Xerox Corporation

Ronald A Millar

Charles B Abate

A small amplitude, low frequency, signal is added to the focus servo signal controlling the position of the objective lens of an optical disc storage system after focus acquisition has been achieved, so as to produce a slight defocusing effect of the light spot on the disc. The resulting variations in the read back signal is utilized by a synchronous detection circuit scheme to extract magnitude and polarity information of the focus offset present. That information is fed back to the focus servo signal to null out the focus offset, thereby assuring continual accurate focusing of the laser beam even under conditions of changes in the mechanical alignment of the optics of the storage system and/or the d.c. stability of the electronics of the storage system.

Automatic focus offset correction system

The synchro-servo zones of the data blocks of an optical disc track are provided with information which will permit the generation of clock synchronization pulses and tracking pulses, with those latter pulses being used with peak detector/sample and hold circuitry of novel design to generate a substantially noise free tracking information servo feedback signal. Specifically, a pair of pulses having respective amplitudes proportional to the displacement of the read/write beam from the center of the track are generated, with the peak amplitudes of those pulses being stored, and then compared to provide, after carrier filtering, a tracking information servo feedback signal which would have an amplitude proportional to the displacement of the read/write beam from the center of the track and a sign or sense indicative of the direction of the displacement. The feedback signal, having the proper corrective sign, could be applied to the tracking adjustment coils of a dual axis optical head or mirror for moving the objective lens or mirror of the optical head so that the read/write beam moves toward the center of the track. A tracking accuracy of less than 0-1 .mu.m has been achieved using this method.

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Sunnyvale, CA, United States of America

Ronald A Millar