Land/Groove Signal and Differential Push-Pull Signal Detection for Optical Disks by an Improved 3-Beam Method

In land/groove recording, it is necessary to detect both a land/groove signal and a differential push-pull signal for stabilization of the tracking servo. The former discriminates between land and groove during track pull-in operation, while the latter compensates for offset of the tracking error signal due to shift of the objective lens. This paper proposes an improved 3-beam method for detecting both signals with the same beam configuration. A 4-segment diffraction grating is employed, in which the grating patterns for two diagonal regions are π/2 out of phase. A land/groove signal is obtained from sub-beam signals, while a differential push-pull signal is obtained from the difference between the main beam and sub-beam signals. Features of this method are the applicability to disks with different track pitches and the robustness against disk eccentricity. A land/groove signal was observed, and the compensation of the tracking error signal offset with shift of the objective lens was confirmed experimentally. These results have demonstrated the validity of this method.