| 摘要 |
Light emitted from a semiconductor laser ( 1 ) is split by a diffractive optical element ( 3 a) into a main beam and sub-beams. The diffractive optical element ( 3 a) is divided into four regions by a straight line parallel to a tangential direction of a disc ( 7 ) and a straight line parallel to a radial direction of the disc ( 7 ). A phase of grating in two regions arranged diagonally and a phase of grating in the other two regions arranged diagonally are different from each other by pi. A focused spot of the main beam and focused spots of the sub-beams are located on the same track. A focusing error signal is detected by a differential astigmatism method, using the main beam and the sub-beams.
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