Position sensing in video disk readout

In video disk systems based on contactless optical readout, the position of the readout light spot is dynamically controlled with the aid of servo systems. The input of the radial or vertical servo systems in the readout unit is fed with a bipolar function of the position error that has to be corrected. This paper describes a method that generates a radial and a vertical error signal by making use of the phase relations between diffracted orders in the far field of the video disk. These phase relations are derived from the rf signals that appear at the different outputs of a composite photodetector. It is shown that in this way auxiliary beams or supplementary optical elements can be avoided in the readout system.