Publications

The performance of a series connected photovoltaic array is limited by the photocell that is illuminated the least. This paper quantifies the effects of single-mode and multi-mode illumination and discusses the design parameters.

Abstract not provided.

There has been little interest in the information associated with the shadows in high resolution Synthetic Aperture Radar (SAR) images. In this paper we give an algorithm for the reconstruction of an object's shape from the shadows cast by the object in a sequence of SAR images. The algorithm is a back-projection type algorithm based on the intersection of solids. The effects of diffraction and synthetic aperture occlusion on SAR shadow resolution are also addressed.

A firing set capable of charging a 0.05 μF capacitor to 1.7 kV is constructed using a 2.5 mm diameter Series Connected Photovoltaic Array (SCPA) in lieu of a transformer as the method of high voltage generation. The source of illumination is a fiber coupled 3 W 808 nm laser diode. This paper discusses the performance and PSpice modeling of an SCPA used in a firing set application.

Abstract not provided.

The optical transfer of power is becoming important for military and industrial applications. The powering of electrical circuitry, sensors and actuators over optical fiber offers immunity from RF, EMI, voltage breakdown, lightning and high voltage hazards. Optical power transfer is being employed in industries such as electric power, communications, remote sensing, and aerospace. In this paper we address issues associated with the illumination of Series Connected Photovoltaic Arrays (SCPA). SCPAs are extremely sensitive to the uniformity of illumination. The performance of a photovoltaic array is dominated by the least illuminated cell. We introduce an analytical model that predicts the performance of a photovoltaic array for an arbitrary illumination. Experimental data on array performance is presented, and general issues associated with the problem of producing uniform illumination are discussed.

Airborne synthetic aperture radar (SAR) imaging systems have reached a degree of accuracy and sophistication that requires the validity of the free-space approximation for radio-wave propagation to be questioned. Based on the thin-lens approximation, a closed-form model for the focal length of a gravity wave-modulated refractive-index interface in the lower troposphere is developed. The model corroborates the suggestion that mesoscale, quasi-deterministic variations of the clear-air radio refractive-index field can cause diffraction patterns on the ground that are consistent with reflectivity artifacts occasionally seen in SAR images, particularly in those collected at long ranges, short wavelengths, and small grazing angles.

Applications requiring injection of a high-power multimode laser into multiple fibers with equal energies, or specific energy ratios, provide unique design challenges. As with most all systems, engineering trades must balance competing requirements to obtain an optimal overall design. This is particularly true when fabrication issues are considered in the design process. A few of these competing design requirements are discussed in this conceptually simple system. This fiber injection system consists of three components; a refractive beam homogenizer, a diffractive beamsplitter, and a fiber array. We show the design process, starting with first-order design, for an example fiber injection system that couples a high-power YAG laser into seven fibers. Design goals include high efficiency, good beamsplitting uniformity, compact overall size, maximum mode filling of the fibers, and low cost of fabrication and assembly.

Abstract not provided.

Synthetic Aperture Radar systems are being driven to provide images with ever-finer resolutions. This, of course, requires ever-wider bandwidths to support these resolutions in a number of frequency bands across the microwave (and lower) spectrum. The problem is that the spectrum is already quite crowded with a multitude of users, and a multitude of uses. For a radar system, this manifests itself as a number of 'stay-out' zones in the spectrum mandated by regulatory agencies; frequencies where the radar is not allowed to transmit. Even frequencies where the radar is allowed to transmit might be corrupted by interference from other legitimate (and/or illegitimate) users, rendering these frequencies useless to the radar system. In a SAR image, these spectral holes (by whatever source) degrade images, most notably by increasing objectionable sidelobe levels, most evident in the neighborhood of bright point-like objects. For contiguous spectrums, sidelobes in SAR images are controlled by employing window functions. However, those windows that work well for contiguous spectrums don't seem to work well for spectrums with significant gaps or holes. In this paper we address the question "Can some sorts of window functions be developed and employed to advantage when the spectrum is not contiguous, but contains significant holes or gaps?" A window function that minimizes sidelobe energy can be constructed based on prolate spheroidal wave functions. This approach is extended to accommodate spectral notches or holes, although the guaranteed minimum sidelobe energy can be quite high in this case.

In many applications, the ability to monitor the output of a capacitive discharge circuit is imperative to ensuring the reliability and accuracy of the unit. This monitoring is commonly accomplished with the use of a Current Viewing Transformer (CVT). In order to calibrate the CVT, the circuit is assembled with a Current Viewing Transformer (CVR) in addition to the CVT and the peak outputs compared. However, difficulties encountered with the use of CVRs make it desirable to eliminate the use of the CVR from the calibration process. This report describes a method for determining the calibration factor between the current throughput and the CVT voltage output in a capacitive discharge unit from the CVT ringdown data and values of initial voltage and capacitance of the circuit. Previous linear RLC fitting work for determining R, L, and C is adapted to return values of R, L, and the calibration factor, k. Separate solutions for underdamped and overdamped cases are presented and implemented on real circuit data using MathCad software with positive results. This technique may also offer a unique approach to self calibration of current measuring devices.

We present a technique for determining non-linear resistances, capacitances, and inductances from ring down data in a non-linear RLC circuit. Although the governing differential equations are non-linear, we are able to solve this problem using linear least squares without doing any sort of non-linear iteration.

Abstract not provided.

Imaging systems such as Synthetic Aperture Radar collect band-limited data from which an image of a target scene is rendered. The band-limited nature of the data generates sidelobes, or ''spilled energy'' most evident in the neighborhood of bright point-like objects. It is generally considered desirable to minimize these sidelobes, even at the expense of some generally small increase in system bandwidth. This is accomplished by shaping the spectrum with window functions prior to inversion or transformation into an image. A window function that minimizes sidelobe energy can be constructed based on prolate spheroidal wave functions. A parametric design procedure allows doing so even with constraints on allowable increases in system bandwidth. This approach is extended to accommodate spectral notches or holes, although the guaranteed minimum sidelobe energy can be quite high in this case. Interestingly, for a fixed bandwidth, the minimum-mean-squared-error image rendering of a target scene is achieved with no windowing at all (rectangular or boxcar window).

Superresolution concepts offer the potential of resolution beyond the classical limit. This great promise has not generally been realized. In this study we investigate the potential application of superresolution concepts to synthetic aperture radar. The analytical basis for superresolution theory is discussed. In a previous report the application of the concept to synthetic aperture radar was investigated as an operator inversion problem. Generally, the operator inversion problem is ill posed. This work treats the problem from the standpoint of regularization. Both the operator inversion approach and the regularization approach show that the ability to superresolve SAR imagery is severely limited by system noise.

Superresolution concepts offer the potential of resolution beyond the classical limit. This great promise has not generally been realized. In this study we investigate the potential application of superresolution concepts to synthetic aperture radar. The analytical basis for superresolution theory is discussed. The application of the concept to synthetic aperture radar is investigated as an operator inversion problem. Generally, the operator inversion problem is ill posed. A criterion for judging superresolution processing of an image is presented.

The three most important factors effecting the difficulty of a beam shaping problems were discussed. These factors were scaling, smoothness, and coherence. Algorithms were developed to counteract these factors encountered in the design of any beam shaping system.

Industrial, military, medical, and research and development applications of lasers frequently require a beam with a specified irradiance distribution in some plane. A common requirement is a laser profile that is uniform over some cross-section. Such applications include laser/material processing, laser material interaction studies, fiber injection systems, optical data image processing, lithography, medical applications, and military applications. Laser beam shaping techniques can be divided into three areas: apertured beams, field mappers, and multi-aperture beam integrators. An uncertainty relation exists for laser beam shaping that puts constraints on system design. In this paper the authors review the basics of laser beam shaping and present applications and limitations of various techniques.

Injection of high power, multi-mode laser profiles into a fiber optic delivery system requires controlling a number of injection parameters to maximize throughput and minimize concerns for optical damage both at the entrance and exit faces of the fiber optic. A simple method for simultaneously achieving a compact fiber injection geometry and control of these injection parameters, independent of the input source characteristics, is provided by a refractive lenslet array and simple injection lens configuration. Design criteria together with analytical and experimental results for the refractive lenslet array and short focal length injection lens are presented. This arrangement provides a uniform spatial intensity distribution at the fiber injection plane to a large degree independent of the source mode structure, spatial profile, divergence, size, and/or alignment to the injection system. This technique has application to a number of laser systems where uniform illumination of a target or remote delivery of high peak power is desired.