Optical Disk Servo AGC Using Laser Power Control
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-01
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.