Hirosuke Furuta

Improved Holographic Recording Techniques for Data-Page Rewriting

Novel holographic recording configurations capable of selective erasure of multiplexed holograms for data-page rewriting in holographic storage are presented. By using a half wave plate and a polarizing beam splitter as the phase control device of the interference pattern in holographic recording/erasure procedure, comparatively large-area and finespun data-pages may be recorded, selectively erased, and rewritten in simple operations. We experimentally demonstrate the selective erasure of multiplexed data-pages in photorefractive iron-doped LiNbO3 crystals, and show the effectiveness of the configuration. We also propose a method to improve the in-plane uniformity of the erasure making use of the diffuser in the optical path of the object beam which is employed for dispersing the object image and the phase conjugate readout technique.

Selective Erasure of Holograms in Photorefractive and Photopolymer Materials for Rewritable and Secure Data Storage

A selective erasure/update method of multiplexed photorefractive and photopolymer holograms by use of the double Mach–Zehnder (DMZ) interferometric arrangement is proposed. The DMZ arrangement is used as the optical phase-control device for holographic recording/erasure/update operations, which can produce a pair of π-phase-shifted interference patterns with a simple optical setup. We intend to erase the holograms recorded in not only the inherently rewritable photorefractive crystal but also the photopolymer material generally used for the write-once (i.e., unerasable) holographic recording. It is experimentally confirmed that our method can erase the write-once type photopolymer holograms as well as the photorefractive holograms, although there will be a certain degree of restriction on the rewrite cycles limited by the monomer density, the degree of multiplexing and so on.

Selective erasure of speckle-multiplexed holograms by use of a double Mach-Zehnder interferometric arrangement.

A novel method to selectively erase and update speckle-multiplexed holograms in photorefractive crystals by use of a double Mach-Zehnder (DMZ) interferometric arrangement is presented. The DMZ arrangement automatically produces a pair of pi-phase-shifted interference patterns used for holographic recording, erasure, and update operations with a fairly simple optical configuration that consists of a commonly used dielectric multilayer beam splitter and two mirrors. The recording and the erasure conditions required for erasing a photorefractive hologram quickly and completely are discussed by calculating the temporal property of the hologram buildup and decay using the time-dependent coupled-wave equations. An experiment is also performed, in which arbitrary holograms in speckle-multiplexed holograms are selectively erased and updated with the simple DMZ optical configuration.

Determination of free‐carrier‐enhanced emission rate of carriers in depletion‐edge region by constant capacitance technique

The rate of majority‐carrier emission at the trap level located in the edge of the depletion layer is enhanced by the influence of a free‐carrier tail. This causes a nonexponential change in transient capacitance with time after the application of a filling pulse. The emission rates of the midgap level in GaAs and the Au acceptor level in Si are measured by the constant capacitance technique which eliminates the additional nonexponential factor due to the high trap concentration. The apparent emission rate determined from the initial slope of transient voltage increases with decreasing the difference of the depletion widths in the emission and capture processes. The enhanced emission rate due to the free‐carrier tail is observed even on the sample of the low carrier concentration. Fitting the calculated emission rate to the experimental results enables the estimation of the capture cross section of majority carriers in the trap level as well as the emission rate.

Trap filling measurements to determine the crossover point of Fermi level and midgap level in GaAs by constant capacitance technique

Trap filling measurements on the midgap level in GaAs are carried out under constant capacitance conditions in the emission process. The transient voltage after application of a filling pulse is measured with varying capture time. A method to obtain the distance λ1 from the depletion edge to the crossover point of the midgap level and the quasi‐Fermi level is proposed for the case of high trap concentration. Fitting the calculated λ1 to the experimental results enables estimation of the capture cross section of electron in the midgap level.

Burst error reduction in rewritable-type holographic memory using scattering recording and phase conjugate retrieving

We propose a novel method to overcome burst error of rewritten data in rewritable type holographic memory. We experimentally exhibit that our method is effective for reduction of the unevenness of the reproduction image.

Spatial arrangement of speckle-multiplexed holograms using directional control of shift selectivity with holographic diffuser

We propose a method for directional control of the shift selectivity of the speckle-multiplexed holograms using the holographic diffuser whose surface profile varies with the orthogonal direction. The method improves the tolerance of the spatial multiplexed holograms for the repositioning error in the holographic storage. In addition, we utilize the nature that the shift selectivity in the propagation direction of the reference beam is intrinsically relaxed, and experimentally implement the spatial arrangement of the multiplexed holograms in both the “controlled” and the “intrinsically relaxed” directions. We show that the high quality output is obtained using our method by the measurement of the BER of the data-pages that are retrieved from the spatially arranged multiple holograms.

A New Recovery Method for Attenuated Holograms in Selective Data Page Rewriting

We propose a novel method of optically recovering holograms that are inevitably attenuated by selective rewriting of a data page in multiplexed holograms. We prove the effectiveness of our hologram recovering method experimentally. We also numerically investigate the recovery degree of multiplexing with consideration of the optical and electric noises, so that we can reveal the maximum recoverable degree of multiplexing under the condition in which such noises exist. Then, we show that the recoverable degree of multiplexing can be improved by setting the appropriate recovery duration.

Enhancement of Repositioning Tolerance by Directionally Controlled Shift Selectivity of Speckle-Multiplexed Holograms

A method for directional control of shift selectivities of speckle-multiplexed holograms with a diffuser whose diffusion angle varies with the diffusion direction is proposed to enhance the tolerance of recorded holograms to repositioning error in holographic storage. The directional dependence and controllability of shift selectivities of holograms are experimentally investigated using various diffusers. By retrieving 50 speckle-multiplexed holograms with deliberate deviations from the original recording position and by measuring bit error rate, it is also confirmed that the tolerance to repositioning error is improved.

Triple Resonance Optical Filter with Three Ripples in Passband Area

The experiment on the Fabry–Perot (F–P) triple resonance optical filter is reported. Flat passband can be achieved owing to the three subtle ripples which appear in the passband. Furthermore the passband loss and width can be arbitrary tuned by changing the reflectivity of mirrors. In order to experimentally generate the triple resonance, it was required that the resonance frequencies of the three F–P single resonators, whose optical lengths were 300 µm, were matched within several GHz. The passband width of 26 GHz and the smallest loss of 0.2 dB were obtained. The sharp cutoff characteristic was also achieved successfully. 1.5–10 dB shape factor was evaluated at 0.60. These experimental data well coincide with the theoretical values.