Abstract

There are several techniques that can be used for focus-error detection in an optical data storage device. Astigmatic, knife-edge, critical-angle prism, pupil obscuration, and spot-size detection are common techniques (1)(2)(3). These methods sense the focus error by manipulating reflected light from the disk and creating an electrical focus-error signal (FES) with sectioned detectors. If a continuously pregrooved disk is used, the reflected light also contains diffracted orders that are used to provide a tracking-error signal (TES). It is difficult to completely separate the focuserror information from the tracking-error information, regardless of the focus-error detection method. The residual amount of TES observed in the FES is called cross talk. Prikryl (4) has modeled the sensitivity of several focus-error detection methods to sources of cross talk. Stahl (5) has modeled the sensitivity of astigmatic focus-error detection. The works of Prikryl and Stahl indicate that spot-size focus-error detection is probably a good method for eliminating cross talk. In this paper, we discuss the characteristics of a differential spot-size measurement technique, which has better cross-talk rejection than the single-detector spot-size measurement technique. Similar differential techniques have been presented in the literature (6), but they have not been analyzed with respect to cross talk.

© 1991 Optical Society of America