Apparatus and method for focus control
Control apparatus and method are disclosed for shifting focus on a multilayered optical medium using closed loop servo operation. The outputs of an astigmatic optical pickup are independently modified by a control signal generator according to a predetermined time varying pattern to produce a deceptive error signal that changes the apparent focal offset seen by a servo circuit. While in closed loop mode, the servo tracks the shifting apparent focal plane from a first information layer of the medium toward a second information layer. Thereafter the control signals are removed, and the servo locks focus on the second information region.