| 摘要 |
PURPOSE:To use simultaneously and easily one optical system to perform tracking and auto-focusing without degrading the light use efficiency by using three semiconductor lasers. CONSTITUTION:Laser luminous fluxes from three semiconductor lasers on a substrate 1 are led to photo detectors 9, 9', and 9'' respectively after passing a lens 3. The detector 9' detects an information signal to reproduce a picture on a television 11. The differential output of photodetectors 9 and 9'' drives a galvanomirror 4 to allow the laser luminous flux to follow up an information track varying in the direction of an arrow. The detector 9' is set on a rear-side focal surface 15 of a converging lens 5, and the detector 9 is set in the positive side, and the detector 9'' is set in the negative side. If a video disc 6 is deviated from the focus of the lens 5, the difference between output signals from photodetectors 9'' and 9 is positive or negative. This difference signal from low-pass filters 12 and 12' is used to drive the lens 5 by a driving circuit 14, thereby keeping the length between the lens and the disc constant to hold the focus on the disc surface. |
