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. In 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 and layer distance, respectively. When rays with spherical aberration converge, the focus of the far-axis ray deviates from that of the near-axis ray, and gradually moves towards the optical axis. Based on this phenomenon, we demonstrated that spherical aberration can be detected by subtracting the focus-error signal of the far-axis ray from that of the near-axis ray. The resulting spherical aberration signal can be used in a phase-control device, such as an expander lens actuator or a liquid-crystal phase shifter, to produce a dynamic servo controller that can compensate for spherical aberration.