OPTICAL DATA STORAGE MEDIUM

摘要

PROBLEM TO BE SOLVED: To clearly read recorded data by providing a focusing device for focusing a radiation to the selected one of plural data surfaces and a receptor for responding to a return radiation from a medium. SOLUTION: A controller seaks a desired data surface by making a motor move a lens 210 in a positive displacement direction and a peak detector detects the two peaks of focus error signals in respective data layers. The controller counts the peaks and decides an accurate data surface where a light beam 202 is to be focused. At the time of reaching the desired data surface, the controller makes the motor position the lens 210 so as to make the focus error signal be between the two peaks relating to the surface. Then, the motor is controlled by using the focus error signal and a zero point focus error signal is searched between the two peaks. That is, a positive gradient signal is tracked so as to obtain accurate focusing.