| 摘要 |
PURPOSE:To achieve sure control with a simple optical system, by splitting a photodetector into four around an optical axis, using a knife edge for focus control and detecting an output difference through the difference among the light amount to be reflected for track control. CONSTITUTION:An output light from a laser 1 is irradiated on a disc 9 via an objective lens 2, a 1/4 wavelength plate 3, a condenser lens 4, and a reflected light is irradiated to a detector 6' via a beam splitter 5. Since a knife edge 8 is located at the image forming point and the amount of light to upper and lower two sets of photodetectors of the detector 6' is changed depending on the distance between the disc 9 and the pickup, this is used for focus control. Since the amount of light of the detector 6' to left and right photodetectors is changed depending on the shift of center of information pits 10, tracking is made by using this. |
