Modeling high density microholographic data storage: Using linear, quadratic, thresholding and hard clipping material characteristics Balázs Gombköt } o a, * , Zsolt Nagy b , Pál Koppa a , Em } oke L } orincz a a Department of Atomic Physics, Budapest University of Technology and Economics, Budafoki út 8, H-1111 Budapest, Hungary b Holografika Kft. Pf. 100, H-1704 Budapest, Hungary article info Article history: Received 19 November 2007 Received in revised form 21 March 2008 Accepted 22 April 2008 Keywords: Holographic and volume memories Volume holographic gratings Inhomogeneous optical media abstract Crosstalk related raw signal-to-noise ratio (SNR) and bit error rate (BER) of high density bitwise micro- holographic data storage is investigated by numerical modeling. Scattering and diffraction of light is calculated in non-paraxial scalar approximation. A multiple thin slice implementation of the perturbative volume integral equation is used, which can be easily parallelized. The effect of bit and track spacing, and the different local characteristics of the holographic recording material on the SNR, BER and diffraction efficiency are investigated. The results show that these lateral spacing parameters have much more effect on crosstalk noise than the number of layers. Using two-photon, thresholding or hard clipping materials generates less crosstalk noise at the same data density than a linear material, and the dynamic range of these materials can be used more effectively resulting in higher single microhologram diffraction efficiencies. Ó 2008 Elsevier B.V. All rights reserved. 1. 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 reflec- tive microholographic volume gratings instead of pits has a benefit from the 3D shift selectivity of holographic readout, i.e. neighbor- ing bits/holograms are read out not only partially but phase mis- matched 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 im- age of the addressed bit, where the backscattered/reconstructed signal beam is being focused. Due to similar data encoding and bit- wise storage, the required opto-mechanical system is highly com- patible 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 [2–7] to fulfill the requirements of present and future holographic data storage systems. Experimental results on the method also ex- ist in the literature [8–10]. In this paper, we present a numerical model of microholograph- ic 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 ra- tio (SNR) and bit error rate (BER) values are obtained from energy histograms produced by the model. Several parameter configura- tions 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 in- ter-layer crosstalk, material dynamic range and single bit diffrac- tion efficiency is also discussed. 2. 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 fo- cused readout beam scans the middle data layer in the exact loca- tions of the bits by shifting the layer in the lateral directions. The summed intensity of the backscattered light after the confocal fil- ter is registered, and the collected data set is compared to the set of bits written in order to build an energy histogram of the ON and OFF bits. The histogram is further analyzed for deriving BER and SNR values. The simulation consists of three parts: the first part generates the focused writing and reading beams using a wave-optical lens 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.04.057 * Corresponding author. Tel.: +36 1463 1457; fax: +36 1463 4180. E-mail address: gombkoto@eik.bme.hu (B. Gombköt} o). Optics Communications 281 (2008) 4261–4267 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom