Multilayer volume holographic optical memory

Optimization of Blue Photorefractive Properties and Exponential Gain of Photorefraction in Sc-Doped Ru:Fe:LiNbO3 Crystals

Sc:Ru:Fe:LiNbO3 crystals were grown from congruent melt by using the Czochralski method. A series of LiNbO3 crystals (Li/Nb = 48.6/51.4) with 0.1 wt% RuO2, 0.06 wt% Fe2O3 and various concentrations of Sc203 were prepared. RF1 and RF4 refers to the samples containing 0 mol% Sc203 and 3 mol% Sc203, respectively. The photorefractive properties of RF4 were measured by Kr+ laser (λ = 476 nm blue light): ηs = 75.7%, τw = 11 s, M/# = 19.52, S = 2.85 cmJ−1, Γ = 31.8 cm−1 and ∆nmax = 6.66 × 10−5. The photorefractive properties of five systems (ηs, M/#, S, Γ and ∆nmax) under 476 nm wavelength from RF1 to RF4 continually increased the response time, while τw was continually shortened. Comparing the photorefractive properties of Sc (1 mol%):Ru (0.1 wt%):Fe (0.06 wt%): LiNbO3 measured by Kr+ laser (λ = 476 nm blue light) with Sc (1 mol%):Fe (0.06 wt%):LiNbO3 measured by He-Ne laser (633 nm red light), ηs increased by a factor of 1.9, Vw (response rate) increased by a factor of 13.9, M/# increased by a factor of 1.8 and S increased by a factor of 32. The ∆nmax improved by a factor of 1.4. A strong blue photorefraction was created by the two-center effect and the remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers. The photorefractive properties (ηS, τw, M/#, S, ∆nmax) were increased with an increase in Sc3+ ion concentration. Damage-resistant dopants such as Sc3+ ions were no longer resistant to damage, but they enhanced the photorefractive properties at the 476 nm wavelength. The experimental results clearly show that Sc-doped two-center Ru:Fe:LiNbO3 crystal is a promising candidate blue photorefraction material for volume holographic storage. Sc-doped LiNbO3 crystal can significantly enhance the blue photorefractive properties according to the experimental parameters. Therefore, the Sc:Ru:Fe:LiNbO3 crystal has better photorefractive properties than the Ru:Fe:LiNbO3 crystal.

Multilayer recording holographic data storage using a varifocal lens generated with a kinoform

A multilayer recording method using a varifocal lens generated with a kinoform is presented. In this recording method, a focus position is axially displaced by adding a defocus phase to a phase modulation pattern, which consists of a random phase mask and a computer-generated reference pattern. Shift selectivity and multiplexed recording are experimentally investigated in coaxial holographic data storage. Experimental results show that the proposed method allows the recording of holograms along an optical axis without any mechanical movement.

Direct rapid-prototyping fabrication of computer-generated volume holograms in the millimeter-wave and terahertz regime

Computer-generated volume holograms (CGVHs) are gradient refractive index (GRIN) devices that consist of a superposition of multiple periodic diffraction gratings. Fabrication of these components for the visible range is difficult due to the small length-scale requirements but is more tenable in the terahertz (THz), as the length scales become more practical (≥ 10^-5 m). We successfully utilized polymer-based 3D additive rapid-prototyping technology to fabricate, to our knowledge, the world's first 3D THz CGVH in approximately 50 minutes, using $12 of consumables. This demonstration suggests that this technique could be extended to fabricate THz volumetric optics with arbitrary electromagnetic profiles.

Review of Random Phase Encoding in Volume Holographic Storage

Random phase encoding is a unique technique for volume hologram which can be applied to various applications such as holographic multiplexing storage, image encryption, and optical sensing. In this review article, we first review and discuss diffraction selectivity of random phase encoding in volume holograms, which is the most important parameter related to multiplexing capacity of volume holographic storage. We then review an image encryption system based on random phase encoding. The alignment of phase key for decryption of the encoded image stored in holographic memory is analyzed and discussed. In the latter part of the review, an all-optical sensing system implemented by random phase encoding and holographic interconnection is presented.

Three-dimensional shift selectivity in reflection-type holographic disk memory with speckle shift recording

Three-dimensional shift selectivity of a reflection-type hologram with speckle shift recording is investigated experimentally and numerically. We build an experimental setup consisting of lenses with numerical apertures of 0.28 and an iron-doped LiNbO(3) with a thickness of 0.5 mm. The experimental results show that three-dimensional selectivity has a size of 0.97 microm x 0.97 microm x 8.8 microm in diffraction efficiency. We also develop a volume holographic memory simulator to evaluate the experimental results. The simulator can quantitatively evaluate bit error rate, signal-to-noise ratio, and diffraction efficiency. Numerical results are in good agreement with the experimental results. The experimental and numerical results indicate that three-dimensional shift multiplexing can increase the storage capacity.

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.

Theoretical and experimental studies of hologram multiplexing that uses a random wave front generated by photorefractive beam fanning

A hologram multiplexing technique-that uses random wave fronts generated by photorefractive beam fanning is investigated. A storage photorefractive crystal generates various random wave fronts to be used as reference beams without the external diffusers such as ground glass and multimode optical fiber that are generally employed. We experimentally demonstrate hologram multiplexing with six images and show that the stored holograms can be selectively retrieved. We also simulate photorefractive beam fanning inside a BaTiO3 crystal, in particular regarding the correlation properties of the fanning beams for the first time to our knowledge, and reveal the conditions of incidence of an object beam and a reference beam required for suppressing image degradation, implementing low-cross-talk retrieval, and producing a large number of stored holograms.

Folded shift multiplexing

Shift multiplexing is a holographic recording method that uses a spherical reference wave. We extend the principle to a thin slab of holographic material that acts as a waveguide. Total internal reflection folds the reference spherical beam in one dimension. We demonstrate that the shift selectivity with the folded spherical beam is independent of the slab thickness but depends instead on the numerical aperture of the coupled spherical wave. A shift selectivity of 0.5 microm has been achieved with a 1-mm-thick LiNbO3 crystal and 50 high-definition data pages are recorded with this method.

Three-dimensional shifting selectivity of random phase encoding in volume holograms

We analyze and demonstrate the three-dimensional shifting selectivity of volume holograms based on random phase encoding with ground glass. Under weak coupling, the diffraction characteristic is caused by the phase difference between the reference and the reading light. We find that the shifting selectivity is different for different shifting directions, which include laterally horizontal, laterally vertical, and longitudinal directions. The shifting selectivity depends on the diameter of the region of illumination on the random phase plate, the thickness of the hologram, and the distance between them.

Sparse modulation coding for increased capacity in volume holographic storage

In page-oriented memories, data pages commonly consist of comparable numbers of on and off pixels. Data-page sparsity is defined by reduction of the number of on pixels per page, leading to an increased diffracted power into each pixel. When page retrieval is dominated by a fixed noise floor, the number of pages in the memory is limited by the pixel diffraction efficiency. Sparsity increases the number of storable pages while reducing the amount of user information per page. A detailed analysis of sparsity in volume holographic memories shows that the total memory capacity can be increased by 15% by use of data pages that contain on average 25% on pixels. Sparsity also helps to reduce the effects of interpixel cross talk by strongly reducing the probability that worst-case pixel patterns (e.g., blocks of on pixels with a center off pixel) will occur in the data page. Enumeration block coding techniques provide construction of sparse-data pages with minimal overhead. In addition, enumeration coding offers maximum-likelihood detection with low encoding-decoding latency. We discuss the theoretical advantages of data-page sparsity. We also present experimental results that demonstrate the proposed capacity gain. The experiment verifies that it is practical to construct and use sparse-data pages that result in an overall user capacity gain of 16% subject to a page retrieval bit-error rate of 10(-4).