Shift-multiplexed self-referential holographic data storage

Volume-recorded hologram modeling, point-spread function analysis, and segmented adaptive equalization for holographic data storage

The rapid growth in digital information throughout the world has demanded storage with a high capacity and high transfer rate. Holographic data storage (HDS) is one promising candidate for next-generation data storage because of its high potential in capacity and transfer rate. However, high-volume HDS usually suffers from inter-signal interference (ISI). Here, we first develop volume recording and reproduction models for HDS. We reveal through optical simulations based on the developed models that ISI differs not only between pages but also between areas on the same page. Then, we develop an advanced equalization technique called segmented adaptive equalization (SAE) that takes into account inter-area ISI difference. We can improve signal quality much more than conventional equalization techniques by using SAE. We are confident that SAE can enhance the realization of HDS.

Rigorous Bragg condition for volume holographic gratings with anisotropic distortion

Volume holograms have Bragg selectivity because of the volume effect and are used for holographic data storage (HDS), which represents next-generation optical storage. High capacity can be realized in HDS because multiple holograms recorded within the same volume can be retrieved selectively using Bragg selectivity by varying the readout conditions. However, it is inhibited to retrieve holograms via the holographic grating distortion that results from volume or refractive index changes of the medium. We therefore derived the rigorous Bragg matching condition when anisotropic distortion occurred in-plane and the thickness direction of the holographic grating. We also evaluated the effects of readout compensation with angle and wavelength adjustments based on the derived Bragg matching condition using numerical analysis, and the condition's effectiveness was clarified.

Reconstruction of non-error magnetic hologram data by magnetic assist recording

Hologram memory is expected to be the next-generation of optical data storage technology. Bismuth-substituted yttrium iron garnet is typically used for rewritable magnetic hologram media. The diffraction efficiency of magnetic holography depends on the Faraday rotation angle, but the experimental diffraction efficiency is not as high as that expected from calculations. This difference could be caused by incomplete magnetization reversal at the recorded region. In this study, we investigated the effects of magnetic assist (MA) recording through numerical simulation and experiment to improve the diffraction efficiency and the resulting reconstructed images. The improvement of diffraction efficiency was more effective in garnet films thinner than the width of a fringe, and a suitable value of the assist magnetic field was identified for the improvement. In addition, MA recording improved the intensity of reconstructed images and broadened the non-error recording conditions to the low energy region. This technique shows promise in improving the reconstructed quality of magnetic hologram data.

Recording procedures for high-quality signal readout in self-referential holographic data storage

Two methods are proposed to improve the readout quality of signals in self-referential holographic data storage (SR-HDS), in which no reference beam is required to record and read digital data holographically: the off-the-focus (OtF) method and the oversampling additional pattern (OsAP) method. The focal point is located outside of the recording medium in the OtF method, and the signal pattern is recorded with an additional pattern that possesses a higher spatial frequency than the signal pattern in the OsAP method. Experimental results show that both methods are effective for drastically improving the signal-to-noise ratio (SNR). In addition, through numerical simulation, it can be observed that the readout quality is improved by the achievement of homogeneous hologram distribution.

Diffraction model of peristrophic multiplexing with spherical reference wave

Multiplexing recording is a primary contributor to determining the recording density in holographic data storage. Therefore, many different kinds of recording methods have been proposed. Among them, the method that utilizes spherical waves as reference waves is characterized by the ability to enable multiplexing recording only by moving (shifting or rotating) the recording medium. In our research, we propose a theoretical diffraction model of peristrophic multiplexing with a spherical reference wave and evaluate the diffraction efficiency; this multiplexing recording method has incorporated spherical reference waves in rotation of the media. Additionally, we verify the effectiveness of the model by comparing it with experimental results.