Efficient method for controlling the spatial coherence of a laser.

An efficient method to tune the spatial coherence of a degenerate laser over a broad range with minimum variation in the total output power is presented. It is based on varying the diameter of a spatial filter inside the laser cavity. The number of lasing modes supported by the degenerate laser can be controlled from 1 to 320,000, with less than a 50% change in the total output power. We show that a degenerate laser designed for low spatial coherence can be used as an illumination source for speckle-free microscopy that is nine orders of magnitude brighter than conventional thermal light.

Coupled lasers: phase versus chaos synchronization.

The synchronization of chaotic lasers and the optical phase synchronization of light originating in multiple coupled lasers have both been extensively studied. However, the interplay between these two phenomena, especially at the network level, is unexplored. Here, we experimentally compare these phenomena by controlling the heterogeneity of the coupling delay times of two lasers. While chaotic lasers exhibit deterioration in synchronization as the time delay heterogeneity increases, phase synchronization is found to be independent of heterogeneity. The experimental results are found to be in agreement with numerical simulations for semiconductor lasers.

Phase locking of lasers with self-stabilized minimal coupling.

A novel configuration for phase locking two ring lasers with self-stabilized minimal exchange of power between them is presented. We show experimentally that losses introduced between the lasers are self compensated in order to maintain minimal power exchange between them. The experimental results are in good agreement with numerical results.

Enhanced coherence of weakly coupled lasers.

We present the phase-locking and coherence properties between two weakly coupled lasers. We show how the degree of coherence between the two lasers can be enhanced by nearly 1 order of magnitude after taking into account the effects of coupling on both their phases as well as their amplitudes. Specifically, correlations between synchronized spikes in the amplitude dynamics and the phase dynamics of the lasers allow for an interference pattern with a fringe visibility of 90%, even when the coupling strength is far below the critical value and they are not phase locked.

Rapid laser solver for the phase retrieval problem.

Tailored physical systems were recently exploited to rapidly solve hard computational challenges, such as spin simulators, combinatorial optimization, and focusing through scattering media. Here, we address the phase retrieval problem where an object is reconstructed from its scattered intensity distribution. This is a key problem in many applications, ranging from x-ray imaging to astrophysics, and currently, it lacks efficient direct reconstruction methods: The widely used indirect iterative algorithms are inherently slow. We present an optical approach based on a digital degenerate cavity laser, whose most probable lasing mode rapidly and efficiently reconstructs the object. Our experimental results suggest that the gain competition between the many lasing modes acts as a highly parallel computer that could rapidly solve the phase retrieval problem. Our approach applies to most two-dimensional objects with known compact support, including complex-valued objects, and can be generalized to imaging through scattering media and other hard computational tasks.

Anisotropic Poisson's ratio and compression modulus of cortical bone determined by speckle interferometry.

Young's modulus and Poisson's ratios of 6mm-sized cubes of equine cortical bone were measured in compression using a micro-mechanical loading device. Surface displacements were determined by electronic speckle pattern-correlation interferometry. This method allows for non-destructive testing of very small samples in water. Analyses of standard materials showed that the method is accurate and precise for determining both Young's modulus and Poisson's ratio. Material properties were determined concurrently in three orthogonal anatomic directions (axial, radial and transverse). Young's modulus values were found to be anisotropic and consistent with values of equine cortical bone reported in the literature. Poisson's ratios were also found to be anisotropic, but lower than those previously reported. Poisson's ratios for the radial-transverse and transverse-radial directions were 0.15+/-0.02, for the axial-transverse and axial-radial directions 0.19+/-0.04, and for the transverse-axial and radial-axial direction 0.09+/-0.02 (mean+/-SD). Cubes located only millimetres apart had significantly different elastic properties, showing that significant spatial variation occurs in equine cortical bone.

Modeling supra-molecular helices: extension of the molecular surface recognition algorithm and application to the protein coat of the tobacco mosaic virus.

Geometric matching of molecular surfaces appears to be essential for the formation of binary molecular complexes and of supra-molecular aggregates. The structure of a binary complex is characterized by the best geometric match, whereas the structure of an aggregate is characterized by the best combined match, i.e. the sum of all the internal matches in the system. We describe a method to identify and quantify the binary matches between molecules and then use them to form the supra-molecular helices and evaluate them. This method is applied to the single protein subunit of tobacco mosaic virus. It successfully predicts the structure of the helical protein coat of the virus and the structure of the disk that is formed as the initial step in the virus assembly process. It also predicts structural intermediates, between disk and helix, which explain how the disk can transform into a helix without dissociating into subunits.

Phase locking of lasers with intracavity polarization elements.

New configurations for phase locking several laser beams with intracavity polarization elements are presented. With this configuration we demonstrated efficient phase lock of up to 24 ND:YAG laser beams with only two polarization beam displacers.

Color correction in planar optics configurations.

Color correction in planar optics configurations can be achieved by resorting to gradient-index rather than uniform-refractive-index substrates. The basic configuration, principle of correction, and calculated and experimental results are presented. The results reveal that, with an appropriate refractive index distribution along the thickness of the substrates, the color can be corrected over a wavelength range up to 155 nm depending on incidence angles.

Intracavity coherent addition of 16 laser distributions.

The efficient intracavity coherent addition of 16 separate laser Gaussian mode distributions is presented. The coherent addition is achieved in a multichannel pulsed Nd:YAG laser resonator by use of four intracavity interferometric beam combiners. The results reveal 88% combining efficiency with a combined output beam of nearly pure Gaussian distribution.

Intracavity coherent addition of single high-order modes.

We report on efficient intracavity coherent addition of several single high-order mode distributions in a multichannel laser resonator. The phase locking and coherent addition is achieved by using an intracavity interferometric beam combiner. The principle, configuration, and experimental results with pulsed Nd:YAG Laguerre-Gaussian TEM01 and TEM02 laser beam distributions are presented. The results reveal more than 95% combining efficiency with a nearly pure high-order mode output beam distribution in both free-running and Q-switched operation.

Rotation-invariant correlation with incoherent light.

We have incorporated an algorithm for rotation invariance, based on circular-harmonic expansion, into a pattern recognition system that uses incoherent light. Simulations and experimental results reveal that the algorithm can provide adequate peak-to-background ratios regardless of the input rotation.

Spatial multiplex tomography.

We present a new method for the synthesis of sections of a 3-D object from an encoded record of its projections. This record is prepared by spatially multiplexing selected projections of the original object onto a photographic film. The film is then inserted into a coherent optical analyzing system, where an image of one planar section is generated; only a single plane of object is in sharp focus, while the rest of the object is blurred. The location of the sharp plane may be chosen arbitrarily with no need to recall the original object. Analysis, supported with experimental verification, is presented.

Fringe formation in two-wavelength contour holography.

This paper presents a description of fringe formation in two-wavelength contour holography. Approximations are introduced primarily by restricting the recording wavelength difference with the aid of a tunable dye laser and by observing the contour fringes through a controlled viewing system aperture. Several holographic recording and readout arrangements are presented for which explicit and conveniently interpretable contour fringe formulations are derived. Finally, the results from a set of experiments performed with each of the arrangements are offered as experimental evidence supporting the analysis.

Parallel image transmission by a single optical fiber.

A nonscanning technique in which an image of a two-dimensional input transparency is transmitted in parallel through a single, round, step-index optical fiber is described. To ensure that the output display is not smeared, the input light distribution is first spectrally and spatially coded, and after transmission through the optical fiber it is decoded. Both coding and decoding operations are achieved solely by means of conventional, passive optical components.

Image restoration using volume diffraction gratings.

The angular selectivity of volume diffraction gratings is exploited to form a filter that alters the relative amplitude and phase of selected frequency components to improve the output imagery of blurred data.

Multicolor wavefront reconstruction.

Two and three primary colors derived from an He-Ne gas laser and an argon gas laser were employed in recording and reconstructing holograms. For the tricolor case it is possible to reconstruct a three-dimensional multicolor image which possesses almost all the natural hues of the original object. Each wavelength generates an independent fringe system that is recorded on a photographic plate with a thick emulsion that constitutes a three-dimensional recording medium. In reconstruction, each fringe system diffracts light in a manner satisfying the Bragg relation for a particular reconstructing wavelength. If the reconstruction wavelengths are the same as the original wavelengths used to record the fringe systems, the result is a multicolor reconstruction possessing few or no ghost images. In our experiments, the angle between the object beam and the reference beam was greater than 100 degrees , and the photographic plates were oriented so that the fringe surfaces were approximately perpendicular to the emulsion surfac This minimized the deleterious effects of single-color, ghost image formation and shrinkage during development. Finally, a method of synthesizing multicolor scenes using a multiple-exposure recording with one wavelength and reconstructing with several wavelengths is described.

Effects of film nonlinearities in holography.

A study of the effects of photographic nonlinearities in recording coherent wavefronts is reported. Particular emphasis is placed on holography using the two-beam interferometry technique. A phenomenological model which provides the mathematical formulation for describing the effects of nonlinearities is described. The model includes a zero-memory nonlinearity which represents experimentally derived transmittance-exposure curves for various photographic emulsions. An analysis of this model reveals many interesting phenomena which are supported experimentally. In particular, the nonlinearity of the film generates false targets, causes weak signal suppression, and can introduce additional noise as a result of spectral folding. Experimental results, verifying the analysis, are presented.

Thick absorption recording media in holography.

The response of holograms recorded in thick absorption media is evaluated in terms of diffraction efficiency, angular orientation sensitivity, and wavelength sensitivity. Calculated results showing the relation between these measures of response and various hologram recording parameters are presented. These calculations are supported with experimental data obtained with thick photographic emulsions and various photochromic materials. In addition, experimental results are given which demonstrate the use of thick recording media in holographic data storage and multicolor wavefront reconstruction.

All-optical neural network with inhibitory neurons.

A Hopfield-type neural network with only inhibitory interconnections is shown to perform similarly to networks with both inhibitory and excitatory interconnects. With the use of only inhibitory neurons the need to subtract light intensities is eliminated, so an all-optical neural network can be realized. This realization is achieved with a liquid-crystal light valve for implementing a two-dimensional array of 16 inhibitory neurons and an array of subholograms for the interconnections. Two stable states that can be retrieved from noisy optical inputs are demonstrated.