| 摘要 |
PURPOSE:To improve the tracking performance by setting the area and position of an interference light receiving face of 1st and 2nd photodetectors in a way that an interference region is not stepped out from the original interference region even it is shifted thereby decreasing the offset. CONSTITUTION:The photodetecting means detecting '0'th order diffracted light, plus 1st order diffracted light and minus 1st order diffracted light has a 1st photodetector whose photodetecting face 11 is arranged in the interference region of the '0'th order interference light and minus 1st order diffracted light and a 2nd photodetector whose photodetecting face 12 is arranged in the interference region of the '0'th order interference light and minus 1st order diffracted light, and the area and position of the photodetecting face of the 1st and 2nd photodetectors are set so that the photodetecting face is not parted from the interference region even if the interference region is shifted by the shift of the objective lens. Then the 1st photodetector detects the luminous quantity of the interference region of the '0'th order and plus 1st order diffracted light from a medium and the 2nd photodetector detects the luminous quantity of the interference region of the '0'th and minus 1st order diffracted light from a medium, and the photodetecting face of the 1st and 2nd photodetectors is not parted from the interference region even if the interference region is shifted by the shift of the objective lens. |
