Photosensing Arrays with Improved Spatial Resolution

The spatial resolution obtainable in a photosensing array used for optical imaging may be limited by the diffusion of photogenerated carriers within a uniformly doped semiconductor even if other components of the optical system are optimized and scattered light is reduced. A technique has been developed to improve the spatial resolution for critical applications by incorporating subsurface electric fields that accelerate the photogenerated carriers toward or away from the surface so that the carriers are prevented from diffusing to distant photosensing elements. The subsurface fields are obtained by incorporating suitable dopant concentration gradients into the structure. In one structure fabricated the subsurface field was formed by using a heavily doped buried layer and a lightly doped epitaxial film over a lightly doped substrate, all of the same conductivity type. This structure is compatible with the incorporation of other semiconductor devices in the same monolithic substrate. The technique has been applied to an array of photodiodes in a silicon integrated circuit, but the principle is directly applicable to other types of photosensing arrays, such as charge-coupled devices (CCD's), and other semiconductor materials.     

Ambiguity resolution in self-cohering arrays

Two approaches to resolving the phase ambiguity associated with phase multilateration of self-cohering antenna arrays are described and the probability of ambiguity error is derived for each approach. For the minimum least square error method an efficient computational technique is introduced which permits element position uncertainties as large as one wavelength in the presence of phase measurement errors in the order of one rad. The multiple frequency method permits element position uncertainties significantly larger than one wavelength, at some increase in bandwidth. The probability of ambiguity error is shown to be acceptably small if the root mean square (rms) phase measurement errors are in the order of 0.5 rad or smaller.     

Analysis of spatial smoothing with uniform circular arrays

We analyze spatial smoothing with uniform circular arrays (UCAs). In particular, we study the performance of the Root-MUSIC with smoothing in the presence of correlated sources, finite data perturbations, and errors in the transformed steering vector that arise due to some approximations made while extending the Root-MUSIC and smoothing to UCA. Expressions are derived for the asymptotic performance of the Root-MUSIC with smoothing applied to the transformed UCA data. An attempt has been made to bring out the impact of both the forward and forward-backward smoothing. We consider UCAs with isotropic as well as directional sensors in our study. Computer simulations are provided to demonstrate the usefulness of the analysis     

Performance analysis of spatial smoothing with interpolated arrays

The interpolated spatial smoothing algorithm is a computationally efficient method for estimating the directions of arrival (DOAs) of signals, some of which may be perfectly correlated. It extends the spatial smoothing method to arbitrary array geometries. A statistical performance analysis of the algorithm is presented. Closed-form expressions for the covariance matrix of the DOA estimation errors are derived using a perturbation analysis. Evaluating these expressions for specific cases and comparing them to the Cramer-Rao lower bound for the DOA estimates provides insight into the statistical efficiency of this algorithm. The formulas for the error covariance are quite general and can be specialized to provide results for other DOA estimation algorithms as well     

Microwave holographic imaging method with improved resolution

A multifrequency holographic microwave imaging method with high resolving capability in both azimuth and range directions, is described. The method incorporates a synthetic aperture approach in pulse radar systems, originally designed for the pulse-echo imaging, in order to improve azimuth resolution. The imaging technique places no constraint on the waveform of a transmitted signal in use and allows the three-dimensional imaging. This paper develops 1) the general formula, which describes the relation between the transmitted and received signals, based on Kirchhoff diffraction theory, 2) a test function, which is the kernel of a linear operation for producing object images from received signals, and 3) a filter design technique for compressing the durations and suppressing the sidelobe levels of received signals. Numerical image reconstructions are presented for demonstrating high resolution capability of the method.     

Single-mode optical-fibre index profiling with improved resolution

A method is given which enables one to double the resolution of the transverse interferometric technique for the index profiling of an optical fibre. This method makes use of the measured data of the phase and the deflection function of the fibre. It should find applications in the profiling of single-mode fibres which have small core diameter and the measuring instruments are limited in resolution.     

Synthesis of antenna arrays with spatial and excitation constraints

Synthesis of antenna arrays subject to spatial and excitation constraints to yield arbitrarily prescribed patterns in both the mean-squared and minimax sense are discussed. The spatial constraints may require that the interelement spacings be greater than a prescribed value or that the element locations lie within a specified region. The excitation constraints are of the form where the current-taper ratio is constrained to be less than or equal to a prescribed value. The technique employed consists of reducing the constrained optimization problem into an unconstrained one by the use of simple transformations of the independent variables. In such cases where explicit transformations are not available, a created response surface technique (CRST) has been used to convert the constrained optimization problem into a series of unconstrained optimizations. The optimization has been carried out using a nonlinear simplex algorithm. Numerical examples are given wherein both the linear and circular arrays are synthesized subject to constraints.     

Source localization with distributed sensor arrays and partial spatial coherence

Multiple sensor arrays provide the means for highly accurate localization of the (x,y) position of a source. In some applications, such as microphone arrays receiving aeroacoustic signals from ground vehicles, random fluctuations in the air lead to frequency-selective coherence losses in the signals that arrive at widely separated sensors. We present performance analysis for localization of a wideband source using multiple, distributed sensor arrays. The wavefronts are modeled with perfect spatial coherence over individual arrays and frequency-selective coherence between distinct arrays, and the sensor signals are modeled as wideband, Gaussian random processes. Analysis of the Cramer-Rao bound (CRB) on source localization accuracy reveals that a distributed processing scheme involving bearing estimation at the individual arrays and time-delay estimation (TDE) between sensors on different arrays performs nearly as well as the optimum scheme while requiring less communication bandwidth with a central processing node. We develop Ziv-Zakai bounds for TDE with partially coherent signals in order to study the achievability of the CRB. This analysis shows that a threshold value of coherence is required in order to achieve accurate time-delay estimates, and the threshold coherence value depends on the source signal bandwidth, the additive noise level, and the observation time. Results are included based on processing measured aeroacoustic data from ground vehicles to illustrate the frequency-dependent signal coherence and the TDE performance.     

Microwave radiometer spatial resolution enhancement

Scanning radiometer data processing can allow enhancing the limited intrinsic spatial resolution. This is important for data fusion. Mathematically, the problem to be solved is an inverse ill-posed problem. In this paper we compare the classical Backus-Gilbert inversion method with the truncated singular value decomposition (TSVD) one. A one-dimensional intercomparison is accomplished using an hypothetical sensor configuration. Results show the superiority of TSVD inversion method.     

Microwave frequency measurement with improved measurement range and resolution

An approach is proposed and demonstrated to improve the measurement range and resolution of a microwave frequency measurement system. Two optical wavelengths are modulated by a microwave signal in a Mach-Zehnder modulator (MZM). The optical output from the MZM is sent to a dispersive element to introduce different chromatic dispersions, leading to different microwave power penalties. A fixed relationship between the microwave power ratio and the microwave frequency is established. The microwave frequency is estimated by measuring the microwave powers. A new measurement range is defined with an improved measurement range and resolution. Analysis is performed and confirmed by an experiment.     

Kernel based visual tracking with variant spatial resolution model

Kernel based tracking (KBT) has achieved considerable success in the visual tracking field owing to its simplicity and speediness. But the target's scale and rotation parameters cannot be estimated efficiently within its own framework. To complement the ability of KBT, both Cartesian and Log-polar co-ordinates are used as a joint tracking coordinate. By using the target's space-variant resolution model, the proposed algorithm can estimate the target's scale and rotation parameters.     

An improved woodward method for separate arrays synthesis

Based on the property of sample function, the Woodward synthesis method for separate arrays is improved. The improved method is justified theoretically, and the sample function of the flat-topping and 30 degree cosecant-squared space segment pattern on radar phased array antenna is synthesized using it. Through simulation and analysis, it can be used fewer elements to make the same synthesis effect on the pattern with the traditional Woodward method. And if the same number of elements is used, the synthe-sized pattern using the improved method will have better approach to the sample function, narrower beam width, and higher gain.     

A novel quadrilinear CCD for high resolution line arrays

This paper presents a novel quadrilinear CCD, allowing both serial and parallel clocking with one set of busbars common to inner and outer registers on the same side of the array. This is made possible by the special organization of the charge transfer from inner to outer registers. Line arrays having 384, 808, 1728, and 3456 sensors have been fabricated successfully on a pitch of 8 µm; the sensor nonuniformity is less than ±5 percent and the transfer loss is 1.10^-5in both inner and outer registers.     

Microoptical phased arrays for spatial and spectral switching

This article describes two optical devices based on linear arrays of micromirrors. The first is a phased array of micromirrors that can be rotated as well as translated vertically to maintain coherence across the array. We demonstrate experimentally that such micromirrors are capable of high-diffraction-efficiency phased-array scanning of laser beams. The second device is a Gires-Tournois (1969) interferometer with a micromirror array that provides tunable phase modulation for the multitude of partially reflected beams within the interferometer. We demonstrate experimentally that the MEMS-GT interferometer can operate as a tunable deinterleaver for dense wavelength-division multiplexed fiber optic communication.     

Imaging with solid immersion lenses, spatial resolution, and applications

Solid immersion microscopy, similar to liquid immersion microscopy, extends the diffraction limit by filling the object space with a high refractive index material, such as glass (index of refraction n=1.5-2), sapphire (n/spl sim/1.8), and semiconductor materials (n/spl sim/3), which shrink the wavelength of light. But solid immersion technique can achieve significantly higher spatial resolution since the refractive indices of available solids can be much higher than those of liquids (n=1.3-1.5). Besides high spatial resolution, solid immersion microscopy also possesses all the good properties of far-field imaging, such as high transmission efficiency and parallel imaging capability, which make it outstanding among beyond-the-diffraction-limit optical imaging techniques. In this paper, we discuss, from an experimental point of view, the resolution limit of solid immersion microscopy and the implementation of such technique in various applications.     

Improved angular resolution for spatial smoothing techniques

Spatial smoothing techniques are used to estimate the directions of coherent arrivals. The performance of these techniques deteriorates rapidly as the coherent arrivals become closely spaced. The author examines the resolution performance of the existing forward-only and forward/backward spatial smoothing techniques. It is shown that the resolution for coherent signals can be improved by squaring array covariance matrices before forming smoothed array covariance matrices     

High spatial resolution optical fibre mode profiling

Near-field scanning optical microscopy is used in emission mode to measure the mode profile of singlemode fibre. Excellent agreement is obtained between the mode profile of a singlemode fibre measured using emission and collection modes and qualitative agreement obtained between measurements made on a multimoded fibre and a simple theoretical model.     

The spatial resolution limit of electron lithography

As the size of microcircuit elements continues to decrease it is of importance to attempt to determine what fundamental factors might limit the width, and spacing, of lines fabricated by electron lithography when using conventional polymer resists. In this paper a new Monte Carlo simulation, which considers the contribution of both primary and secondary electrons, is used to model two special situations. Firstly, the case of a thin, freestanding, resist is considered to examine the factors which set a limit to the minimum attainable line width. Secondly, the case of a thin resist on a bulk substrate is considered to calculate the exposure conditions required for the highest resolution lithography in this condition.     

The resolution threshold of a direction-finding algorithm fordiversely polarized arrays

Subspace-based algorithms for narrowband direction-of-arrival (DOA) estimation require detailed knowledge of the array response (the array manifold) and assume that the noise covariance matrix is known up to a scaling factor. In practice, these quantities are not known precisely. Resolution and estimation accuracy can degrade significantly when the array response or the noise covariance deviate from their nominal values. We examine the resolution threshold of a recently proposed subspace-based algorithm for direction finding with diversely polarized arrays. We study finite sample effects, and the effects of modeling errors (errors in the array manifold or the noise covariance), on the resolution threshold. A comparison is made between the resolution thresholds of the MUSIC algorithm (for uniformly polarized arrays) and the proposed algorithm (for diversely polarized arrays)