| 摘要 |
PURPOSE:To attain a high SN ratio even if double refraction remains, by controlling an optical path length variable wavelength plate so that the amplitude of a reproduced signal, which is obtained by subjecting the reflected light separated by a polarizing beam splitter to photoelectric conversion, is maximum. CONSTITUTION:A laser light is irradiated to a ruggedness 22 of an information disc 16 through a collimate lens 12, a polarizing beam splitter 13, a Babinet- Soleil wavelength plate 21, and an object lens 15. The reflected light passes through the polarizing beam splitter 13 and etc. and is converted to an electric signal 28 by a photodetector 18. The amplitude of this signal is detected by an amplitude detecting circuit 23 and is compared with a reference amplitude value 24 in a comparator 25, and a wavelength plate driving device 27 is driven in accordance with an obtained difference signal, and the wedge-shaped wavelength plate 21 is shifted to change the thickness so that the amplitude of the signal 28 is maximum. Thus, the quantity of detection light is made maximum to eliminate its variance. |
