| 摘要 |
PROBLEM TO BE SOLVED: To detect deviation in substrate thickness and also to obtain an excellent ratio of signal to noise of RF signals even for a disk to which the RF signals are not preliminarily recorded. SOLUTION: Exist light from a semiconductor laser 1 is divided into three rays of light of zero-order light being a main beam and ±1st-order diffracted rays being sub beams by a diffraction optical element 3, and a focus error signal is detected from each of the main beam and the sub beams. Since distributions of light intensity in the main beam and the sub beams when making incident on the object lens 6 differ by the operation of the diffraction optical element 3, zero cross points of the focus error signals in the main beam and the sub beams are deviated when the deviation in substrate thickness exists in the disk 7. The deviation in substrate thickness of the disk 7 is detected based on the deviation of the zero cross point of the focus error signal. |
