SYSTEM AND METHOD FOR COMPENSATING FOCUS OF DISTANT VIEW SHIFT OF BEAM

PURPOSE: To strictly attain keeping a focal point of a laser beam by positioning an objective lens at a low power operation of a laser source by a digital signal processor, and focusing the laser beam on a disk surface. CONSTITUTION: A signal processor 34 samples a currently normalized focus error signal directly before a transition to a high power operation and stores it in a memory. And, in order to correct a focus displacement presented directly after a laser source 13 makes a transition from a low power output level to a high power output level, a signal processor 34 samples the last normalized focus error before a lens 20 moves, and stores it in a memory. Further, two normalized focus errors are combined, and an offset correction signal is generated for adjusting the focus error signal when an erasing operation is balanced. Thus, the laser beam can optimally be focused on a disk surface during erasing operation.