Yoshinao Taketomi

Incremental recording for photorefractive hologram multiplexing.

We investigate an incremental recording technique for multiplexed hologram storage in photorefractive crystals, in which each hologram is recorded with multiple short exposures. The performance is theoretically compared with that of scheduled (single exposure per hologram) recording. Our analysis shows that this technique systematically controls the signal uniformity and can also decrease the total recording time. We present an experimental demonstration with LiNbO(3) using a binary orthogonal phase-code addressing technique.

Dynamic photorefractive optical memory

We investigate an architectural approach to dynamic three-dimensional volume storage that circulates holograms between two photorefractive crystals. Introduction of an optical amplifier into the system increases the effective write-erase asymmetry of the crystals and permits the amplification of the recalled images. This memory architecture is experimentally shown to provide write-erase storage with robustness to multiple optical readouts.

Multiplex holography in strontium barium niobate with applied field

We characterize the performance of photorefractive Sr0.6Ba0.4Nb2O6 under an externally applied voltage. We compared the field dependences of the gain coefficient, response time, sensitivity, and diffraction efficiency with predictions of Kukhtarev’s solution [ Sov. Tech. Phys. Lett.2, 438 ( 1976)] for two-wave interaction, using material parameters obtained at zero voltage. Plane-wave holograms were superimposed in the crystal with angular multiplexing. The hologram capacity (or minimum diffraction efficiency) increased by an order of magnitude with a 15-kV/cm applied field.

Reflection hologram for reconstructing deep images

A method to record deep and blurless images as a reflection hologram is proposed. A slit was used in the recording process in the same manner as for a rainbow hologram. The reconstructed image was monocolor and could be observed from wide longitudinal angles when it was illuminated by an extended white-light source such as a fluorescent lamp. One can easily obtain multicolor images by multiplexing the holograms with different wavelengths.

Dynamics of a composite grating in photorefractive crystals for memory application

The dynamics of a composite grating that consists of an original grating and a newly superimposed grating are investigated for memory applications by solution of coupled-wave equations and photorefractive-material equations with the following initial conditions: the original grating has an arbitrary refractive index modulation, an arbitrary phase shift exists between the original and the superimposed grating, and there is an externally applied dc electric field. The effect of beam coupling, including fringe bending on the composite grating, is investigated by numerical simulation. Simplified analytical solutions that neglect beam coupling are derived, showing good agreement with experimental results. The investigation also shows that a selective erasure process in the presence of an external electric field is made possible by provision of an appropriate constant phase shift. Analytical solutions of the composite-grating dynamics are used to analyze both scheduled and incremental recording dynamics with an external electric field. It is shown that scheduled recording results in a nonuniform relative phase distribution among multiplexed gratings. In contrast, the incremental recording converges to a uniform relative phase distribution and indicates that selective erasure may be easily implemented for fast memory update.

Moving grating for enhanced holographic recording in cerium-doped Sr(0.6)Ba(0.4)Nb(2)O(6).

We demonstrate that holographic recording in photorefractive materials can be improved by using a moving grating to reduce energy coupling. In cerium-doped Sr(0.6)Ba(0.4)Nb(2)O(6) under an applied dc electric field, experimental and theoretical results show that the proper grating velocity can maximize the modulus and the real part of the spacecharge field while reducing the imaginary component of the space-charge field (and energy coupling) to zero. Avoiding energy transfer between the recording beams allows us to maintain maximum contrast throughout the crystal, producing a uniform, high-index-modulation grating with enhanced diffraction efficiency and superposition properties.

Moving grating and dc external field in photorefractive GaP at 633 nm

We demonstrate that the photorefractive effect in GaP crystal at 633 nm can be enhanced using an externally applied dc field and a moving grating. A two-beam coupling gain coefficient of 2.5 cm(-1) and a steady-state phase-conjugate reflectivity of 1.9% were obtained.

Effects Of Applied Voltage On Holographic Storage In SBN:60

We characterize the holographic storage characteristics of photorefractive SBN:60 (Sr0.6Ba0.4Nb2O6) under an externally applied electric field. The field dependence of the recording response time and sensitivity, gain coefficient, steady-state diffraction efficiency, and erasing response time were measured. Kukhtarevs band transport model is shown to predict the asymmetric erase time / write time behavior of SBN:60. Using these results, we estimate as a function of applied field the number of equal diffraction efficiency holograms which can be superimposed in the crystal.

Reflection hologram for reconstructing deep images: errata.

On p. 1725 of Ref. 1, in the 20th line in the righthand column, the printed phrase two-step time diffraction should read as two-step diffraction.