Nyquist Aperture Research Articles

High-speed focus servo control system using Nyquist aperture mounted on a triaxial actuator for holographic disc drive system

We have developed a holographic data storage system using angular-multiplexing recording that has a large capacity of 2 TB and a data transfer rate of 1 Gbit / s. To read a hologram recorded on a holographic disc at high speed, it is necessary to control the relative position shift between a hologram recorded on a disc and a readout optical system. However, in angular-multiplexing data storage systems, the optical system has no moving mechanism, unlike conventional optical discs such as Blu-ray Discs™. Therefore, it is difficult to control the disc position with high precision and to reduce the settling time of the positioning because the holographic disc has no physical structure for the position control mechanism to control the relative position shift. In the systems, a disc mounted on a disc stage is moved to a position where the signal-to-noise ratio of the reproduced hologram becomes the maximum value. Hence, we present a servo control system based on a shift of the Nyquist aperture using a real-time position error signal to control the disc position. In experiments, a positioning accuracy of ±15 μm and a settling time of 8.5 ms for our angular-multiplexing data storage system using a nonlinear parabolic focus position error signal have been observed.

Technique for positioning hologram for balancing large data capacity with fast readout

The technical difficulty of balancing large data capacity with a high data transfer rate in holographic data storage systems (HDSSs) is significantly high because of tight tolerances for physical perturbation. From a system margin perspective in terabyte-class HDSSs, the positioning error of a holographic disc should be within about 10 µm to ensure high readout quality. Furthermore, fine control of the positioning should be accomplished within a time frame of about 10 ms for a high data transfer rate of the Gbps class, while a conventional method based on servo control of spindle or sled motors can rarely satisfy the requirement. In this study, a new compensation method for the effect of positioning error, which precisely controls the positioning of a Nyquist aperture instead of a holographic disc, has been developed. The method relaxes the markedly low positional tolerance of a holographic disc. Moreover, owing to the markedly light weight of the aperture, positioning control within the required time frame becomes feasible.

Linear phase encoding for holographic data storage with a single phase-only spatial light modulator

A linear phase encoding is presented for realizing a compact and simple holographic data storage system with a single spatial light modulator (SLM). This encoding method makes it possible to modulate a complex amplitude distribution with a single phase-only SLM in a holographic storage system. In addition, an undesired light due to the imperfection of an SLM can be removed by spatial frequency filtering with a Nyquist aperture. The linear phase encoding is introduced to coaxial holographic data storage. The generation of a signal beam using linear phase encoding is experimentally verified in an interferometer. In a coaxial holographic data storage system, single data recording, shift selectivity, and shift multiplexed recording are experimentally demonstrated.

An image filter based on primary frequency analysis to improve the bit error rate in holographic data storage systems

The primary frequency analysis method is proposed to design an image filter for applications in holographic data storage systems that use a Nyquist aperture. We designed the filter by analyzing the frequency of the data patterns, requiring neither the point spread function of the system nor iteration. The component frequencies that constitute the data pattern were calculated, and the modulation transfer function (MTF) determines that the primary frequency components was boosted. This filter boosts the primary frequency region where the modulation transfer function (MTF) <0.5 to reduce the bit error rate (BER). The performance of the designed image filter was verified by both simulation and experimentation. The proposed system exhibited an improvement in BER from 0.0215 to 0.0070 for a Nyquist factor of 1.1 and from 0.0061 to 0.0017 for a Nyquist factor of 1.2. These results are expected to contribute to the design of holographic data image filters.

홀로그래픽 정보 저장 장치의 데이터 페이지 주파수 특성 및 광학 시스템 표현

The selective frequency analysis method is suggested in the holographic data storage system with the nyquist aperture to reduce the size of hologram. The image filter was designed with many different methods to improve the bit error rate caused by the nyquist aperture. In previous the methods of image restoration for HDS, an iteration time and a highly precise point spread function were necessary. In this paper, we describe the optical system with analytic method. Thereby, we expect our result help the researchers to design the filter.

Coaxial Holographic Memory with Designed Reference Pattern on the Basis of Nyquist Aperture for High Density Recording

We propose the designed reference pattern on the basis of the Nyquist aperture for a coaxial holographic memory and investigate its recording performance by numerical simulations. By using the designed reference pattern, the Fourier power spectrum of a reference beam spreads uniformly within the Nyquist aperture, thereby an interference efficiency, a signal-to-noise ratio (SNR) and a symbol error rate (SER) are improved relative to conventional method with a random binary phase mask. Moreover, in the case of applying the 1.25 times aperture than the Nyquist size, the proposed method can record data pages with higher SNR and lower SER than that of conventional method with 2 times aperture than the Nyquist size. Numerical results imply that our proposed method can record data pages at a smaller area of recording media than that of conventional method.

Increasing the storage density of a page-based holographic data storage system by image upscaling using the PSF of the Nyquist aperture

We introduce an image upscaling method that reduces bit errors caused by Nyquist apertures. Nyquist apertures used for higher storage densities generate optical aberrations and degrade the quality of the image that is recorded on the medium. Here, to correct the bit errors caused by the Nyquist aperture, an image upscaling method is used to restore the degraded image in the enhanced spatial frequency domain using its point spread function (PSF) as a restoration filter. The proposed method reduces the bit error rate (BER) significantly and hence allows higher storage densities.

Optimisation of data density in Fourier holographic system using spatial filtering and sparse modulation coding

Summary A computer model is presented that can be used to determine the image formed by a 8-f Fourier imaging system. With the model we optimised data density in a thin film Fourier holographic arrangement. Results of the simulations were experimentally verified in the holographic memory card system. We optimised the capacity in function of sparsity and Fourier filtering close to the Nyquist aperture. We achieved a data density of 1.1-bit/μm2. Our results suggest that the sparse pages combined with spatially filtered Fourier spectrum lead to better performance compared to balanced codes.

Channel modeling and estimation for intrapage equalization in pixel-matched volume holographic data storage

We present two different channel models (the magnitude model and the intensity model) for a pixel-matched volume holographic data storage system that employs the 4-focal-length architecture. First, a framework to describe the channel models is developed. We evaluate the linearity of the channel models by comparing data values obtained from diffraction-limited interference with data values predicted by the channel models. The models are evaluated for linearity and equalization gain under different storage and read-back conditions, such as fill factors, apertures, and contrast ratios. Bit error rate results obtained by use of linear equalization methods in conjunction with the channel models developed are also presented. Our results suggest that the magnitude model leads to better performance when the fill factors are small, whereas the intensity model appears to be more appropriate for the high-fill-factor cases. The magnitude model, when suitable, appears to provide a storage density improvement of as great as 65%, whereas the intensity model seems capable of providing as much as 15% density gain through deconvolution. The optimum aperture for storage seems to be close to the Nyquist aperture.