Characterization of Magneto-Optical Media and Systems

This dissertation is concerned with the characterization of both the magneto-optic (MO) media and optical system used in MO recording. Amorphous rare earth-transition metal (RE-TM) thin films give rise to magnetic domain walls that are not smooth, but possess varying degrees of jaggedness. A figure-of-merit for domain wall jaggedness could be used to rank films with respect to their suitability for use in MO recording, since domain wall jaggedness has been shown to increase readout noise. Using a specially-constructed static tester, the measured fractal dimension of MO domain walls provides this figure of merit. The basic theory of fractal structures, two measurement techniques, and data from MO samples is presented. At the system level, accurate focusing and tracking is required to reliably and repeatedly write and read data on the media, while track position and focus are maintained. Three focusing and tracking methods are analyzed using scalar diffraction theory, including the effects of residual aberrations and misalignments on their performance. Feedthrough, the false focus error signal due to track crossing in pre-grooved media is also examined and its origin in the astigmatic method is compared with the cause of feedthrough in the obscuration method. The performance of the double astigmatic method, a novel differential method that eliminates feedthrough caused by astigmatism is analyzed. Birefringence of the polycarbonate disk substrate affects tracking and data readout. The focus offset between the position for the best tracking error signal, and that for maximum data readout is shown to be caused by birefringence-induced astigmatism. The effects reducing the track pitch, proper choice of groove depth and pre-format mark depth and their effect on the track crossing signal are investigated. An experimental static focusing/tracking testbed is described along with example data showing the dependence of the tracking error signal on track pitch, objective numerical aperture, groove depth, and polarization orientation, as well as feedthrough performance for the astigmatic focusing/push-pull tracking method.