| 摘要 |
PURPOSE:To extract asymmetrically light with simple constitution without using any knife edge by forming a notch in a part of an optical lens which detects the reflected light of the beam irradiated to an information recording medium. CONSTITUTION:The laser light emitted from a semiconductor laser 12 is condensed through a collimator lens 13, a polarization beam splitter (PS) 15, a lambda/4 plate 16, and an objective lens 17 onto a recording medium 4. The reflected light advances reverse to pass through the PS 15 and a half mirror 18 and is condensed by an optical lens 24 onto a photodetector 26, by which the out of focus is detected. The light past the notch 24a of the lens 24 is not detected by the photodetctor 26 and does not contribute to the detection of the out of focus. The light asymmetrical with respect to the optical axis is condensed on the photodetector 26. The need for a knife edge for extracting the asymmetrical light is thus eliminated and the constitution is simplified. |
