| 摘要 |
PURPOSE: To improve an S/N of a tracking error signal by gradually changing a grating interval of a diffraction grating for forming three beams and magnifying the size of side spots in the tangential direction to an optical disk. CONSTITUTION: When a laser beam 2 passes through the diffraction grating 1, a main spot 5 consisting of 0th-order diffraction light and two side spots 6a and 6b consisting of±1st-order diffraction light are generated. A pit train 3 is irradiated by the main spot 5, while right and left sides of the pit train 3 are symmetrically irradiated respectively by the side spots 6a and 6b. A divided photodetector for receiving reflected light is provided with one pair of tracking photodetection parts which detect differentially the side spots 6a and 6b respectively, so as to output the tracking error signal. As to the diffraction grating 1, the grating intervals are formed gradually narrower in the tangential direction A to the optical disk. The side spots 6a and 6b become slender, and hence a high frequency component in the tracking error signal caused by a pit signal is reduced.
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