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Optical Disk Servo AGC Using Laser Power Control

IP.com Disclosure Number: IPCOM000038790D
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 49K

Publishing Venue

IBM

Related People

Arter, NK: AUTHOR [+2]

Abstract

In an optical disk, Automatic Gain Control (AGC) for the tracking and focus servos is achieved by controlling the laser power to maintain the sum of the tracking and focus detector outputs at a constant level. In an optical disk drive, there are two servos - one for focusing and one for tracking. The following applies to both tracking and focus, but only tracking will be used as an example. The tracking servo derives its error signal from the difference in light incident upon two halves, A and B, of a photodetector. For the servo to operate properly, that is, be stable and have the desired gain and phase margin, it must have the correct open-loop gain. One of the terms in the open-loop gain is the difference in light incident upon the two halves of the detector versus the amount of off-track error signals.

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Optical Disk Servo AGC Using Laser Power Control

In an optical disk, Automatic Gain Control (AGC) for the tracking and focus servos is achieved by controlling the laser power to maintain the sum of the tracking and focus detector outputs at a constant level. In an optical disk drive, there are two servos - one for focusing and one for tracking. The following applies to both tracking and focus, but only tracking will be used as an example. The tracking servo derives its error signal from the difference in light incident upon two halves, A and B, of a photodetector. For the servo to operate properly, that is, be stable and have the desired gain and phase margin, it must have the correct open-loop gain. One of the terms in the open-loop gain is the difference in light incident upon the two halves of the detector versus the amount of off-track error signals. However, the difference in the light incident on the two halves of the detector, for a given amount of off-track, is a function of the total light incident on the tracking detector. Thus, the open- loop gain of the servo is a function of the total amount of light on the detector. A simple, inexpensive way of accomplishing AGC is to monitor the sum of the tracking and focus detectors, and to vary the laser current to keep this sum at a preset level. A block diagram is shown in the figure. For low reflectance disks or for inefficient heads, laser power is increased so that the total light on the detector remains co...