| 摘要 |
<p>According to the invention, an astigmatism is given to a light beam (1) which is focused onto a recording medium (202) and the reflected lights modulated by two kinds of pits (203, 204) formed on the recording medium surface are detected by a photo-detector (205). By focusing the light beam (1) having the astigmatism by a focusing lens (2), two focal lines (3, 4) which cross perpendicularly each other are formed at remote positions. A circle of least confusion (5) is formed at an almost center between the focal lines (3, 4). Two kinds of pits or marks) (203, 204) are previously formed on the recording medium surface in order to apply the same modulation to the light beam (1) when the recording medium surface is located at a circle of least confusion position (5) and to apply different modulations to the light beam (1) when the recording medium surface is deviated from the circle of least confusion position(5). The reflected light which was reflected by the recording medium surface and passed through the focusing lens (2) is detected by the photo-detector. The signal modulated by at least one pit or mark of the two kinds of pits or marks is extracted from the outputs of the photo-detector (205). A focus error detection signal in which a point when the recording medium surface is located at the circle of least confusion position (5) between both of the focal lines (3, 4) is set to an in-focus (or zero point) is obtained from the phase or level of the extracted signal. According to the invention, since the focus error detection signal is obtained by using the total output of the photo-detector (205), it is sufficient to arrange the photo-detector into the light beam of the reflected light which passed through the focusing lens (2) so as to detect the total reflection light. The focus error detection signal which is hardly influenced by the deviation of the optical axis of the reflected light on the photo-detector surface and by the change in light intensity distribution can be derived. Thus, there is no need to match the optical axes of the center of the photo-detector (205) and the reflected light and the apparatus can also be easily assembled. Moreover, the focusing objective point corresponds to the circle of least confusion position (5) at which the cross section of the light beam (1) between two focal lines (3, 4) becomes a circle and is the zero point position of the focus error detection signal. The focusing objective point can be autonomously adjusted.</p> |
