Spherical aberration detection in the optical pickups for high-density digital versatile discs

A new method of detecting the spherical aberration of a ray is described. I n a high-density digital-versatile disc system achieved by means of a high- numerical-aperture objective lens system or a multilayer disc system, it is important to reduce the aberrations caused by substrate thickness error an d layer distance, respectively. When rays with spherical aberration converg e, the focus of the far-axis ray deviates from that of the near-axis ray, a nd gradually moves towards the optical axis. Based on this phenomenon, we d emonstrated that spherical aberration can be detected by subtracting the fo cus-error signal of the far-axis ray from that of the near-axis ray. The re sulting spherical aberration signal can be used in a phase-control device, such as an expander lens actuator or a liquid-crystal phase shifter, to pro duce a dynamic servo controller that can compensate for spherical aberratio n.