Differential wax-wane focus servo technique applied to optical recording systems.

Abstract

This dissertation investigates a new focus error detection technique, i.e., differential wax-wane focus servo, for use in optical recording systems. A combination of scalar diffraction modeling and experiment is used to quantify its performance. A brief review of optical recording is given. The performance of an optical drive is linked directly to its servo system. Performance parameters of focus error signals are discussed. A few popular focus and tracking error detection techniques are summarized. We concentrate on the differential wax-wane focus error detection technique. In order to accurately predict its performance, a scalar diffraction model is employed. Diffraction modeling results, such as detector alignment tolerance and effect of aberrations, are presented. Comparisons to the astigmatic and pupil obscuration focus error detection technique are also given. The gain of the differential wax-wane technique is two times that of single wax-wane. The lock-on range is about three times that of most other techniques in the configuration we studied. Without any aberration, it has 0.04 μm of tracking crosstalk amplitude compared to about 0.25 μm in other techniques. Even if aberrations are present, the crosstalk can be eliminated by adjusting the detector and changing the electronic gain of the individual channels. The overall performance of the differential wax-wane technique is better than the other techniques. The design and alignment of an optical head are summarized. The optical head is used to test the modeling results. One factor that affects optical drives significantly is aberration. A phase-shifting interferometer is used to measure the wavefront of our optical head. The measured wavefront is implemented in our diffraction model, and an excellent agreement between experimental and modeling results is achieved. We also present preliminary studies of a multiple beam optical head. A model that combines ray trace and diffraction propagation is used to analyze the focused beam on the disk. The focus-error signal of off-axis beams are discussed. The effects of disk tilt are also given. By using a two-dimensional array, the number of channels available can be increased with the same optics. Possible disk formats and a tracking scheme for multi-beam optical recording using a two-dimensional array are also discussed.