International Business Machines Corporation

David W Greenwell

Munro K Haynes

Easily-fabricated, high track density, size-tolerant and efficient sector servo patterns are provided for optical disks. Easily-fabricated and efficient codes are provided for use with sector tracking optical disk servo systems. Marks larger than the spot size are used to avoid resolution problems and the limits of photolithographic processes. Independence from line-width variations is achieved by balancing. The features used for servo information are in every case in the form of pits or depressions in the substrate surface, such as could be produced by exposing a photoresist through a mask, followed by etching. The servo detectors sense the changes in reflectance from the disk owing to the cancellation of light waves from the two differing depths. The optimum depth is one-quarter wavelength in the substrate, so that the contributions from the two depths will be exactly out of phase. This produces maximum signal, but it also means that no data can be expected to be written or read in the servo sectors. When the spot is far away from a depression, or is completely inside a large depression, the disk reflectance is at its normal maximum value. When the spot is exactly centered on the edge of a large depression, the reflectance is very close to zero, depending on the exact difference in depths.

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Essex Junction, VT, United States of America

David W Greenwell