Aplanatic condition required to reproduce jitter-free signals in an optical digital disk system

Abstract

We present the optical condition required for precise reproduction of electric signals with digital data coded on an optical disk as a combination of the length of pits and the interval between them. We approximate the modulation of photocurrent by the interference between the zeroth- and first-order beams diffracted by the pits, and express the mean-squared waveform distortion of the readout signal as a function of the squared sum of the phase transfer function and the modulation transfer function. We established both by measurement and by prediction using waveform distortion (both measurement and theory agreeing well) that jitter is highest in the presence of tangential coma, next highest in the presence of spherical aberration. We conclude, therefore, that an optical digital disk system must be aplanatic for jitter-free reproduction of digital data.