Cardinal Warde

Anisotropic scattering in photorefractive crystals

A well-defined ring of anisotropically scattered light is observed when a linearly polarized laser beam is incident upon a birefringent photorefractive crystal such as BaTiO3, LiNbO3, and LiTaO3. An analysis is presented that accurately predicts the cone angle, polarization, and the location of the discontinuities in these rings. Unlike the photovoltaic model proposed recently by others, the analysis presented herein is based on the standard photorefractive theory and is independent of the photovoltaic activity of the crystal. The analysis also illustrates the strict phase matching and electro-optic tensor symmetry constraints that must be satisfied in order to observe beam coupling and normal and anisotropic self-diffraction. Preliminary experimental results are presented for BaTiO3 and LiNbO3 and are found to be in excellent agreement with standard photorefractive theory.

Optical studies of cerium doped yttrium aluminum garnet single crystals

Single crystals of undoped and cerium‐doped yttrium aluminum garnet have been pumped with laser radiation of 220‐ and 266‐nm wavelength. A broadband defect emission at approximately 300 nm was observed, similar to that obtained with cathodoluminescence, although decreased in magnitude. Evidence of a transfer of energy from these defect states to the Ce+3 states was inferred from the photoluminescence decay time measurements. We have associated the absorption spectra from 200–300 nm with the defect excitation spectrum, including the peaks at 225 and 270 nm.

Microchannel spatial light modulator

A sensitive, high-speed, optically addressed spatial light modulator is being developed for real-time optical dataprocessing applications involving low-level-control light signals. This device is expected to have a sensitivity of better than 10(-3) microJ/cm(2), a framing rate of about 1 kHz, and a spatial resolution of approximately 20 line pairs/mm. The time response, sensitivity, spatial resolution, and other operating characteristics are being studied in a first-generation device; preliminary results are reported.

Optical information processing characteristics of the microchannel spatial light modulator

The microchannel spatial light modulator (MSLM) is a versatile, highly sensitive, and optically addressed modulator that is well suited for low-light level real-time optical information processing. The image processing operations that can be achieved with the MSLM include contrast reversal, contrast enhancement, edge enhancement, image addition and subtraction, analog and digital intensity level thresholding, and binary level logic operations such as AND, OR, EXCLUSIVE OR, and NOR. Several of these operations are demonstrated herein. Recent prototype MSLMs have exhibited a halfwave exposure of 2.2 nJ/cm(2), an optical information storage time of more than two months, and a framing rate of 40 Hz with full modulation depth (200 Hz with 20% modulation depth). The role of secondary electron emission in the operation of the MSLM is discussed, and design modifications that would yield a spatial resolution of ~10 cycles/mm at the 50% point on an MTF curve are proposed.

Temporal characteristics of single-scatter radiation

An efficient and versatile model that describes the temporal characteristics of scattered radiation when single-scatter conditions prevail is proposed and developed. The model is based on the focal radii property of a prolate spheroid, and for an impulsive source located at one focal point and an observer at the other, it associates all scattering events occurring on a given prolate spheriodal surface with the same transit time. All interactions between the radiation and matter are classified as either elastic scattering or absorption, and expressions are obtained for the intensity and the collected energy as a function of time at the observation point. The model is applied to isotropic, Rayleigh, and Mie-type scattering; the single-scatter phase function for Mie-type scattering is approximated by the Henyey-Greenstein function. For simple source and observer geometries, closed-form expressions are obtained for the intensity in the isotropic and Rayleigh cases. Finally, the model is applied to examples which typify middle ultraviolet radiation propagating in the earth’s atmosphere.

Defect luminescence in cerium‐doped yttrium aluminum garnet

The absorption and photoluminescence of lattice defects and of the cerium ion (Ce+3) in cerium‐doped yttrium aluminum garnet have been measured after annealing single crystals of this material in various reducing atmospheres. The absorption in the 200–300‐nm wavelength range is observed to decrease as the partial pressure of oxygen (PO2) in the annealing atmosphere decreases, while both the cerium emission at 550 nm and the lattice defect luminescence at 300 nm increases. The lattice defect emission appears to be associated with electrons located on oxygen vacancies.

High beam-coupling gain and deep- and shallow-trap effects in cobalt-doped barium titanate, BaTiO 3 :Co

A series of cobalt-doped barium titanate (BaTiO3) crystals were grown; these crystals have the highest beam-coupling gain reported to date, 7.9 cm−1, measured by using the smallest electro-optic coefficient of BaTiO3. The intensity dependence of the absorption and the gain indicate that there are at least two active photorefractive species. These samples show light-induced absorption and negative absorptive coupling, which is indicative of crystals with deep and shallow traps. However, doping with cobalt diminishes the intensity dependence of the electro-optic gain and increases the electro-optic and absorptive coupling, which indicates that cobalt is a deep-level photorefractive dopant. The higher gain is attributed to an increase of the total effective trap density and to an increase in the deep- and shallow-trap intensity-dependent factor η(I), with higher cobalt doping.

Defect-property correlations in garnet crystals. VI: The electrical conductivity, defect structure, and optical properties of luminescent calcium and cerium-doped yttrium aluminum garnet

The electrical and optical properties of calcium and cerium‐doped yttrium aluminum garnet (Ca,Ce:YAG) have been studied. Ca,Ce:YAG is a mixed ionic and electronic conductor with an ionic conductivity activation energy of 4.3 eV. Evidence of cluster formation with a consequent higher‐than‐expected activation energy is presented. The cerium normally enters the crystal as Ce+4, but it may be converted to Ce+3 under reducing atmospheres at elevated temperatures.

Operating Modes Of The Microchannel Spatial Light Modulator

The fundamental operating characteristics and materials limitations of the optically addressed microchannel spatial light modulator (MSLM) are discussed. The role of secondary electron emission in the operation of the device is stressed, and some of the write, cycling, and readout modes, and their limitations, are described. In addition, the limitations of the inherent space-domain image processing operations of the device are discussed.

Real-time joint spectral-spatial matched filtering

Abstract A technique for real-time joint spectral-spatial matched filtering is presented. The technique involves the real-time conversion of an input color image into a set of sampled monochromatic coherent images through the use of a tricolor grid and real-time spatial light modulator. These color-coded monochromatic images are then processed by a multi-channel, coherent matched filtering subsystem. The advantage of this technique over prior matched filtering methods is its ability to use both color and shape in recognizing objects.