| 摘要 |
PURPOSE:To make an optical head small-sized and inexpensive by providing a spot shape converting means for verying the shape of a spot formed on an information storage medium, on an optical path of an optical beam reaching the information storage medium from a light source. CONSTITUTION:When recording and reproducing are executed, a slit 27 for converting a spot shape is not interposed in an optical path 28. When executing erasion, a pair of slit plates 27b, 27b having a deep groove 27a move along the X direction from both sides of the optical path 28, and the slit 27 is interposed in the optical parh 28. As a result, immediately before an objective lens 29, a spot shape 40 on a far field surface to a recording layer 31 becomes long in the X direction and short in the Y direction, and the spot shape on the recording layer 31 becomes an elliptical shape having a long axis in the Y direction and a short axis in the X direction. In accordance with this variation, a spot shape 38 is also varied, but as for said spot shape 40, the size is varied only in the Y direc tion, therefore, the size veriation of the spot does not occur in the X' direction of a photodetector 33, and accordingly, when executing erasion, too, a track shift can be detected stably. |
