Terumasa Ito

Volume Holographic Recording Using Spatial Spread-Spectrum Multiplexing

A new recording method called spatial spread-spectrum (SSS) multiplexing, which is particularly suitable for holographic disc recording, is proposed. Multiplexing holograms by phase modulation and demodulation onto the signal beams themselves (not onto reference beams) provides a new degree of freedom in the design of holographic storage systems, thereby making it possible to improve storage density by combining with other multiplexing methods. In our newly proposed recording geometry with spatial filtering, uncorrelated noise coming from undesired pages can be efficiently deflected away from a detector. Calculations show that the theoretically achievable multiplicity of the SSS recording system with spatial filtering is improved up to 70 when the numerical aperture (NA) of the signal beam is 0.1.

Coherent Parallel Copying of Holograms Recorded by Spatial Spread-Spectrum Multiplexing

In this paper, we propose a new parallel hologram copying system in which multiplexed holograms are coherently copied from a master medium to a copy medium. The diffraction efficiency in spatial spread-spectrum multiplexing, which is a type of phase multiplexing based on phase modulation onto signals, can be significantly enhanced by coherent parallel copying. We theoretically and experimentally showed that the diffraction efficiency of M multiplexed holograms can be increased up to 1/M of that of a single hologram, which is an improvement by a factor of M compared with the 1/M2 formula in conventional multiplexing systems.

Shift selectivity of spatial spread-spectrum holographic recording system

Volume holographic recording is a promising solution for next- generation optical disc storage that has a high capacity more than 1 TB. This huge capacity is achieved by superimposing many holograms, each of which has millions of bits, at the same recording spot. We proposed a new technique, Spatial Spread Spectrum (SSS) multiplex recording. Unlike conventional multiplex holography based on Bragg effect of thick holograms, our technique utilizes spatial phase modulation and demodulation of the signal beam itself with a random diffuser to address the multiplexed page data. SSS multiplexing is additionally combined with other multiplexing methods, and provides further improvement of the total capacity of holographic storage. In this paper we experimentally verify the basic recording and readout feasibility, and investigate the shift selectivity and the aligning margin of the SSS holographic recording that are an important factor to determine the tolerance against vibration. It is shown that a clear 2-dimensional image is successfully reconstructed from the hologram even in the case the central part of the diffused signal beam is blocked in recording, and that a sharp shift selectivity about 5 microns was obtained by a diffuser with a diffusion angle of 15 degree, and the aligning margin for a sufficient SNR was approximately 1 micron.

Fault tolerant function of dynamic refreshing holographic memory with shutter-less optical feedback circuit

The optical system, consisting of two photorefractive memories and a shutter-less optical feedback circuit, will be demonstrated to function as data mirroring. This function is known to automatically detect the data dropout and restore data, using unimpaired data in another memory, in the event that part or all of the data in either of them were lost for some reason. This memory system also can cope with a damaged hologram, a result of reading beams, which is a disadvantage of rewritable photorefractive memory, to ensure non-destructive holographic reading. It can be achieved by using no electronic circuits or mechanical structures; our optical experimental method in particular obtains this basic action.

Quantum Effect on Helium Adsorbed in Y Zeolite

Heat capacities of 4He adsorbed in Y zeolite, in which there is a three-dimensional(3D) network of channels of only about 10A in diameter, have been measured at 0.6 nc, the heat capacities are well described with sum of two parts, αT2+β or αT2+γT where α, β and γ are constants depending on n. This result is interpreted in terms of the coexistence of a 2D solid and a gas or a liquid in the small channels.

Noise reduction strategy in high-speed replication of holographic memories using photorefractive amplification

We propose a high-speed replication system using photorefractive amplification for holographic memories. Also, we show a noise reduction strategy that enables to ensure a high signal-to-noise ratio of replicated holograms by using a transient gain of photorefractive amplification.

Intrapage crosstalk in one-beam holographic recording system using a blazed grating

We analyzed intrapage crosstalk characteristics in the one-beam holographic recording system in which a hologram is recorded with a signal and its diffraction beam split by a blazed grating. Calculations show that low-crosstalk holographic reconstruction can be achieved even with a non-modulated reference beam by optimizing the optical system.

High-Speed Holographic Read-Only Memory Replication Systems with Two-Wave and Four-Wave Photorefractive Amplifier

Copying speed is an important characteristic for optical read-only memory (ROM) replication systems. The copying speed of holographic ROM replication is, however, limited by small energy efficiency of the optical system due to the small diffraction efficiency of multiplexed holograms. In this paper we propose new holographic ROM replication systems with a photorefractive amplifier, and analyze the speed gain performance. We improve energy efficiency significantly and speed up replication by amplifying weak diffraction signal beams using photorefractive wave mixing. Our new theory and numerical calculations revealed that achievable speed gain can be evaluated from only a single dimensionless parameter that is the product of the three as follows: (i) the pump beam intensity ratio in the amplifier, (ii) the ratio of the photopolymer and photorefractive sensitivities, and (iii) the dynamic range per hologram of the copy medium. In current holographic recording systems, a practical copying speed gain of more than 10 is achievable with currently available photorefractive materials.

Nondestructive readout of a photorefractive hologram by phase-conjugate copying in a one-crystal configuration

We propose a novel phase-conjugate copying method for nondestructive readout of a volatile photorefractive hologram. In the one-crystal configuration, two holographic memories and a mutually pumped phase conjugator (MPPC) are formed within a single photorefractive crystal, instead of using multiple crystals. Two memories share the same hologram and complement each other in refreshing the hologram. A MPPC suppresses fanning noise and automatically aligns the wavefront of the reference and readout beams. We found the optimum configuration to achieve nondestructive readout from calculations and geometric consideration. In the experiments with a BaTiO(3) crystal, a continuous readout of 20 times longer than the recording time was achieved.

Recording schedule for partially coherent hologram multiplexing in a photorefractive medium

The exposure schedule for partially coherent hologram multiplexing, in which data pages are multiplexed by multiple signal beams and a single reference beam, is investigated in detail for the case of a pi/2 phase-shifted photorefractive medium. We found that the optimum recording schedule for partially coherent multiplexing cannot be determined by the classical recording schedule theory because of time-constant errors induced by partially coherent interaction between a reference beam and self-diffraction signal beams. To overcome the issue, we derive a modified recursion equation that accounts for the time-constant errors, and we also propose a novel iterative recording-schedule correction algorism for finding the optimum solution. In the calculation with hologram multiplicity of 30 and photorefractive coupling strength of 3.0, we could successfully obtain a flat diffraction-efficiency profile after the second recursion.