RING-LENS FOCUSING AND PUSH-PULL TRACKING SCHEME FOR OPTICAL DISK SYSTEMS

An experimental comparison of the ring-lens and the astigmatic techniq ues of generating a focus-error signal (FES) in optical disk systems r eveals that the ring lens generates a FES curve over two times steeper than that produced by the astigmat. Partly because of this large slop e and partly because of its diffraction-limited behavior, the ring-len s scheme exhibits superior performance characteristics. In particular, the undesirable signal known as feedthrough (induced on the FES by tr ack crossings during the seek operation) is substantially lower than t hat observed with the astigmatic method. The ring lens is easy to alig n and has reasonable tolerance for positioning errors.