Characterization of a Focus Sensor Using Far-Field Diffracted Waves for Holographic Data Discs

The characteristics of a fringe-scanning-type focus sensor, which uses the interference of far-field diffracted waves, were studied by experiments and simulation. The focus sensor's capture range by experimentally obtained corresponded well to the simulated value and the offset was successfully maintained around zero against the photodetector's position shift (±150 µm in the radial direction and ±500 µm in the tangential direction on the projected beam area of 8 mm diameter). Also we confirmed an optimal photodetector location that provides the best focus under the existence of spherical aberration.