Norihiko Ishii

Compensation of Interference Fringe Distortion Due to Temperature Variation in Holographic Data Storage

Photopolymer materials are feasible for holographic recording media. However, these materials shrink owing to photopolymerization and interference fringes recorded in them distort. In addition, temperature variation causes shrinkage and expansion of these materials and thus distorts recorded interference fringes. This distortion degrades reconstructed image quality and decreases the signal-to-noise ratio of the reproduced data. We applied adaptive optics controlled by a genetic algorithm to compensate for the distortion and improved the reconstructed image quality at 25 and 30 ?C ambient temperature. Under these conditions, the signal-to-noise ratio of reproduced data was more than 4 dB. Furthermore, we evaluated the distortion due to the temperature variation by using a medium angle and the wavefront of the reference beam. We found that the distortion caused by anisotropic shrinkage is slight; consequently, an optimised wavefront at 25 ?C can compensate for the interference fringe distortion and increase the signal-to-noise ratio by adjusting only the medium angle even if a temperature variation occurs. Adaptive optics can thus be used to compensate for interference fringe distortion caused by shrinkage and expansion due to temperature variation.

Half-data-page insertion method for increasing recording density in angular multiplexing holographic memory

We have developed a method to use a half-size data page between two full-size data pages to increase the recording density in angular multiplexing holographic memory up to 1.5× as much as the conventional angular multiplexing sequence. In our recording sequence, the full- and half-size data pages are alternately multiplexed. This is because each plane wave from various points in a data page has different angular selectivity. A half-size data page has higher angular selectivity than a full-size data page. The required angular intervals were estimated by numerical simulation taking holographic medium tilt into account. Also, an angular multiplexing experiment using the half-data-page insertion method resulted in a low bit error rate of the order of 10(-3), which is sufficient for practical use.

Control of Angular Intervals for Angle-Multiplexed Holographic Memory

In angle-multiplexed holographic memory, the full width at half maximum of the Bragg selectivity curves is dependent on the angle formed between the medium and incident laser beams. This indicates the possibility of high density and high multiplexing number by varying the angular intervals between adjacent holograms. We propose an angular interval scheduling for closely stacking holograms into medium even when the angle range is limited. We obtained bit error rates of the order of 10-4 under the following conditions: medium thickness of 1 mm, laser beam wavelength of 532 nm, and angular multiplexing number of 300.

Integrated Simulation Technique for Volume Holographic Memory Using Finite-Difference Time-Domain Method

We developed an integrated simulation technique for volume holographic memory. Volume holographic gratings were expressed as the permittivity distribution in the media. The distribution derived from the interference fringes of a reference beam and an object beam. These fringes were calculated using the angular spectrum of plane waves, instead of the diffraction integral formula. The finite-difference time-domain method was used to simulate the light wave propagation in the volume holographic gratings. The diffracted electromagnetic waves on the observation plane were obtained by the irradiation of the reference beam from the source plane. The simulation of an angularly multiplexed hologram was performed. The simulation results showed that two different data pages are reproduced separately at the same angle as those recorded in a numerical computation.

GeSbTe Phase Change Material for Blue-Violet Laser at High Linear Speed

High-speed recording and read-out were carried out under the condition of numerical aperture (NA) 0.65 and substrate thickness 0.6 mm using a blue-violet laser of 405 nm. Using a GeSbTe phase change material of the crystal nucleus generation dominant type, we obtained a carrier-to-noise ratio (CNR) of 50 dB with 0.24 µm mark and a DC erase ratio of 30 dB at a velocity of 18 m/s. It was proven that the realization of the 100 Mbps data transfer rate was possible by optimizing record compensation.

Method of Phase Compensation for Holographic Data Storage

The characteristics of holographic recording in the presence of phase noise caused by air disturbances and vibrations were studied. Diffraction efficiencies were calculated by a finite-difference time-domain method. Simulation results showed that the amplitudes of fringes decrease with increasing phase noise level, and thus, diffraction efficiencies decrease. A novel method of phase compensation for holographic data storage was proposed. Interference fringes were impinged on a charge-coupled device (CCD) camera symmetrically positioned relative to a recording medium, and the fringes in the medium were estimated using the fringes on the camera. Phase information was obtained by fast Fourier transform, and a phase modulator was set in the reference path and driven to reduce the phase error, so that the phase on the camera was stabilized. By this method, the standard deviation of the phase error was reduced to less than 1/10 that obtained by a conventional method. The diffraction efficiencies of angle-multiplexing in a photopolymer were measured and the relative multiplexing number with feedback control was 50% larger than that without it. As the holographic data storage device records the phase, the proposed method is an important and feasible technology.

Improved Signal-to-Noise Ratio Using Phase Compensation in Shift- and Angle-Multiplexed Holographic Data Storage

The phase compensation method and the use of a speckled reference beam was proposed for use in a shift- and angle-multiplexed holographic data storage system. As phase is random in a speckled reference beam, we multiplexed a plane wave with a speckled reference beam. Using this method enabled us to suppress phase fluctuations, and we thus increased the signal-to-noise ratio (SNR) by 2.2 dB in digital data. The theoretical limit of bandwidth and resolution of phase was also considered. The bandwidth of the phase modulator was made the limit of the control bandwidth in the current system. Widening it enabled us to create a more stable holographic data storage system with a higher SNR.

Dual-page reproduction to increase the data transfer rate in holographic memory

To increase the reproduced data transfer rate in holographic memory, we have investigated simultaneous reproduction of two data pages. By irradiating s- and p-polarization reference beams whose angle gap is equal to the angle between the neighboring data pages in angle-multiplexed holograms, two different data pages can simultaneously be reproduced with a bit error rate low enough to decode. This technology is effective to double the data transfer rate in holographic memory.

Optical compensation for hologram distortion using wavefront interpolation in angle-multiplexed holograms

Distortion of the hologram may occur when the photopolymer material used in the medium shrinks or expands. We analyzed interference fringe distortion for plane waves and a reference beam with an angular gap between recording and reproducing for the purpose of compensating for the distortion. We found that the wavefronts that could compensate for the distortion could approximately be obtained by linear interpolation of such angle-multiplexed holograms. We recorded 80 data pages with the angle-multiplexing method and obtained an optimized wavefront to compensate for hologram distortion on the first, fortieth, and eightieth data pages using adaptive optics with genetic algorithms and linear interpolated wavefronts at the other data pages. The calculation time for 80 wavefronts to compensate for distortion fell to 3/80th of that of having to calculate optimizations for all pages. The bit error rates were lower than 1.0 × 10−2 on all data pages reproduced using these wavefronts.

Angular Spacing Control for Segmented Data Pages in Angle-Multiplexed Holographic Memory

To improve the recording density of angle-multiplexed holographic memory, it is effective to increase the numerical aperture of the lens and to shorten the wavelength of the laser source as well as to increase the multiplexing number. The angular selectivity of a hologram, which determines the multiplexing number, is dependent on the incident angle of not only the reference beam but also the signal beam to the holographic recording medium. The actual signal beam, which is a convergent or divergent beam, is regarded as the sum of plane waves that have different propagation directions, angular selectivities, and optimal angular spacings. In this paper, focusing on the differences in the optimal angular spacing, we proposed a method to control the angular spacing for each segmented data page. We investigated the angular selectivity of a hologram and crosstalk for segmented data pages using numerical simulation. The experimental results showed a practical bit-error rate on the order of 10-3.