Ring-lens focusing and push-pull tracking scheme for optical disk systems.

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