Interleaving and error correction in volume holographic memory systems

Two-Dimensional Interference Estimator with Parallel Structure for Holographic Data Storage Channel

Recently, holographic data storage (HDS) has attracted significant attention as the next-generation storage technology because of its large capacity and high-speed transfer capabilities. However, two-dimensional (2D) inter-symbol interference (ISI) is a critical problem encountered in HDS. Therefore, in this study, we propose a 2D interference estimator to prevent the 2D ISI. In addition, we propose a parallel structure to exploit horizontal interference (HI) and vertical interference (VI) effectively. After detecting the HI and VI, the signal is combined using an average function. The simulation results indicate that the proposed model can significantly improve the performance of HDS.

Decision feedback equalizer for holographic data storage

Holographic data storage (HDS) has attracted much attention as a next-generation storage medium. Because HDS suffers from two-dimensional (2D) inter-symbol interference (ISI), the partial-response maximum-likelihood (PRML) method has been studied to reduce 2D ISI. However, the PRML method has various drawbacks. To solve the problems, we propose a modified decision feedback equalizer (DFE) for HDS. To prevent the error propagation problem, which is a typical problem in DFEs, we also propose a reliability factor for HDS. Various simulations were executed to analyze the performance of the proposed methods. The proposed methods showed fast processing speed after training, superior bit error rate performance, and consistency.

Improvement of bit error rate and page alignment in the holographic data storage system by using the structural similarity method

Although widely recognized as a promising candidate for the next generation of data storage devices, holographic data storage systems (HDSS) incur adverse effects such as noise, misalignment, and aberration. Therefore, based on the structural similarity (SSIM) concept, this work presents a more accurate locating approach than the gray level weighting method (GLWM). Three case studies demonstrate the effectiveness of the proposed approach. Case 1 focuses on achieving a high performance of a Fourier lens in HDSS, Cases 2 and 3 replace the Fourier lens with a normal lens to decrease the quality of the HDSS, and Case 3 demonstrates the feasibility of a defocus system in the worst-case scenario. Moreover, the bit error rate (BER) is evaluated in several average matrices extended from the located position. Experimental results demonstrate that the proposed SSIM method renders a more accurate centering and a lower BER, lower BER of 2 dB than those of the GLWM in Cases 1 and 2, and BER of 1.5 dB in Case 3.

Improving signal-to-noise ratio by use of a cross-shaped aperture in the holographic data storage system

A cross-shaped aperture is proposed to improve signal-to-noise ratio (SNR) in the holographic data storage system (HDSS). Both simulated and experimental results show that higher SNR can be achieved by the cross-shaped aperture than traditional square or circular apertures with the same area. A maximum gain of 20% in SNR is obtained for the optimized cross-shaped aperture. The sensitivities to pixel misalignment and magnification error are also numerically compared.

Low-complexity pixel detection for images with misalignment and interpixel interference in holographic data storage

This paper presents an efficient solution to recovering data pixels of images that have undergone optical and electrical channel impairments in holographic data storage systems. The channel impairments considered include interpixel interference, three types of misalignment, and noise. The proposed misalignment-compensation scheme, consisting of realignment and rate conversion, can effectively eliminate misalignment with more than 84% reduction in additions and 74% reduction in multiplications. In addition, several low-complexity techniques are introduced to reduce the complexity of a two-dimensional maximum a posteriori pixel detection method by up to 95% and do so with negligible degradation in detection performance.

Irregular repeat-accumulate codes for volume holographic memory systems

We investigate the application of irregular repeat-accumulate (IRA) codes in volume holographic memory (VHM) systems. We introduce methodologies to design efficient IRA codes. We show that a judiciously designed IRA code for a typical VHM can be as good as the optimized irregular low-density-parity-check codes while having the additional advantage of lower encoding complexity. Moreover, we present a method to reduce the error-floor effect of the IRA codes in the VHM systems. This method explores the structure of the noise pattern in holographic memories. Finally, we explain why IRA codes are good candidates for the VHM systems.

Low-density parity-check codes for volume holographic memory systems

We investigate the application of low-density parity-check (LDPC) codes in volume holographic memory (VHM) systems. We show that a carefully designed irregular LDPC code has a very good performance in VHM systems. We optimize high-rate LDPC codes for the nonuniform error pattern in holographic memories to reduce the bit error rate extensively. The prior knowledge of noise distribution is used for designing as well as decoding the LDPC codes. We show that these codes have a superior performance to that of Reed-Solomon (RS) codes and regular LDPC counterparts. Our simulation shows that we can increase the maximum storage capacity of holographic memories by more than 50 percent if we use irregular LDPC codes with soft-decision decoding instead of conventionally employed RS codes with hard-decision decoding. The performance of these LDPC codes is close to the information theoretic capacity.

Volume holographic data storage at an areal density of 250 gigapixels/in.(2)

One thousand volume holographic data pages, each containing 1x10(6)pixels , are stored in a common volume of LiNbO(3) :Fe by use of the 90 degrees geometry. An effective transverse aperture of 1.6 mm x 1.6mm , realized by repetition of this experiment at each of the eight surrounding locations, results in a demonstrated areal density of 394pixels/mum (2) (254 Gpixels/in. (2)) . Short-focal-length Fourier optics provide a tightly confined object beam at the crystal; the reference beam is angle multiplexed. Data pages retrieved with a 1024 x 1024 CCD camera are processed to remap bad spatial light modulator pixels and to compensate for global and local pixel misregistration and are then decoded with a strong 8-bits-from-12-pixels modulation code. The worst-case raw bit-error rate (BER) before error correction was 1.1x10(-3) , sufficient to deliver a user BER of 10(-12) at an overall code rate of 0.61 user bits per detector pixel. This result corresponds to 1.08% of the well-known theoretical volumetric density limit of 1/lambda(3) .

Information-based optical design for binary-valued imagery

Applications such as optical data storage, optical computing, and optical interconnects require optical systems that manipulate binary-valued images. Such an optical system can be viewed as a two-dimensional array of binary communication channels. This perspective is used to motivate the use of pagewise mutual information as a metric for optical system analysis and design. Fresnel propagation and coherent imaging both are analyzed in terms of mutual-information transmission. An information-based space-bandwidth product is used to analyze the trade-off between the numerical aperture and the number of image pixels in a coherent 4f system. We propose a new merit function to facilitate information-based optical system design. Information maximization and bit-error-rate minimization both are possible with the new radially weighted encircled-energy merit function. We demonstrate the use of this new merit function through a design example and show that the information throughput is increased by 8% and the bit-error rate is reduced by 36% when compared with systems designed with traditional criteria.