A Complete Study Of Differential Wax-wane Focus Servo Technique

We concentrate on differential wax-wane focus servo technique that is insensitive to many of the errors found in other schemes. Servo optics are illustrated in Figure 1. The beam reflected from optical disk is focused by the detector lens onto two detectors, detector 1 is slightly inside focus, and detector 2 is slightly beyond focus. The quad detector is offset from the center of the beam to give the FES algebra, where a is an electronic gain factor that can be any number larger than one.