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. We used scalar diffracton model to study its performance. We studied beam propagation in the optical system, the focus error signal, the detector alignment tolerance, the tracking error signal and crosstalk. The effect of aberration on the above parameters is modeled in detailed. The differential wax-wane technique has several advantages over a single wax-wane focus servo technique. The gain is two times higher, the lock-on-range is better defied, and the linearity is ten times better in terms of RMS deviation. It is also insensitive to track rotation and disk tilt.