Modeling high density microholographic data storage: Using linear, quadratic, thresholding and hard clipping material characteristics
Introduction
Microholographic data storage [1] is one of the best candidates for future high capacity optical memories. DVD based bitwise data storage systems are able to have up to four 2D data layers, and eight layer systems exist in laboratory environment offering 200 GByte user capacity per disk. Further growth of the number of layers is strongly limited by inter-layer crosstalk, besides, beam filtering and disk manufacturing is also a critical issue. Using reflective microholographic volume gratings instead of pits has a benefit from the 3D shift selectivity of holographic readout, i.e. neighboring bits/holograms are read out not only partially but phase mismatched as well, and therefore their contribution to the detector signal is weaker. Using a so called confocal filter further improves the suppression of crosstalk. The filter is placed to the optical image of the addressed bit, where the backscattered/reconstructed signal beam is being focused. Due to similar data encoding and bitwise storage, the required opto-mechanical system is highly compatible with existing DVD technology. The microholograms are generated by the interference of two counter propagating focused beams in an appropriate recording material. Being a critical issue, such holographic materials are intensely researched worldwide [2], [3], [4], [5], [6], [7] to fulfill the requirements of present and future holographic data storage systems. Experimental results on the method also exist in the literature [8], [9], [10].
In this paper, we present a numerical model of microholographic data storage, which is able to simulate the crosstalk of more than 20 data layers on a single desktop computer. The method can be adapted to parallel computation effectively and only 2D arrays have to be stored in the computer memory. Raw signal-to-noise ratio (SNR) and bit error rate (BER) values are obtained from energy histograms produced by the model. Several parameter configurations were tested regarding bit spacing, track spacing, layer spacing distances, and material characteristics. The results show good agreement with our previous modeling tool [11]. The relation of inter-layer crosstalk, material dynamic range and single bit diffraction efficiency is also discussed.
Section snippets
Optical modeling
The 3D model of the optical setup can be seen in Fig. 1. The counter propagating focused object and reference beams expose the microholographic gratings one-by-one inside the recording layer, when an ON bit is to be stored. During the readout the focused readout beam scans the middle data layer in the exact locations of the bits by shifting the layer in the lateral directions. The summed intensity of the backscattered light after the confocal filter is registered, and the collected data set is
Results with linear material characteristics
Using the above described scalar wave-optical model of microholographic data storage we have performed simulations with the following parameters. The wavelength was 405 nm, the numerical aperture of the system and the beams was 0.6, and the sampling distances were 200 nm by 200 nm by 50 nm along the x, y and z coordinates, respectively. With these parameters the confocal filter has a radius of 1.2 × rAiry = 1.2 × 411.75 nm = 494.1 nm. The number of data layers varied from 7 to 23. More layers (and
Non-linear recording materials
The typical candidates of present and future holographic data storage systems are photopolymer materials. Generally, both polymerization and monomer diffusion has to be modeled together with exposure schemes, but in a large scale system model simple and fast running methods are needed. The net result of the two effects in a photopolymer is some degree of saturation of the material when exposed to light. In our microholographic system the holograms are reflective along the optical axis, so the
Summary and conclusion
The presented results show that the multilayer thin slice implementation of the perturbative electromagnetic volume integral equation in first-order approximation combined with other scalar diffraction methods is an applicable tool for modeling high density microholographic data storage. The simulation can be adopted to parallel computing easily. The exposure of the 3D set of microholographic gratings and the scattering of the reading beam on the resulting complex grating together with confocal
Acknowledgements
This research has been supported by the European Union under the project Microholographic Data Disk for Archival Storage (MICROHOLAS) IST 511437, and the Hungarian National Office for Research and Technology under project HEF06-2-BMEHOLO1.
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