Focus Sensing Method Using Far-Field Diffracted Waves and Its Application to Holographic Data Discs

The basic characteristics of a focus sensing method using far-field diffraction were studied by simulation and experiment. A proposal is given for sampling four points from the periodical structure, discretely separated from each other by a 1/4 pitch and imprinted on the reference plane in the optical disc. Simulation results gave an independent well-shaped focus error signals on the photo detector location, within the interference of 0th and ±1st order. The following experiment presented bipolar signals around the focus from the photo detector without micro adjustment. Also a collinear holographic data disc system, to be implemented with this method, is proposed for positioning the signal and reference beams separately with focal accuracy and providing higher disc interchangeability.