Generally, optical pickups for recording and reproducing information via a magneto-optical disk detect the reflected light beams whose polarization angle is rotated by means of a magneto-optic effect (called the Kerr effect) of the recording portion of the magneto-optical disk, to decode the recorded information. To more specifically describe the detection process, FIG. 1 shows a conventional simple optical pickup for recording and reproducing the information of the magneto-optical disk.
In FIG. 1, the laser beam emitted from a laser diode 1 is a linearly polarized wave. The light beam of the linearly polarized wave advances parallel to the beam axis by means of collimating lens 2. A beam-shaping separation prism 3 shapes the cross-sectional surface of the light beam into a true circle. Then, the shaped light beam is separated so that one part thereof is transmitted to a monitor photo detector 4 and the remainder continues along a straight path. This portion of the linear polarized light beam which is not redirected by beam shaping separation prism 3 is collected in a magneto-optical disk 6 via an objective lens 5. The light beam reflected from magneto-optical disk 6 reversely passes through objective lens 5. Then, part of the reversely passed light is transmitted directly to laser diode 1 via beam-shaping separation prism 3 and the remainder thereof is transmitted forward through a focus lens 7. The part of the reflected light beam which passes through focus lens 7 is reflected at a reflective knife-edge prism 8 to proceed to a half-wavelength plate 9. While the remainder thereof advances directly to a focusing photo detector 10. The light reflected from the recording portion of magneto-optical disk 6 has a linearly polarized wave, with its polarization angle rotated by the Kerr effect. Half-wavelength plate 9 passes the linear polarized wave whose polarization angle is rotated. The reflected light beam which passes half-wavelength plate 9 is again polarization-separated into two components by polarized beam splitter 11, so that the two separated light beams respectively arrive at two reproduction photo detectors 12 and 13. The reproduction signal for the information recorded on the magneto-optical disk is obtained by a differential signal between photo detectors 12 and 13. That is, the conventional optical pickup for the magneto-optical disk polarization-separates the reflected light beam whose polarization angle is rotated, and detects the differential signal due to the difference of the quantity of light between the two components so as to decode the information.
In such a conventional optical pickup, part of the reflected light beam, i.e., that at the unrecorded portion of the magneto-optical disk, is again incident upon the laser diode. Accordingly, since the light oscillation of the laser diode is very unstable, noise is generated. Therefore, to suppress such noise generation, the laser diode should be driven with a costly high-frequency oscillator. Also, since the light beam is separated in each of the beam-shaping separation prism, knife-edge prism and polarized beam splitter, the quantity of light which reaches the respective photo detection elements becomes very small. Thus, the signal-to-noise ratio is low. When the fluctuation of the quantity of light is severe, errors are generated in the servo control of the objective lens.
According to a technology disclosed in U.S. Pat. No. 5,115.420, the signal-to-noise ratio can be improved to strengthen the function of the servo control of the objective lens. That is, the wasted light beam is intercepted by a mirror at one side of a beam splitter, and is phase-coherently combined with the light reflected from a recording medium. However, even with the above technology, it is inevitable that part of the reflected light beams is again incident upon the laser diode, and problems still persist in the generation of servo control errors due to fluctuations in the quantity of light (intensity).
The above problems reside in the fact that the detection of a control signal for objective lens focusing and tracking servo operation, is impossible using only a reflected light beam whose polarization angle is rotated.