| 摘要 |
PURPOSE:To prevent the degradation of a tracking error signal for defocusing by inserting a mask provided with a pair of symmetrical slits to the front of a detector. CONSTITUTION:A mask 9 having two slits consisting of apertures which are parallel with the center line, where the 0th-order diffracted light and the + or -1st- order diffracted light of a light beam overlap, and are symmetrical with respect to this center line is inserted in front of a two-divided detector 6. The width of each aperture of the mask 9 is made wider than 1/10 of the width of overlapping between th e 0th-order diffracted light and the + or -1st-order diffracted light and is made narrower than 1/2 of said width. By this constitution, the vicinity of the part where the position difference between the 0th-order diffracted light and the + or -1st-order diffracted light is not changed is detected to detect the tracking error signal, thus preventing the degradation of the tracking error signal for defocusing.
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