Focussing system and methods for multi-track optical disk apparatus

A method of correcting the focus in apparatus for simultaneously reading multiple tracks of an optical disk, the apparatus including a light source providing a wide-area field of illumination that covers multiple tracks, an image detector, an objective, and a servo-system that moves the objective, the method comprising steps of:

• providing a cylindrical lens;
• providing a plurality of focus detectors generating outputs;
• positioning the plurality of focus detectors spaced equidistant apart around a periphery of the image detector, each one of the plurality of focus detectors located at a position on an edge of a light beam reflected from the optical disk corresponding to the wide-area field of illumination;
• positioning the cylindrical lens between the objective and the plurality of focus detectors, so that an out-of-focus condition causes distortion of the light beam reflected from the optical disk;
• and
• computing an error signal from the outputs of the plurality of focus detectors to drive the servo-system responsive to the distortion of the light beam reflected from the optical disk.

In apparatus for simultaneously reading multiple tracks of an optical disk, the apparatus including a light source providing a wide-area field of illumination that covers multiple tracks, an image detector, an objective, and a servo-system that moves the objective, the improvement comprising a focussing system including:

• a plurality of focus detectors spaced equidistant apart around a periphery of the image detector, each one of the plurality of focus detectors located at a position on an edge of a light beam reflected from the optical disk corresponding to the wide-area field of illumination, the focus detectors generating outputs;
• a cylindrical lens disposed adjacent to the light source, so that an out-of-focus condition causes distortion of the light beam reflected from the optical disk;
• and
• means for computing an error signal from the plurality of focus detectors to drive the servo-system responsive to the distortion of the light beam reflected from the optical disk.

In apparatus for simultaneously reading multiple tracks of an optical disk, the apparatus including a light source providing a wide-area field of illumination that covers multiple tracks, an image detector, an objective, and a servo-system that moves the objective, the improvement comprising a focussing system including:

• a plurality of focus detectors spaced equidistant apart around a periphery of the image detector, each one of the plurality of focus detectors located at a position on an edge of a light beam reflected from the optical disk corresponding to the wide-area field of illumination, the focus detectors generating outputs;
• a cylindrical lens interposed between the objective and the plurality of focus detectors, so that an out-of-focus condition causes distortion of the light beam reflected from the optical disk;
• and
• means for computing an error signal from the plurality of focus detectors to drive the servo-system responsive to the distortion of the light beam reflected from the optical disk.