| 摘要 |
The spot diameter D3 of a side beam 21 imaged on a disc 5 is set to at least 2.5 times as large as that of a main beam. Via this configuration, an output signal from a detector detecting a reflected light via a side beam, that is, a signal representing a different in the output from photo-detection devices split by split lines in the radial contains a negligible track cross component caused by a beam spot crossing a track. The signal substantially contains a DC offset component alone caused by the shift of an objective lens in the radial direction under tracking control. Thus, by subtracting the output signal of the detector from the push-pull signal of the main beam, a tracking signal substantially containing no DC offset.
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