Browse Prior Art Database

Fine Focusing Onto an Optical Disk

IP.com Disclosure Number: IPCOM000039600D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

Arter, NK: AUTHOR [+3]

Abstract

When focusing a laser of an optical head onto a rotating grooved optical disk, the focusing lens is dithered up and down at a frequency much higher than that of the disk's rotation. The signal used to achieve this lens dither is compared to the dither component of the head's read signal. The phase relationship between these two signals is used to achieve proper focus. Fig. 1 shows the device. Fig. 2 shows two relative positions of the lens and the groove/land area of the disk, and the dithered read signal which results from the two relative positions. Once approximate focus has been achieved, and laser light 10 is tracking either groove 11 or land 12 on the surface of disk 13, more precise focusing is accomplished. The intensity of return beam signal 14 indicates the amount of light being reflected from the disk.

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Fine Focusing Onto an Optical Disk

When focusing a laser of an optical head onto a rotating grooved optical disk, the focusing lens is dithered up and down at a frequency much higher than that of the disk's rotation. The signal used to achieve this lens dither is compared to the dither component of the head's read signal. The phase relationship between these two signals is used to achieve proper focus. Fig. 1 shows the device. Fig. 2 shows two relative positions of the lens and the groove/land area of the disk, and the dithered read signal which results from the two relative positions. Once approximate focus has been achieved, and laser light 10 is tracking either groove 11 or land 12 on the surface of disk 13, more precise focusing is accomplished. The intensity of return beam signal 14 indicates the amount of light being reflected from the disk. In the case of tracking on lands 12, as lens 114 comes into better focus, signal 14 increases in magnitude, and as it goes out of focus, it decreases in magnitude. The opposite occurs when tracking on the grooved portion of the disk. The best point of focus is when the laser light does not fall on the adjacent lands (or grooves) but is concentrated in the region being tracked. The problem of improper focus is solved by injecting an oscillatory or dither signal 20 into focus servo 21. The frequency of signal 20 is much higher than that of the rotation of disk 13. This oscillation causes lens 114 to move (see arrow 22), c...