a position of a beam, focused on the optical storage medium, on a track of the optical storage medium, from the beams reflected from the optical storage medium; a second diffractive element; a sensor lens for generating an astigmatism in the beams reflected from the optical storage medium; and photodetecting means including main detection means for receiving a first beam passing through the first and second diffractive elements without change, and two first sub detection means for receiving the second beam, wherein the second diffractive element diffracts part of beams reflected from other layers, rather than a current layer of the optical storage medium on which recording or reproduction is currently performed, thus preventing the diffracted beams from being incident on the photodetecting means.