| 摘要 |
PURPOSE: To reduce the disturbance generated accompanying traversing of a light spot across a guide groove in an optical disk by making a difference signal between the products of outputs of photodetectors in diagonal positions respectively in a quadripartite photodetector arranged in the vicinity of a converging point of reflected light from an optical recording medium a focus displacement signal. CONSTITUTION: Light emitted from a semiconductor laser 1 is passed through a collimator lens 2, a beam splitter 3 and a rising prism 4 and is converged on the optical disk 6 by an objective lens 5. Its reflected light goes back and is reflected by the beam splitter 3 to pass through a condenser lens 7, and is the subjected to astigmatism by a cylindrical lens 8 and is incident upon the quadripartite photodetector 9. The outputs a, b and c, d of the photodetector 9 are inputted to multipliers 11a and 11b in a signal calculating circuit 10 respectively, and the products of the outputs of the photodetectors in the diagonal positions a×b and c×d are inputted to a differential amplifier 12, and its output is made to be the focus displacement signal.
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