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We propose using “iLabs,” which relies on the digitalization of an experiment and a virtual display with photographs, as a solution to overcome the lack of scalability of realistic online experimentation and facilitate educational conversations.

A grating-based Talbot-Lau X-ray interferometer provides three imaging modalities, namely attenuation, differential phase contrast, and dark field. Of these, dark-field imaging is uniquely capable of detecting and characterizing micron-scale fine structure in an object via small-angle scattering that reduces fringe visibility. In Part 1 [Opt. Express 29, 40891 (2021)10.1364/OE.447794], we formulate a statistical optics model that predicts the change in visibility, or dark-field signal, as a function of the statistical properties of the scattering object as well as its location within the interferometer. In Part 2, we demonstrate use of this model by simulating an object comprising a random collection of scattering microspheres placed in an X-ray grating interferometer designed to operate at 28 keV. The statistical optics results are validated by numerical Fourier optics simulations.

A grating-based Talbot-Lau X-ray interferometer provides three imaging modalities, namely attenuation, differential phase contrast, and dark field. Of these, dark-field imaging is uniquely capable of detecting and characterizing micron-scale fine structure in an object via small-angle scattering that reduces fringe visibility. Several empirical studies have been published showing the utility of this imaging modality for a wide range of applications. There also exists a more limited set of theoretical papers, based primarily on wave-optics formulations. In this two-part paper we present a comprehensive statistical optics model of the dark-field effect. In Part 1, we develop the theoretical underpinnings of the model with an emphasis on a scattering object comprising a random collection of microspheres, and in Part 2 [Opt. Express 29, 40917 (2021)10.1364/OE.447798], we provide a variety of example simulation results.

We present evidence-based design principles for three different UV-C based decontamination systems for N95 filtering facepiece respirators (FFRs) within the context of the SARS-CoV-2 outbreak of 2019–2020. The approaches used here were created with consideration for the needs of low- and middle-income countries (LMICs) and other under-resourced facilities. As such, a particular emphasis is placed on providing cost-effective solutions that can be implemented in short order using generally available components and subsystems. We discuss three optical designs for decontamination chambers, describe experiments verifying design parameters, validate the efficacy of the decontamination for two commonly used N95 FFRs (3M, #1860 and Gerson #1730), and run mechanical and filtration tests that support FFR reuse for at least five decontamination cycles.

Volume holographic data storage is a promising alternative to current surface storage technologies, since it can provide simultaneously large digital storage capacity, fast data transfer rates, and short access times [1]. This is achieved by storing multiple holographic data pages in a common recording volume, while retaining the ability to retrieve any individual page. Multiplexing of data pages can be performed either at one wavelength, using angular or phase-encoded multiplexing, or at different wavelengths, using wavelength multiplexing. This investigation focuses on phase-encoded multiplexing, which for example allows direct binary arithmetic operations on recorded pages and enables data encryption [2].

Page-access optical memories are being investigated for the next generation of digital storage due to their potential for simultaneously achieving high capacity, fast transfer rates, and short access times. Examples of such memories are systems based on volume holography[1], two photon 3D recording[2], and spectral hole burning[3]. A common feature of these systems is that data is input using a spatial light modulator (SLM) and the output signal is imaged onto a CCD array. A typical arrangement for a digital volume holographic memory is shown in Fig. 1. To meet the capacity and transfer rate goals of a commercial system, page sizes on the order of one million pixels are required. The need to image pixel-to-pixel from the SLM to the CCD array over such a large page imposes tight tolerances on the optical design. Deviations from ideal imaging typically result in degraded performance. In future systems, signal processing methods will be required to overcome the limitations of the optical components. We describe a technique for data detection in page-acces memories that extends the well-known method of sequence detection from digital communications to two dimensions. The technique, which we term Decision Feedback Viterbi Algorithm (DF-VA) Detection improves the bit-error-rate (BER) performance of systems without ideal optics. Use of such a detection scheme can allow the use of less expensive, lower-complexity imaging optics, or, alternatively, can lead to higher information density per data page.

The performance of digital holographic data storage (DHDS) systems is primarily limited by the recording medium, as other components have been developed for consumer applications. In this invited paper we review state-of-the-art DHDS systems, fixing issues, and fundamental tradeoffs between materials and device characteristics. Recent results, including stored and retrieved video clips will be presented.

The lack of a suitable recording material has long been the Achilles heel of holographic storage. In contrast to the other system components (such as SLM, laser and detector array) which are generally available, although they may not be optimized for this particular application, the ideal material has yet to be developed. As a result, after 30 years of on-and-off development, no commercial digital holographic storage system has yet been deployed To address this bottleneck, the PRISM (photorefractive information storage materials) consortium was formed in 1994 to focus on the development of a suitable recording material and to understand how storage system trade-offs are affected by each candidate material system.

We discuss the experimental performance of a digital holographic data storage device and architectural and materials issues related to achieving large capacity and low bit error rates.

The combination of large electro absorptive effects in semiconductor multiple quantum wells (MQW) and photorefractive-like transport offers the promise of producing fast, efficient thin film dynamic holographic materials for such applications as compact free space interconnects and high speed optical processors.1

Summary not available at time of printing.

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The objective of this tutorial is to explore similarities and differences between the design of conventional and integrated optical elements.

To avoid erasure of holograms, recorded in photorefactive materials, during readout several fixing techniques have been used.

The diffraction efficiency of a grating during recording and readout can be quite different from a simple exponential or double exponential behaviour predicted by the Kukhtarev model.

In this review we discuss photorefractive effects associated with holographic storage in ferroelectrics from a materials, optical architecture and systems viewpoint. As a guide to the discussion a holographic storage system based on phase encoding for multiple superposition of interconnect patterns is presented. System performance is evaluated in terms of storage capacity, cross talk between superimposed patterns, S/N ratio, erasure upon readout, recording, erasure, and cycle times, and sensitivity. These performance characteristics are intimately related to materials properties and photorefractive processes in ferroelectrics. For example, cross talk between superimposed interconnect patterns is dependent, among other things, upon striations in the medium, the specifics of the multiplexing scheme, and the application of fixing procedures. Material inhomogeneities may be overcome by using phase conjugate readout techniques, and phase encoding allows many more holograms to be superimposed than is practical with angular encoding. Fixing may be achieved by recording at elevated temperatures, and the efficiency and duration of readout is affected by application of an electric field. These phenomena are re viewed in terms of the dependence of fundamental electro-optic properties on such parameters as applied field and temperature. Examples are given of experimental and modeling results.

In this presentation we review recent advances in growth, evaluation, and application of photorefractive fibers for optical signal processing and data storage. Optical ferroelectric fibers are attractive alternatives to bulk photorefractives, because optical energy can be confined over longer interaction lengths, and fibers of good optical quality are easier to grow than bulk crystals, at least for certain morphologies. Since the material properties and the applications for which these fibers can be used are closely related, we will discuss both topics. Wave interactions in photorefractive fibers are discussed from a theoretical viewpoint, and applications, such as beam coupling in periodically poled fibers, stimulated back scattering in c-axis SBN fibers, and data storage in a-axis SBN fibers, including consideration of cross talk for multiplexed holograms are presented. The dependence of hologram cross talk on striations and other compositional variations in fibers is considered, and ways to overcome these problems are discussed.

We present the results of a comprehensive electro-optic (EO) study of Sr0.61Ba0.39Nb2O6 at λ = 633 nm. The experiments are conducted in the transverse field configuration, hence the effective linear EO coefficient rc is measured. Uniformly poled samples yield rc = 230 ± 20 pm/V at 25°C and rc = 1600 ± 150 pm/V at 65°C. However, the EO behavior is found to substantially degrade when attempting domain reversal. Imaging a crystal through crossed polarizers during the reversal process shows a very non-uniform birefringence distribution in the vicinity of the original -c electrode face.

Most photorefractive crystals display a measure of imperfection, such as striations, curved surfaces, or other phase-disturbing properties, which distort transmitted images and degrade the fidelity of holographic storage. We consider strontium barium niobate crystals with striations perpendicular to the c-axis. In such crystals, extraordinary polarized light experiences the strongest photorefractive effect but scatters off the striae, degrading the image fidelity. Conventional four-wave mixing, using counterpropagating pump beams, does not completely alleviate the problem, as the pump beams are also scattered by the striae. This difficulty can be overcome by using two true phase-conjugate pump beams. We describe hologram storage in striated strontium barium niobate, using a true conjugate of the reference beam generated by internal self-pumping in a second photorefractive crystal. The conjugate of the reference beam generates a conjugate of the signal beam, which then reproduces the stored image with high fidelity. This technique enables the use of imperfect crystals in photorefractive applications. We explore uses of this technique for holographic data storage.

We identify three separate regimes in the evolution of self-pumped phase conjugation in photorefractive media. These are evident from the time evolution of transmission, omnidirectional scatter, and phase-conjugate reflection intensities measured experimentally. Initially, beam fanning occurs, manifested by depletion of the transmitted light to one-third of its initial value. This is followed by a gradual increase in the omnidirectional scatter intensity caused by spreading of the light paths in the crystal, until almost all the crystal is flooded. Only much later does the phase-conjugate beam appear and grow to half of the input intensity at the expense of both the transmitted and scattered light. The three processes persist for different durations: fanning is appreciably faster than the other two processes. Each regime is characterized by different beam spread within the crystal, which is observed in top-view images. We postulate physical mechanisms to describe the observed phenomena.

Back-seeded stimulated photorefractive scattering phase conjugation is demonstrated in SBN fibers for the first time. A 7 mm long by 0.8 mm diameter 0.01% Ce:SBN fiber oriented with its c-axis along the length of the fiber is used for the measurement. The seed is provided by placing a retroreflecting screen in the path of the beam that is transmitted by the fiber. Reflectivities of 28% and 49%, accounting for reflection losses, are achieved by using light with wavelengths of 515 nm and 638 nm, respectively. Previously seeded stimulated phase conjugation (SSPC) has been demonstrated in bulk photorefractive BaTiO3,1,2 by using retroreflector arrays. Good fidelity of the back-reflected signal is achieved with reflectivities approaching 60%. The geometry for optimum results requires that an index-matching fluid surround the crystal; pixel-based image processing-appears to be difficult to achieve in bulk crystals. In the new approach a synthetic material is constructed by bundling SBN fibers. This allows parallel, pixel-based image processing to be accomplished3 with low-power (milliwatt) laser beams. By employing a polarization beam splitter in conjunction with a wave plate, the reflected light can be coupled out without loss in amplitude. We report on the angular sensitivity, wavelength dependence, and amplitude threshold needed for SSFC to occur in these fibers.

Photorefractive fibers of SBN provide an attractive medium for storing information. The information can be in the form of a 2-D image, called a page, and multiple pages may be stored in a single fiber by holographic recording. The multiple pages are distinguished by angular multiplexing of the reference wave. In this paper we discuss experimental results regarding the number and quality of these pages stored in SBN fibers of different diameters, lengths, and recording arrangements. The data storage capacity of these fibers is calculated taking into account crosstalk and recording resolution.

A model describing the image irradiance of two images produced by a multiple grating reflection type focusing element is developed. The gratings may be formed simultaneously or sequentially, and the recording medium can change thickness during processing. In the algorithm, the spot size and location of the image are first calculated by ray tracing from each surface grating in the multiplexed grating element. The efficiencies of these rays are then computed with a multiple grating model which allows for coupling between the fields in the grating volume. Finally, the efficiency of each ray is weighted by a factor corresponding to the intensity profile of the reconstruction beam. The sum of these weighted rays per unit area in the image plane determines the relative image irradiance. A comparison is then made between the image irradiance produced by experimental multiple focused elements and the calculated irradiance pattern. The model predicts the relative profile and the size of the irradiance patterns.

The volume holography process in a sensitized fluid is studied. The object of the research is to use forced Rayleigh scattering methods in the measurement of velocity gradients in fluids. The process is divided into three major steps: writing of the hologram; evolution of the hologram under the effect of fluid motion; and reading of the hologram by optical means. The last two steps are discussed with special emphasis on the effects of convection and diffusion as they relate to the hologram deformation and decay.

The application of volumetric diffraction gratings in photochromic materials to the measurement of velocity gradients in fluid flows is presented. The process involves the writing of a crossbeams hologram by two coherent pulsed beams that intersect inside a fluid to which a photochromic dye has been added. The hologram is continuously read at the proper Bragg angle by a third beam of lower intensity and different wavelength. The effect of flow shearing is to change the main orientation of the plane of the fringes of the hologram, which in turn reflects in a change of the angular properties of the readout beam. This is carried out until the hologram fades out by diffusion or it moves out of the Bragg condition. We show the optimization of the writing of the cross-beam grating in an optically thick material (like the photochromies used) as well as the wavelength for the most efficient reading of such gratings. Velocity gradients in a carefully controlled flow are shown and compared to alternative established methods and theoretical profiles. Finally, we discuss several alternative geometries capable of measuring different components of the velocity gradient tensor as well as multichannel arrangements capable of measuring vorticity components (e.g., ω y = ∂v z /∂x− ∂v z /∂z).

We present an architecture for optically implementing a digital associative memory using a coherent optical system. Bipolar information is holographically phase encoded in a photorefractive crystal and upon readout is interferometrically phase decoded.

SBN is an attractive photorefractive medium, which is difficult to grow in large bulk sizes of good optical quality. Alternatively, at Stanford we grew SBN fibers of lengths exceeding 10 cm and with a diameter of μm. These fibers offer a unique environment for carrying out phase conjugation experiments, such as holographic data storage, self-phase conjugation, optical interconnects, and two-beam coupling.1 We report, to our knowledge, the first photorefractive holographic recordings in an SBN fiber. A plane wave reference and an image carrying object beam are incident on the fiber in a reflection recording geometry, where the grating vector is essentially aligned with the c axis.

Recent development of a computer-controlled Czochralski crystal growth system at Stanford University has allowed us to produce large single-crystal Sr0.6Ba0.4Nb2O6 boules. Since this material is grown in a solid solution, striations are normally present and difficult to suppress. We discuss the nature of the striations and a simple scattering measurement that provides quantitative information about the degree of compositional variation in our samples. The results show that the effect of striations on laser beam propagation is negligible over a wide range of input angles.

A method is presented to enhance defects in periodic objects in real time. A low-intensity reference wave interferes with the Fourier transform of an object beam to form a hologram in a photorefractive crystal. The nonlinear properties of the crystal perform a filtering operation, suppressing the high-intensity Fourier components of the periodic structure, and phase-conjugate read-out results in a defect-enhanced image. The mask to be inspected is placed in the input plane, and the defect-enhanced image appears at the output plane, in a time limited only by the time constant of the crystal. Using ESSO with a total incident light intensity of ~20 mW/cm2, the response time is ~50 ms. Defects of 10 × 100 μm2 have been easily detected with high SNR, and performance limitations are discussed.

Recent interest in neural network models for parallel distributed processing is stimulating much interest within the optics community, in particular, on the feasibility of optical associative memories.1 We illustrate how a set of spatially multiplexed volume holograms recorded in a photorefractive crystal constitutes an optical crossbar switch that globally interconnects a two-layer neural network.2 We focus on the use of the network to provide associative memory and show that the mathematical operations involved in distributed memory storage and recall can be optically implemented in a very natural fashion within the framework of photorefractive holography. These operations include vector outer products, linear combinations of outer-product matrices, and matrix-vector multiplication.

Magnetic resonance imaging techniques can be used to study fluid systems which are difficult or impossible to probe with optical methods. The problem in imaging fluid flows with magnetic resonance has been the length of time necessary to acquire an image. Recent developments in magnetic resonance imaging techniques indicate that much shorter acquisition times may be possible.

Much research has been devoted to image understanding and the automated analysis of images of 2-D scalar data, but comparatively little work has been devoted to understanding higher dimensional data. Experiments and numerical calculations of fluid flows yield large data sets consisting of vector data on 2-, 3-, and 4-D domains. These are often too complicated for manual inspection, manipulation, and display. As with scene analysis, an abstract representation of the data set would greatly facilitate these tasks. Such a representation based on the flow topology can be constructed by locating and characterizing the critical points of the velocity vector field. These points serve as a basis for building a representation of the global topology as determined by the tangent curves of the vector field. This paper describes methods for the implementing this analysis on a computer and results from their application to 2- and 3-D fluid flow data sets.

Two types of conical emission in sodium have appeared in the past. In the first, a single tightly focused laser beam is tuned to the blue side of the D1 line in conditions compatible with self-trapping. The emission is specular, has a cone half-angle of a degree, and occurs on the red side of line center. In the second type, two weakly focused counterpropagating laser beams are tuned to the red side of line center. The emission consists of radial filaments, has a cone half-angle of 8°, and is the same wavelength as the pumps. We report on yet a third type of conical emission which is distinct from the first two types by virtue of its spatial and spectral characteristics. Like the first type, the pumps are tuned to the blue side of line center, the emission has a cone half-angle of a degree and is specular. However, this phenomenon also resembles the second type of conical emission in that two counterpropagating pumps are required, the pumps are weakly focused, and the emission wavelength is the same as the pumps.

A beta switch is a 2 × 2 crossbar cell, which is the building block for many interconnection networks (i.e., Clos-Benes, Spanke). A new optical beta switch design based on the ferroelectric liquid crystal total internal reflection switch is investigated for use in dynamic optical interconnection networks. It exhibits low crosstalk and can be driven by only a few volts. Crosstalk, loss, and scaling characteristics are discussed in the context of the device cascadability.

The polarization properties of multiple exposure speckle patterns recorded in a photorefractive crystal (Bi12SiO20) are exploited to improve the signal-to-noise ratio in real-time speckle metrology and velocimetry. Previous work1 demonstrates the feasibility of real-time photorefractive recording and optical processing of multiple-exposure speckle patterns for metrology and velocimetry applications. The speckle patterns are produced by a coherent imaging system, a scattering object, and a Q-switched Nd:YAG laser. The multiple-exposure is immediately interrogated with a CW laser and processed optically. The resulting fringe pattern contains two-dimensional displacement or velocity information in real-time. The effects of speckle recording on the polarization properties of the photorefractive crystal shows that polarization filtering separates a portion of the signal from the background noise in the optical processor. This produces an increase in the contrast of the fringe pattern in the output plane of the optical processor. Polarization filtering may also reduce the energy density incident on the crystal necessary to produce a signal of given strength. Experimental results demonstrating the improved signal in real-time speckle velocimetry are shown.

Microelectronic chip-to-chip interconnection necessitates channeling information from a large number of independent transmitters to a large number of receiver locations. Limitations of existing electronic interconnection techniques have prompted examination of optical methods of connection. One proposal is to use reflection-type holographic optical elements to direct modulated signals from optical sources to detectors. Obtaining multiple images from a single source requires encoding a set of gratings in the same recording medium. This can be done either sequentially (incoherently) or simultaneously (coherently). In this presentation the results of an experimental and theoretical investigation of simultaneously formed multiple image reflection volume holograms are given.

Although the existence of nonlinear holographic response in phoforefractive materials has been experimentally observed and its presence treated in some theoretical detail,1 standard holographic recording techniques have yielded higher-order grating components substantially weaker than the fundamental linear component. Using the unstationary running wave technique of Huignard and Marrakchi,2 however, we show that these higher grating harmonics may be substantially increased in strength. Furthermore, this enhancement is selective in that there are distinct criteria for the optimal amplification of each grating order. In addition to a theoretical analysis of this effect, we present experimental verification using photorefractive Bi12SiO20 crystals. By measuring the diffraction efficiency of both the fundamental and second harmonic grating components, we observed substantial enhancement of this nonlinear second-order response with respect to the fundamental.

We describe a novel arrangement for obtaining enhanced nondestructive holographic readout in SBN using polarization effects and modification of the applied field between recording and readout. The enhancement is most pronounced for a certain range of spatial frequencies. The readout signal first decreases in strength but subsequently grows above the starting value, approaching 100% efficiency in some cases. Over 6 h of continuous readout have been achieved with efficiency remaining strong enough to provide one billion reads with SNR exceeding 20 dB.