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Fine Focusing Onto an Optical Disk

IP.com Disclosure Number: IPCOM000039602D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Arter, NK: AUTHOR [+3]

Abstract

When focusing the laser of an optical head onto a rotating, grooved optical disk, the reflected read signal from the disk is sampled, and the lens is then moved slightly to refocus. When the head is again at the same circumferential track position (preferably, an unused gap area), the signal is again sampled. The two samples are then compared, and this comparison is used to generate a signal which achieves proper focus. Once approximate focus has been achieved and laser light 10 is tracking either a groove or a land on the surface of disk 11, more precise focusing is accomplished. The intensity of return beam signal 12 indicates the amount of light being reflected from the disk.

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Fine Focusing Onto an Optical Disk

When focusing the laser of an optical head onto a rotating, grooved optical disk, the reflected read signal from the disk is sampled, and the lens is then moved slightly to refocus. When the head is again at the same circumferential track position (preferably, an unused gap area), the signal is again sampled. The two samples are then compared, and this comparison is used to generate a signal which achieves proper focus. Once approximate focus has been achieved and laser light 10 is tracking either a groove or a land on the surface of disk 11, more precise focusing is accomplished. The intensity of return beam signal 12 indicates the amount of light being reflected from the disk. In tracking on the disk's lands, as lens 13 comes into better focus, return signal 14 increases in magnitude, and, as it goes out of focus, the signal decreases in magnitude --- see curve 14. The opposite occurs when tracking on the grooved portion of the disk -- see curve 15. The best point of focus is when the laser light does not fall on the adjacent lands or grooves, but rather is concentrated in the region being tracked. A sample of the amplitude of signal 12 is taken and stored in network
16. An offset voltage 17 is then placed in focus servo 18. When the disk completes one 360o revolution, and is at the same position where the sample was taken, signal 12 is resampled. Comparator 19 compares these two samples. If the comparison indicates an increas...