Substrate Birefringence and its Effects on the Performance of Optical Disk Data Storage Systems.

This dissertation examines several aspects of optical disk data storage with particular emphasis on substrate birefringence and how it affects these data storage systems. In future generations of optical disk systems, to increase the areal density, shorter wavelengths as well as larger numerical aperture objectives will have to be used to reduce the size of the focused spot at the storage layer. With shorter wavelengths and higher numerical apertures, substrate birefringence will become a large enough problem that new methods will have to be developed to eliminate it or compensate for it. The birefringence present in optical disk substrates is due primarily to the material, polycarbonate, and to the manufacturing process, injection molding. Since the read and write beams are focused through the substrate, the wavefronts become aberrated due to this birefringence. In Chapter 3, we investigate the behavior of substrate birefringence as a function of temperature. This study reveals that the in-plane birefringence changes dramatically within the investigated range of temperatures, while the vertical birefringence in good measure remains constant. We suspect that the change in birefringence is due primarily to thermally induced stress in the substrate. Next, in Chapter 4, we investigate a proposed recording scheme for magneto-optical data storage called land/groove recording. In the scheme of land/groove recording in magneto-optical disk data storage systems, it has been shown that an optimum groove depth exists at which the cross-talk from adjacent tracks diminishes. Cross-talk cancellation, however, is very sensitive to various parameters of the system and, in particular, the presence of substrate birefringence can have devastating effects on system performance. In Chapter 5, we will examine some of the more subtle effects of the disk substrate in optical data storage systems such as feedback into the laser diode in compact disk systems. Our modeling of the compact disk system led us to discover a new technique for rapid measurement of the substrate birefringence. Finally, we have investigated a certain kind of manufactured birefringence. High spatial frequency surface -relief gratings on dielectric media can behave as homogeneous birefringent materials if the grating period is less than the wavelength of the incident light. Chapter 6 describes a technique using high refractive index thin films to fabricate submicron gratings that provide very large phase retardations. Appendix A contains detailed instructions for the reader who desires to fabricate this type of grating. (Abstract shortened by UMI.).