Simulation methods for optical disk drive functions.

Abstract

Computer simulations of the optical servo functions of optical disk drives are developed and compared with experimental results. The focus control servo is investigated first, with emphasis on the astigmatic focus detection method. A paraxial ray trace, enhanced to allow tolerance studies of tilted and decentered surfaces, is used to calculate the size and orientation of an astigmatic blur on a quadrant photodetector as a function of focus error. The resulting irradiance distribution is integrated over the detector elements and processed to yield typical focus servo signals. A method for simulating generalized astigmatic focus systems, independent of a particular design, is also shown. The simulation results are used to derive normalized tolerance curves for detector misalignment and spot motion. Alignment diagnostics based on the servo signals are also presented. A wavefront aberration model is also developed and used to investigate the focus servo's performance in the presence of common aberrations. Simulations based on diffraction theory are used to investigate the radial tracking servo. Both scalar and vector diffraction theories are considered. The scalar theory is found to be adequate in most cases, while offering a large advantage in computational efficiency. A model for computing the signals detected by scanning the microscopic features of the disk is developed using the optical cross transfer function that describes the imaging characteristics of partially coherent systems. This model is used to investigate push-pull and three beam tracking. Aberrations, data patterns, detector misalignment, and pregroove profile are all examined for their effects on the servo signals. Crosstalk between the focus and tracking error detection is also briefly considered, and a possible extension of the radial tracking servo model to investigate this phenomenon is suggested.