Gary W. Carhart

Adaptive phase-distortion correction based on parallel gradient-descent optimization

We describe an adaptive wave-front control technique based on a parallel stochastic perturbation method that can be applied to a general class of adaptive-optical system. The efficiency of this approach is analyzed numerically and experimentally by use of a white-light adaptive-imaging system with an extended source. To create and compensate for static phase distortions, we use 127-element liquid-crystal phase modulators. Results demonstrate that adaptive wave-front correction by a parallel-perturbation technique can significantly improve image quality.

Synthetic imaging:?nonadaptive anisoplanatic image correction in atmospheric turbulence

We introduce a synthetic-imaging technique that can be applied to correct anisoplanatic images degraded by atmospheric turbulence. This method is based on local image-quality analysis applied to a large set of short-exposure images and can be considered a generalization of the frame-selection technique. Experimental results obtained for atmospheric data demonstrate the efficiency of the synthetic-imaging technique in improving image quality.

Image quality criteria for an adaptive imaging system based on statistical analysis of the speckle field

We introduce image quality criteria for organizing feedback control in adaptive imaging systems. These image quality criteria are dependent on the Fourier spectrum of the image and can be obtained optically with a coherent optical system. Digital processing of the image plane intensity distribution is not required. We present experimental results, along with corresponding numerical simulations, that demonstrate the potential effectiveness of these criteria for adaptive correction of phase-distorted extended-source images.

Adaptive imaging system for phase-distorted extended source and multiple-distance objects

We introduce an incoherent adaptive imaging system based on optimization of an image quality metric measured using a coherent optical system. Experimental results and numerical simulations are presented that demonstrate adaptive correction of phase-distorted extended source images containing objects located at multiple distances.

Optical simulation of phase-distorted imaging systems: nonlinear and adaptive optics approach

We discuss a technique based on nonlinear and adaptive optics for simulation of phase-distortion effects in imaging systems. This technique uses a nonlinear two-dimensional optical feedback system to produce a controllable spatially and temporally varying chaotic intensity distribution. The intensity pattern is converted into a spatially varying thin phase screen using an optically addressed spatial phase modulator. A chaotic phase distortion is then introduced into an imaging systems output image by propagation through the phase screen. A deformable mirror with computer control is used for simulation of large-scale phase distortions.

Parallel perturbation gradient descent algorithm for adaptive wavefront correction

An adaptive imaging system with a liquid crystal phase modulator having 127 individually addressed hexagonal elements is experimentally studied. The system operation is based on direct optimization of an image quality metric dependent on image plane intensity distribution. For optimization of the image quality metric we applied a modified version of a stochastic perturbation gradient descent algorithm. Experimental results demonstrated the efficiency of the algorithm for high- resolution adaptive wavefront correction in an imaging system. A modification of the algorithm that significantly accelerates algorithm convergence is suggested and studied by numerical simulation.

Turbulent Phase Screen for the Study of Imaging System Performance

Abstract We introduce a nonlinear optical technique for the creation of small-scale phase distortions in coherent optical systems. This technique uses the nonlinear response of a liquid-crystal light valve placed in a two-dimensional optical feedback system to produce a controllable spatially and temporally varying chaotic intensity distribution. The intensity pattern is converted into a spatially varying thin phase screen using a second liquid-crystal light valve. A chaotic phase distortion is then introduced into the output image of a coherent imaging system by propagation through the phase screen.

Optimum classification of correlation-plane data by Bayesian decision theory

A multimodal model for correlation-plane distributions generated by composite filters is presented. From this model a statistical classifier referred to as a composite Bayesian classifier is developed. By exploiting the Gaussian behavior of correlation-plane data, this classifier concisely represents multimodal distributions as composite algebraic functions. These multimodal distributions, each of which is constructed by superposition of many normal distributions, are used to partition a vector signal space into optimum classification regions derived from Bayess likelihood ratio test. For the purpose of validating the multimodal model, expected performance for the training images is derived from calibration data and compared with observed performance.

Composite filter trees and image recognition via binary search

Construction rules for building composite binary phase-only filters (CBPOFs) designed for use at the nodes of a binary tree have been developed using upweighted superposition algorithms. This paper describes the use of this method to build a four-target, rotation-invariant CBPOF bank on which binary tree searches can be performed. Each of the four trees in this filter bank consists of 256 simple filters (BPOFs) and 254 composites. Sequential search of the 256 simple filters is replaced by a binary search which uses at most 26 composites and 4 simple BPOFs. An image recognition system utilizing the filter bank has been developed, assembled, and evaluated by simulation and experiment. The empirical results obtained using a hybrid optical correlator with computer-controlled magneto-optic spatial light modulators (MOSLMs) at the input and filter planes are presented.

Adaptive correction of phase-distorted extended-source images

An experimental model of an adaptive imaging system for compensation of large-scale phase distortions under conditions of strongly anisoplanatic imaging is studied. We investigate adaptive compensation of phase-distorted extended source images using new types of image quality criteria. These image quality criteria are dependent on the Fourier spectrum of the image, and can be obtained using a coherent optical system. For adaptive control of a ten electrode continuously deformable bimorph mirror, simple gradient algorithms for image quality criteria were applied. We experimentally demonstrate the efficiency of large-scale phase distortion compensation for extended and complex targets.