PRE-GROOVED OPTICAL DISC FOR HIGH DENSITY INFORMATION STORAGE
Original Publication Date: 1985-Jun-30
Included in the Prior Art Database: 2004-Apr-04
Xerox Disclosure Journal
Optical disc high density information storage systems are well known. The inforrnation is read by a focuSsed laser spot which is carefully servoed to maintain focus and tracking. The tracking, due to the extremely small spot size and shallow depth of focus, is difficult. One well-known method proposed is the Phillips Tri-Beam System. In this system, deviation of the light spot from the center of the track is measured by splitting the laser beam into three parts with a diffraction grating, thereby obtaining one zero order beam and two first order beams. These three beams impinge upon the information track simultaneously and continuously. The light reflected from the two first order (outer) beam spots is collected separately by their respective photodiode detectors. The difference in the output of the two outer detectors is a measure of the deviation of the central light spot from the center of the track.