PURPOSE:To simplify the entitled optical system by using a half mirror penetrating reflected light flux into a photo detector and suppressing the back-talk noise of the light radiated from a semiconductor laser by the negative feedback of a signal which has detected a part of the radiated light. CONSTITUTION:Light radiated from a semiconductor laser 1 is made to parallel light flux 4, which is divided into parallel light flux 18 and irradiating light flux 4, which is divided into parallel light flux 18 and irradiating light flux 6 by a half mirror 17. The irradiating light flux 6 is reflected on a disk 11, the reflected light flux 7 is divided into reflected light flux 19 and 20 and a data signal is reproduced from the reflected lighr flux 19. In order to reduce back-talk noise which increases when the reflected light 20 is penetrated again into the semiconductor laser 1, the parallel light flux 18 is converted into an electric signal by a photo detector 21 and fed backed negatively to the semiconductor laser 1 through an amplifier 22.
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