INVESTIGATIONS ON HIGH DENSITY HOLOGRAPHIC DATA STORAGE AND CONTENT- ADDRESSABLE SEARCH by BHARGAB DAS

Holographic digital data storage systems are an exciting topic of wide spread research, and have been investigated by many research groups and companies for the next-generation of disk-based optical storage systems. Storage systems based on volume holography inherently have the characteristics of high storage density, fast data transfer rate, short random access time, and content-addressable search. Rapid technological advancements made in various optoelectronic components such as spatial light modulators (SLA4), chargecoupled devices (CCD), complementary metal-oxide semiconductors (CMOS), and the development of new concepts, high quality recording materials, and data detection schemes are expected to make holographic data storage (HDS) systems as commercially viable in the near future. Once commercially realized as a portable device, HDS has the potential to outperform the conventional optical storage technologies such as DVD or BD. HDS is fundamentally different from these earlier optical storage technologies. Its remarkable attributes are achieved by recording information throughout the volume of a recording medium instead on a two-dimensional surface, and also by transferring data in parallel rather than serially. Information bearing two-dimensional data pages are stored as interference patterns within a thick photosensitive optical storage material. At the same time, the use of different multiplexing techniques allows to store a large number of such data pages in the same location of the holographic disk leading to high data densities. Additionally, HDS systems offer the potential for content-based searching in which a simultaneous search of an entire database can be performed through multiple optical correlations between the stored data pages and a search argument. Such an added feature of content-addressable searching with holographic memories is always beneficial considering the high storage densities of — 500 GBytes in a 120 mm disk. Since each storage location on the holographic disk can be searched with a single optical exposure, the holographic search engine (HSE) can potentially search massive data bases orders of magnitude faster than the conventional alternatives. Also, going by the literature on holographic correlators for pattern recognition, it can be observed that the center of attention has been shifted in the direction of developing encoding schemes to generate two-dimensional data pages out of the images to be correlated. These new encoding schemes retain the advantages of volume HDS systems enabling efficient and high speed optical correlations. In a conventional HDS system, information bearing two-dimensional data pages are stored as holograms in the Fourier transform plane or a region close to the Fourier plane which offers the optimal storage density. Also in the vast majority of the previous HDS systems, data has been encoded as a two-dimensional amplitude pattern. Fourier transform of an amplitude-modulated data pattern gives rise to a high-intensity zero-order (dc) spot. The recording materials used for holographic storage have limited dynamic range and hence never suitable for recording this dc spot of the Fourier image. This is because of the fact that during multiplexing, the exposure in this dc spot region exceeds the recording material limits and the resultant holograms are of insufficient quality. Serious efforts have been made in this direction and as a result several methods have been put forward to avoid this undesirable effect of the dc spot in the case of amplitude data pages. This thesis reports a number of contributions towards this direction. In spite of active research carried out over the last few decades, there is still enormous scope for improving the performance of HDS systems. The storage of 1TBytes in a 120 mm disk with data transfer rates near 1Gbps still remains a challenge. We report in this thesis our contributions towards increasing the memory capacity of HDS systems, and also towards performing a faithful content-addressable search with the HDS systems. Initially, we present the results of our investigations on the performance of defocused volume HDS systems in terms of bit-error-rate and content search capability. We have also developed a numerical simulation model of HDS with photopolymer recording material in order to investigate the effects of recording material saturation on the bit-error-rate, the signal-to-noise ratio, and content-addressable search. A few new methods have been designed and implemented in order to enable reliable and accurate content-addressable data search in defocused volume HDS systems. Noteworthy research work is also carried out in the direction of twodimensional data encoding employing purely phase-based data pages. We present the results of our investigations on gray-scale and sparse-gray-scale data pages. Gray-scale encoding increases the code rate of the modulation codes beyond unity, thereby enhancing the capacity of each page and also improving the transfer speed. We have proposed new methods for the implementation of gray-scale and sparse-gray-scale modulation codes with a single SLM working in phase mode. This phase mode representation of gray-level and sparse-gray-level data pages ensures a `homogenized Fourier spectrum' that improves the interference efficiency between the signal and the reference beams for better recording. These different approaches of sophisticated phase utilization have the potential to achieve a significant gain in storage density.