Focus Sensing Method Using Diffracted Waves to Position Spatially Phase-Modulated Beams for Collinear Holographic Data Discs

A focus sensing method using diffracted waves, which are produced by a one-dimensional grating embedded inside a hologram disc, to position spatially phase-modulated beams is proposed. Simulation has been performed to show the effectiveness of the proposed method and has given an offset-free and well-shaped focus error signal. This focus error signal can keep the distance between the focal position of the recorded hologram and the servo layer of the collinear holographic data disc and provides higher disc interchangeability and portability.