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At least 451 records · Page 25

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NASA NTRS1984

Supermode analysis of phase-locked arrays of semiconductor lasers

The optical characteristics of phase-locked semiconductor laser arrays are formulated in terms of the array supermodes, which are the eigenmodes of the composite-array waveguide, by using coupled-mode theory. These supermodes are employed to calculate the near fields, the far fields, and the difference in the longitudinal-mode oscillation wavelengths of the array. It is shown that the broadening in the far-field beam divergence, as well as the broadening of each of the longitudinal modes that were observed in phase-locked arrays, may arise from the excitation of an increasing number of supermodes at increasing pumping levels.

Kapon, E.↗

NASA NTRS1983

Computing the projected area of a spacecraft solar array with matrices

Matrix methods for computing the projected area of a solar array as a function of the rotational position of a spacecraft and of array position on a spacecraft are presented. Formulas are derived which provide the optimum solar array pitch, cant and tracking angles for a given spacecraft configuration and orbit. These formulas are general and applicable to many spacecraft. Formulas are also provided for determining the energy output from an array for a given orbit and the resultant energy available to spacecraft loads. Results are simply obtained and realizable with a hand calculator. The methods above can be extended to the case of computer analysis of solar array shadowing. How this can be done is outlined and results are presented from a spacecraft study program.

E. M. Gaddy↗

NASA NTRS1984

Chirped arrays of diode lasers for supermode control

Nonuniform structures of phase-locked diode lasers, which make it possible to discriminate efficiently against all the higher order array supermodes (lateral modes) are proposed. In these nonuniform arrays, the effective mode index in each channel varies across the array. Consequently, the envelopes of the various supermodes, including the highest order one, differ significantly from each other. Thus, by proper tailoring of the gain distribution across the array, one can conveniently select the fundamental supermode. Such fundamental supermode oscillation is essential in order to obtain single lobe, diffraction limited beams and minimal spectal spread from phase-locked laser arrays.

Kapon, E.↗

NASA NTRS1984

Rate equations analysis of phase-locked semiconductor laser arrays under steady state conditions

Rate equations analysis of phase-locked semiconductor laser arrays has been carried out. It was found that for given (laser) current densities, the photon density distribution in the array elements is that particular one which maximizes the total photon density. The results of this analysis were then combined with the waveguide properties of the laser array waveguide, yielding a basic model of phase-locked diode laser arrays. This model explains the effects of the variation of the current combination through the array elements on its mode structure that were observed recently.

Katz, J.↗

NASA NTRS1984

Controlled fundamental supermode operation of phase-locked arrays of gain-guided diode lasers

Uniform semiconductor laser arrays tend to oscillate in a superposition of their supermodes, thus leading to large beam divergence and spectral spread. Discrimination among the supermodes in phase-locked arrays is discussed theoretically. It is shown that supermode discrimination in gain-guided arrays, in favor of the fundamental supermode, is made possible by the near-field interference patterns which result from the complex optical fields of the gain-guided lasers. A fundamental supermode operation is demonstrated, for the first time, in GaAlAs/GaAs gain-guided laser arrays. This is achieved by control of the current (gain) profile across the array by means of individual laser contacts.

Kapon, E.↗

NASA NTRS1984

Method and apparatus for self-calibration and phasing of array antenna

A technique for self-calibrating and phasing a lens-feed array antenna, while normal operation is stopped, utilizes reflected energy of a continuous and coherent wave broadcast by a transmitter through a central feed while a phase controller advances the phase angles of reciprocal phase shifters in radiation electronics of the array elements at different rates to provide a distinct frequency modulation of electromagnetic wave energy returned by reflection in one mode and leakage in another mode from the radiation electronics of each array element. The composite return signal received by a synchronous receiver goes through a Fourier transform processing system and produces a response function for each antenna element. Compensation of the phase angles for the antenna elements required to conform the antenna response to a precomputed array pattern is derived from the reciprocal square root of the response functions for the antenna elements which, for a rectangular array of NXM elements, is a response function T(n,m). A third mode of calibration uses an external pilot tone from a separate antenna element. Respective responses are thus obtained from the three modes of calibration.

Wu, C.↗

NASA NTRS1985

Langley Research Center Photogrammetric Measurements of Solar Array Dynamics: Preliminary Results

The NASA Langley Research Center participated in the Solar Array Experiment with two primary objectives: (1) to study the structural and control dynamics of a new class of large, lightweight, low-frequency space structures, and (2) to develop technology for remote video measurement of structural motions. The shuttle orbiter's closed circuit television (CCTV) system was used to provide recorded video images of the solar array from four locations in the payload bay, two on the forward bulkhead and two on the aft bulkhead. White reflective targets were placed on the array to provide discrete points at which to track array motion. A dynamic test consisted of a quiescent period in which orbiter operations were inhibited and crew motion restricted, an excitation period, and a free decay period. The orbiter was placed in free drift while in a gravity gradient orientation and dynamics tests were timed to occur at orbital noon so that the Sun would illuminate one side of the array and Earth albedo the other. The CCTV system was turned on during the quiescent period, approximately 3 minutes prior to excitation. Video from all four CCTV cameras was recorded continuously through the excitation and free-decay periods for a total test record of 8 to 10 minutes.

Brumfield, M. L.↗

NASA NTRS1984

Applications of array processors in the analysis of remote sensing images

The architectures, programming characteristics, and ranges of application of past, present, and planned array processors for the digital processing of remote-sensing images are compared. Such functions as radiometric and geometric corrections, principal-components analysis, cluster coding, histogram generation, grey-level mapping, convolution, classification, and mensuration and modeling operations are considered, and both pipeline-type and single-instruction/multiple-data-stream (SIMD) arrays are evaluated. Numerical results are presented in a table, and it is found that the pipeline-type arrays normally used with minicomputers increase their speed significantly at low cost, while even further gains are provided by the more expensive SIMD arrays. Most image-processing operations become I/O-limited when SIMD arrays are used with current I/O devices.

Ramapriyan, H. K.↗

NASA NTRS1985

Coupled-mode analysis of gain and wavelength oscillation characteristics of diode laser phased arrays

The lasing wavelengths and gain characteristics of the modes of phase-locked arrays of channel-substrate-planar (CSP) lasers are presented. The gain values for the array modes are determined from complex coupling coefficients calculated using the fields of neighboring elements of the array. The computations show that, for index guided lasers which have nearly planar phase fronts, the highest order array mode will be preferred. The 'in-phase' or fundamental mode, which produces only one major lobe in the far-field radiation pattern, has the lowest modal gain of all array modes. The modal gain differential between the highest order and fundamental modes is less than 10/cm for weak coupling between the elements.

Butler, J. K.↗

NASA NTRS1984

Two-dimensional infrared detector arrays

An introduction to overall IR array concepts and particular array designs is presented, and examples of laboratory and observational data obtained with IR arrays are given. Hybrid array designs, extrinsic silicon accumulation-mode charge injection devices, and switched-FET or switched-sample photoconductor techniques are summarized. Three recent astronomical observation projects which used IR arrays are examined.

Mccreight, C. R.↗

NASA NTRS1985

Design, performance investigation and delivery of a miniaturized Cassegrainian concentrator solar array

A miniaturized Cassegrainian concentrator (MCC) solar array concept is being developed with the objective of significantly reducing the recurring cost of multikilowatt solar arrays. The desired cost reduction is obtained as a result of using very small high efficiency solar cells in conjunction with low-cost optics. In the MCC single element concept and panel concept, incident solar radiation is reflected from a primary parabolic reflector to a secondary hyperbolic reflector and finally to a 4-millimetr diameter solar cell. A light catcher cone is used to improve off-axis performance. An element is approximately 13-millimeters thick which permits efficient launch stowage of the concentrator system panels without complex optical component deployments or retractions. The MCC elements are packed in bays within graphite epoxy frames and are electrically connected into appropriate series-parallel circuits. A MCC single element with a 21 sq cm entrance aperture and a 20 percent efficient, 0.25 sq cm gallium arsenide solar cell has the same power output as 30-sq cm of 11-percent efficiency (at 68 C) silicon solar cells. The MCC concept provides the potential for a significant reduction in array cost due to a 99 percent reduction in required cell area and a 30 percent reduction in array area relative to planar array of equivalent power.

Patterson, R. E.↗

NASA NTRS1986

On realizations of least-squares estimation and Kalman filtering by systolic arrays

Least-squares (LS) estimation is a basic operation in many signal processing problems. Given y = Ax + v, where A is a m x n coefficient matrix, y is a m x 1 observation vector, and v is a m x 1 zero mean white noise vector, a simple least-squares solution is finding the estimated vector x which minimizes the norm of /Ax-y/. It is well known that for an ill-conditioned matrix A, solving least-squares problems by orthogonal triangular (QR) decomposition and back substitution has robust numerical properties under finite word length effect since 2-norm is preserved. Many fast algorithms have been proposed and applied to systolic arrays. Gentleman-Kung (1981) first presented the trianglular systolic array for a basic Givens reduction. McWhirter (1983) used this array structure to find the least-squares estimation errors. Then by geometric approach, several different systolic array realizations of the recursive least-squares estimation algorithms of Lee et al (1981) were derived by Kalson-Yao (1985). Basic QR decomposition algorithms are considered in this paper and it is found that under a one-row time updating situation, the Householder transformation degenerates to a simple Givens reduction. Next, an improved least-squares estimation algorithm is derived by considering a modified version of fast Givens reduction. From this approach, the basic relationship between Givens reduction and Modified-Gram-Schmidt transformation can easily be understood. This improved algorithm also has simpler computational and inter-cell connection complexities while compared with other known least-squares algorithms and is more realistic for systolic array implementation.

Chen, M. J.↗

NASA NTRS1986

The grid array antenna

Attention is called to the grid array as a possible useful antenna design for UHF clear air radars. This type of antenna integrates radiating elements and the feed network into a single structure so that a fairly large array can be driven from a single feed point. Figures are given which demonstrate the basic principle of the grid array, which are adapted from Conti et al. (1981). Conductors are arranged above a ground plane in a repeating, staggered array of connected rectangles. Each rectangular element is approximately one by one-half wavelength in size. The currents on the conductors at resonance is indicated. The grid array illustrated can be expanded in both vertical and horizontal directions about the feed point by adding additional rectangular conductors. The design eliminates the feed network and would provide a thin, panel-like antenna that could be easily built and transported.

Ecklund, W. L.↗

NASA NTRS1986

Integrated infrared array technology

An overview of integrated infrared (IR) array technology is presented. Although the array pixel formats are smaller, and the readout noise of IR arrays is larger, than the corresponding values achieved with optical charge-coupled-device silicon technology, substantial progress is being made in IR technology. Both existing IR arrays and those being developed are described. Examples of astronomical images are given which illustrate the potential of integrated IR arrays for scientific investigations.

Goebel, J. H.↗

NASA NTRS1987

Means for phase locking the outputs of a surface emitting laser diode array

An array of diode lasers, either a two-dimensional array of surface emitting lasers, or a linear array of stripe lasers, is phase locked by a diode laser through a hologram which focuses the output of the diode laser into a set of distinct, spatially separated beams, each one focused onto the back facet of a separate diode laser of the array. The outputs of the diode lasers thus form an emitted coherent beam out of the front of the array.

Lesh, James R.↗

NASA NTRS1986

Evaluation of multi-spectral linear array detectors for spaceborne applications

A NASA laboratory for evaluating linear detector arrays for remote sensing applications from the visible to shortwave infrared is described. Nominal requirements are presented on the critical parameters of signal-to-noise ratio, modulation transfer function, response linearity, dynamic range, and detector-to-detector response uniformity. Results of tests on a custom developed multi-spectral linear array from RCA and two commercial arrays from Texas Instruments and Reticon for the visible and near-infrared are presented. The custom array was clearly superior in signal-to-noise ratio and dynamic range to its off-the-shelf counterparts. Allthe arrays were found to have somewhat less than desirable modulation transfer functions, but excellent response linearity.

Kalshoven, James E., Jr.↗

NASA NTRS1986

Photovoltaic array loss mechanisms

Loss mechanisms which come into play when solar cell modules are mounted in arrays are identified. Losses can occur either from a reduction in the array electrical performance or with nonoptimal extraction of power from the array. Electrical performance degradation is caused by electrical mismatch, transmission losses from cell surface soiling and steep angle of reflectance, and electrical losses from field wiring resistance and the voltage drop across blocking diodes. The second type of loss, concerned with the operating points of the array, can involve nonoptimal load impedance and limiting the operating envelope of the array to specific ranges of voltage and current. Each of the loss mechanisms are discussed and average energy losses expected from soiling, steep reflectance angles and circuit losses are calculated.

Gonzalez, Charles↗

NASA NTRS1987

A directional array approach for the measurement of rotor noise source distributions with controlled spatial resolution

A special array system has been designed to examine noise source distributions over a helicopter rotor model. The particular measurement environment is for a rotor operating in the open jet of an anechoic wind tunnel. An out-of-flow directional microphone element array is used with a directivity pattern whose major directional lobe projects on the rotor disk. If significant contributions from extraneous tunnel noise sources in the direction of the side lobes are excluded, the dominant output from the array would be that noise emitted from the projected area on the rotor disk. The design incorporates an array element signal blending features which serves to control the spatial resolution of the size of the directional lobes. (Without blending, the resolution and side lobe size are very strong functions of frequency, which severely limits the array's usefulness).

Brooks, T. F.↗