Development and field demonstration of a hardware-compressive fiber-optic true-time-delay steering system for phased-array antennas

We describe the design, fabrication, testing, and antenna-range demonstration of a photonic wavelengthmultiplexed true-time-delay steering system for use with broadband phased-array antennas. The prototype system is based on a unique hardware-compressive architecture and can drive 16 antenna elements over the 0.35-2.1-GHz band with 6-bit angular resolution over a ±45° scan angle.     

Evanescent wave holographic recording in a photopolymer

We present a study of holographic recording with an evanescent reference and a homogeneous (plane) object wave. The grating step was 324 nm. The dependence of the diffraction efficiency on exposure was investigated. The maximum value obtained was 0.01% at 5 mJ cm^?2 exposure. The optimal pre-exposure, needed for grating adhesion to the glass substrate, was 1 μJ cm^?2.     

Modified holographic recording setup for photopolymerizable/flexible recording materials

An improvement in the conventional holographic recording setup has been done for liquid (photopolymerizable) or flexible recording materials. The new vertical recording setup that comprises a horizontal substrate holder permits holographic recording on such materials without positioning them between two substrates, as has been done until now. This setup can be conveniently used for the fabrication of transmission, reflection, Fourier transform, and computer-generated holograms with few simple modifications as indicated.     

Multilayer holographic recording using a two-color-absorption photopolymer

We developed a sensitive two-color-absorption photopolymer in which holograms are recorded by simultaneous irradiation with a 660 nm interference light and a 410 nm gate light. Doped with bis(silyl)pentathiophene as a two-color-photosensitive dye and 2,2-dimethoxy-2-phenylacetophenone as a radical photopolymerization initiator; its matrix contains low-refractive index binding polymers and high-refractive index monomers. The sensitivity and diffraction efficiency of 25 mum thick layers are from 1.2 x 10(-9) to 3.7 x 10(-9) cm2/mJ and from 1% to 4%. We made a three-photopolymer-layer waveguide structure, where each photopolymer layer and high-refractive index adhesive layer serves as a core layer and is sandwiched between two low-refractive index glass substrates that serve as clad layers. Gate light propagated through the adhesive layers, reference and object beams intersected the photopolymer layers, and different diffraction patterns could be written in each layer.     

Shrinkage control in a photopolymerizable hybrid solgel material for holographic recording

We report the correction of the shrinkage observed during UV postrecording curing in a holographic solgel material that was recently achieved by the use of various chemical formulations for the composition of the hybrid supporting matrix. We found that a chemical modification of the matrix noticeably attenuates the shrinkage (from 1.3% to 0.4% of the material's initial thickness with the inclusion of just 20% tetramethylorthosilicate), providing a material with improved stability for permanent data storage applications. The holographic properties of samples with different binders are also reported. In addition, a theoretical study has revealed the way by which to compensate for angular deviation in the Bragg condition during UV postrecording by tailoring the binder shrinkage (s), the maximum refractive-index modulation capability of the photosensitive mixture (deltan), or both.     

Aspheric wave-front recording optics for holographic gratings

The geometric theory of aspheric wave-front recording optics is extended to include the fourth-order groove parameters that correspond to the fourth-order holographic terms in the light-path function. We derived explicit expressions of the groove parameters by analytically following an exact ray-tracing procedure for a double-element optical system that consists of a point source, an ellipsoidal mirror, and an ellipsoidal grating blank. Design examples of holographic gratings for an in-plane Eagle-type vacuum-UV monochromator are given to demonstrate the capability of the present theory in the design of aspheric wave-front recording optics.     

Multichannel holographic recording method for three-dimensional displays

A multichannel holographic recording method is presented for three-dimensional (3D) displays, utilizing pixel-based recording instead of image-based recording in order to realize parallel processing. The proposed approach is composed of two main stages. In the first stage, each two-dimensional (2D) image acquired from multiple viewpoints is partitioned by holographic recording channels (HRC) into nonoverlapping subimages. In the second stage, the corresponding pixels of the subimages are rearranged to constitute an encoding image. The encoding images are recorded simultaneously by each HRC, respectively, so the recording speed is improved significantly. The experimental results have demonstrated that the three-channel system is feasible and the full-parallax hologram reconstructed with white light is acceptable in quality. The three-channel system saves approximately 60% of the recording time in comparison with the single-channel system. More importantly, the proposed method can accomplish a large-scale final hologram composed of multichannel holograms without sacrificing the hologram quality. Several 3D imaging applications such as medical diagnosis and advertisements could benefit from this research.     

Write-once recording for multilayered optical waveguide-type holographic cards

We propose a write-once recording technique for multilayered optical waveguide-type holographic cards. The card medium has a construction created by adding a recording layer and a holographic grating layer to the multilayered optical waveguide composed of core and cladding layers. Individual data for each medium were recorded as an arrangement of optically transparent holes formed in the recording layer. Holograms common to all media were designed in the holographic grating layer so that diffracted lights from the holograms could pass through the holes and focus on an image sensor. We succeeded in write-once recording with a memory capacity potential of more than 128 bits.     

Model of holographic recording in thermoplastic materials

A method for the evaluation of images reconstructed from holograms recorded in thermoplastic materials is reported. The method is based on the use of the experimental modulation transfer function and nonlinear holographic characteristics of the recording material. Calculations have been carried out for high-numerical-aperture holograms of a five-element Ronchi ruling. The quality of the reconstructed image as a function of the recording parameters has been computed. The model predicts that it is possible to optimize holographic recording in these materials.     

Efficient holographic recording in novel azo-containing polymer

We report on periodic structures inscribed through holographic recording by both intensity light patterns and polarization patterns in novel azo-containing polymer poly[oxyethylene({6-[4-(biphenyl-4-azo)phenoxy]hexyl}imino) ethylene-carbonylimino(2-methyl-1,3-phenylene)iminocarbonyl] abbreviated as A-TDI. The kinetic of grating recording and diffraction efficiency of recorded gratings using polarized cw Ar⁺ laser light have been measured for s–s, p–p and s–p polarization configurations. The translation grating technique applied during the light self-diffraction process allowed for investigations of phase and amplitude gratings contributions to light diffraction in the studied polymer. Full reversibility of polarization (s–p) grating recording and observation of a half-period structure development during optical erasure process makes this material a good candidate for further studies as a potential material for dynamic holography purposes.     

In-line recording and off-axis viewing technique for holographic particle velocimetry

Prior approaches (e.g., off-axis holography) to overcoming the limitations of in-line holography for particle fields, namely, intrinsic speckle noise and depth resolution, involved an increased complexity of the optical system. The in-line recording and off-axis viewing (IROV) technique employs a single laser beam to record an in-line hologram, which is then viewed off axis during reconstruction. The signal-to-noise ratio and depth resolution of IROV are higher than conventional in-line holography by an order of magnitude and are comparable with off-axis holography. IROV is a much simpler approach than off-axis holography and is highly promising for holographic particle velocimetry. Measurements of the three dimensional flow velocity field of a vortex ring obtained by an IROV-based holographic particle velocimetry system are presented.     

A study of the effect of a corona discharge on recording of holographic diffraction gratings in the Cu-As2S3 structure

Improvement of the optical recording process in the Cu-As₂S₃ structure in the whole visible band was observed when the structure is simultaneously exposed and charged in the field of a negative corona discharge as compared to the standard direct recording in the above-mentioned structure. It was shown that using the negative corona discharge for recording in these structures allows the holographic sensitivity and diffraction efficiency of the recorded holographic diffraction gratings to be increased several-fold compared to gratings obtained by the standard approach. The results are discussed using the known photoelectrical model.     

Nonlinear signal-processing model for scalar diffraction in optical recording

A nonlinear signal-processing model is derived for the optical recording channel based on scalar diffraction theory. In this model, the signal waveform is written in closed form as an explicit function of the channel bits that are stored on an optical disk, thereby comprising both linear and nonlinear terms. Its explicit dependence on the channel bits makes this model well suited for signal-processing purposes. With the model it is also convenient to assess the importance of nonlinear contributions to the signal waveform. The model is applied for one-dimensional optical storage as well as for two-dimensional (2D) optical storage in which bits are arranged on a 2D hexagonal lattice. Signal folding is addressed as a typical nonlinear issue in 2D optical storage and can be eliminated by recording of pit marks of sizes considerably smaller than the size of the hexagonal bit cell. Further simplifications of the model with only a limited number of channel parameters are also derived.     

Optimization of an acrylamide-based dry film used for holographic recording

A study of the optimization and the characteristics of a dry film photopolymerizable recording material is presented. The effects of intensity, the thickness, and the variation of the concentration of each component have been studied. Diffraction efficiencies of 80%, with energetic sensitivities of 40 mJ/cm(2), have been obtained in photosensitive films of a 35-mum thickness with a spatial frequency of 1000 lines/mm.     

Optical wavelet processor by holographic bipolar encoding and joint-transform correlation

A novel optical wavelet processor based on the techniques of the joint-transform correlator and computer-generated holograms is proposed. A coding technique that is a simplified version of Lee's hologram [Appl. Opt. 9, 639 (1970)] is used to represent positive and negative values for the object signal and wavelet functions. We experimentally demonstrate that wavelet transforms of two different daughter wavelet functions can be simultaneously obtained by the appropriate arrangement of the daughter wavelet functions and the object signal on the input plane.     

Half-data-page insertion method for increasing recording density in angular multiplexing holographic memory

We have developed a method to use a half-size data page between two full-size data pages to increase the recording density in angular multiplexing holographic memory up to 1.5× as much as the conventional angular multiplexing sequence. In our recording sequence, the full- and half-size data pages are alternately multiplexed. This is because each plane wave from various points in a data page has different angular selectivity. A half-size data page has higher angular selectivity than a full-size data page. The required angular intervals were estimated by numerical simulation taking holographic medium tilt into account. Also, an angular multiplexing experiment using the half-data-page insertion method resulted in a low bit error rate of the order of 10(-3), which is sufficient for practical use.     

Fabrication and characterization of methylene-blue-doped polyvinyl alcohol-polyacrylic acid blend for holographic recording

A methylene-blue-sensitized polymer blend of polyvinyl alcohol and polyacrylic acid is fabricated and tested for holographic recording. It was found to have good characteristics such as high sensitivity, storage stability, ease of fabrication, and environmental stability. Optimization of the ratio of polyvinyl alcohol/polyacrylic acid, the sensitizer concentration, pH, energy, diffraction efficiency measurements, etc., have been done. pH is found to have a great influence on the recovery of the dye in this matrix. The results of experimental investigations into the properties of this new material are reported.     

Gelatin layers for holographic purposes: an X-ray diffraction study

In order to overcome uniformity problems in large u.v. holoconcentrators recorded in dichromated gelatin, the structure of hardened gelatin layers has been studied. X-ray diffraction patterns show that layers are formed by individual gelatin chains partly associated as triple-stranded helical rods as those found in native collagen. These rods, in turn, may form fibrillar aggregates. Helical rods and microfibrils are, to a great extent, parallel to the layer surface and their relative amount and packing depend on the hardener as well as on temperature and drying time after coating. X-ray diffraction offers the possibility of estimating in a simple manner the relative amount of triple-helical rods in crystallographic register within the gelatin layers. From these results, a method for obtaining uniform large holograms by means of a slow layer drying process is proposed.     

Analysis and simulations of two-wave mixing in photopolymer holographic recording media

Abstract

The process of two-wave mixing in photopolymer recording materials was investigated theoretically. The diffraction grating already forms during exposition and it may influence the original interference field distribution through diffraction of waves on the refractive index modulation. In order to show this, Kogelnik's coupled wave theory was extended to demonstrate the possibility of energy transfer from one recording wave to the other. The energy transfer and the intensity distribution during the recording process were systematically analysed depending on the boundary conditions. As a next step, the first harmonic model of the transmission grating recording, based on a simple material model, was implemented and solved. The ratio of the input intensities was found to be a crucial parameter and thus extensive simulations for various ratios of intensities were carried out. Modelling implies that the interference field and the refractive index grating just coincide for equal intensities. For intensities differing from unity they do not overlap themselves during the recording process. It has also turned out that the diffraction efficiency of the recorded grating drops against the case where the effects of two-wave mixing are not considered. The results of our analysis and simulation help give a better understanding of the physics of the recording process and proper adjustment of recording parameters in such applications as optical holography and holographic memories.     

System measure for persistence in holographic recording and application to singly-doped and doubly-doped lithium niobate

We define a measure for persistence in holographic recording. Using this measure and the known measures for dynamic range and sensitivity, we compare the performance of singly-doped and doubly-doped LiNbO(3) crystals. We show that the range of performance that can be obtained using doubly-doped crystals is much larger than that obtained using singly-doped ones.