| 摘要 |
PURPOSE:To maintain the best signal-to-noise ratio of a detected light signal by providing a correcting circuit which irradiates a photomagnetic medium with linear polarized luminous flux and outputs an optimum reverse bias corresponding the photodetection quantity of reflected luminous flux, and applying the optimum reverse bias to a photodetecting element. CONSTITUTION:The photomagnetic disk is irradiated with the linear polarized luminous flux and outputs of photodetecting elements 16 and 17 which photodetects the luminous flux split into two by a track detection system 18 are supplied to an adder 25. The mean photodetection quantity 26 of the reflected luminous flux which is added is outputted to the correcting circuit 27. The circuit 27 consists of an A/D converter 28, ROM 29, and D/A converter 30 and the value of the photodetection quantity 26 is converted into such a reverse bias voltage that the S/N ratio of an avalanche photodiode (APD)12 is maximum. The output of the circuit 27 is converted by a DC/DC converter 32 into a necessary reverse bias voltage V2, which is applied to the APD12. The output 33 of the APD12 is amplified by an amplifier 34 to hold the signal-to-noise ratio of the light signal optimum. |
