New tracking servomechanism using phase-shift differential push-pull method for recordable optical disks

We have improved the phase-shift differential push-pull (PS-DPP) tracking method with a multidivision phase-shift grating, which is applicable to various optical disks with different track pitches and does not require the accurate alignment of the three-spot position on a disk. In this paper we presented the properties of this method in recordable optical disks such as phase-change or dye media in which reflectivity greatly changes after recording. In particular, we found the cause of a radial tracking error signal offset occurring at the boundary between recorded and unrecorded tracks. To resolve this issue, we proposed two solutions: one is the optimum calculation procedure for the PS-DPP signal, and another is a newly designed phase-shift grating for recordable disks. We demonstrated the offset cancellation effect of this improved PS-DPP method using a new hologram laser unit for various types of digital versatile discs (DVDs) and showed its applicability to the Blu-ray Disc (BD).