Methods and architecture for rewritable holographic memories
The focus of this thesis relates to issues of concern for rewritable holographic memories, primarily the volatility of recordings made in photorefractive crystals, which are the most likely class of materials to be used for such applications. Holograms written in such crystals tend to gradually deca...
| Summary: | The focus of this thesis relates to issues of concern for rewritable holographic memories, primarily the volatility of recordings made in photorefractive crystals, which are the most likely class of materials to be used for such applications. Holograms written in such crystals tend to gradually decay under illumination, leading to the loss of information. We examine two candidate approaches for dealing with this problem of volatility: the dual wavelength method and the periodic copying technique. We also investigate various potential system architectures for dynamic holographic memories.
In Chapter 2 we analyze the dual-wavelength method, which makes use of different wavelengths for recording and readout to reduce the grating decay while retrieving data. Bragg-mismatch problems from the use of two wavelengths are minimized through recording in the image plane and using thin crystals. We combine peristrophic multiplexing with angle multiplexing to counter the poorer angular selectivity of thin crystals. We successfully store 1000 holograms in this manner and demonstrate a significantly reduced decay rate. However, we find that dark conductivity reduces the effectiveness of this method for nonvolatile readout, and constraints placed on the usable pixel sizes limit this method to moderate storage densities.
In Chapter 3 we examine the periodic copying technique, in which a stored set of holograms is intermittently refreshed to prevent the loss of any information. We show the necessity of using a fixed-time recording schedule with such systems and derive optimum exposure times for maximizing storage capacity. Our analysis includes both purely refreshed memories and memories with active erasure and rewrite capabilities.
From our research in the preceding two chapters, we find periodic copying to be the more complementary of the two approaches, and in Chapter 4 we proceed to study possible memory system architectures that could incorporate the copying technique. We seek to do this while decreasing the system size and increasing the access speed over that of typical holographic systems that have been demonstrated thus far. We find that we can design a compact lensless memory by using the phase-conjugate readout method in combination with a smart-pixel array that combines the functions of a spatial,light modulator and detector array. Rapid random access speeds can be achieved by using laser arrays such as VCSELs (vertical-cavity surface-emitting lasers). We calculate the optimum storage density of this model to be 160bits/[square cm] of system volume.
In Chapter 5 we present experimental results from combining the periodic copying technique with conjugate-readout architectures and demonstrate the operation of a prototype version of the smart-pixel array described earlier. We show that the conjugate readout method yields reconstructed image fidelity at least as good as can be obtained by high-quality imaging systems. We also show the successful refreshment of 25 holograms for 100 cycles with no errors and no appreciable deterioration in image quality. Comparisons with the predictions from Chapter 3 indicate consistency between theory and experiment.
Finally in Chapter 6 we summarize our results and discuss future work to continue this research.
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