Polarization holography and its applications to data storage
Many techniques on holographic mass storage systems have been developed, in which scalar optical data are stored in holographic materials. We discuss two alternative approaches to holographic memory; the use of vector waves for holographic mass storage systems and complex data formats, multi-level phase coding, are presented to increase memory capacity. A novel optical recording technique called "retardagraphy" for recording a retardance pattern of an optical anisotropic object is mentioned, first. In this technique, a polarization pattern of a single beam modulated by an anisotropic object is illuminated onto a polarization-sensitive medium and the retardance pattern is recorded as an optical anisotropy pattern on the medium. Retardance is generated by relative phase difference between two polarization states. Multiple phase difference coding in off-axis holography is also a promising candidate for mass data storage. Some experimental studies are presented.