Inexpensive detector for terahertz imaging.

Glow discharge plasma, derived from direct-current gas breakdown, is investigated in order to realize an inexpensive terahertz (THz) room-temperature detector. Preliminary results for THz radiation show that glow discharge indicator lamps as room-temperature detectors yield good responsivity and noise-equivalent power. Development of a focal plane array (FPA) using such devices as detectors is advantageous since the cost of a glow discharge detector is approximately $0.2-$0.5 per lamp, and the FPA images will be diffraction limited. The detection mechanism of the glow discharge detector is found to be the enhanced diffusion current, which causes the glow discharge detector bias current to decrease when exposed to THz radiation.

Prediction of data stream parameters in atmospheric turbulent wireless communication links.

A unified approach for calculation of information data stream parameters in the atmospheric optical communication channel is presented based on irradiance fluctuations of optical wave propagation through turbulence and on a generalized Ricean K-parameter distribution. The effects of turbulence are described via the well-known Kolmogorov scheme of turbulent structure relaxation in terms of stochastic scintillation theory described by the gamma-gamma distribution along with measurements of the values of the refractive index structure parameter, C(n)(2). The relation between the Ricean parameter K and the signal scintillation parameter sigma(I)(2) is considered to develop a unified description of the corresponding probability density function (pdf) of signal fading within an atmospheric wireless communication link. Through the corresponding pdf and parameter K, signal data stream parameters such as the signal-to-noise ratio (SNR), bit error rate (BER), and capacity of the optical atmospheric channel (C) are estimated. Such an approach permits the reliable prediction of the effects of fading caused by different levels of turbulence and agrees with experimental data observed at different atmospheric levels, at the heights of both 100-200 m and above 1-2 km. It is shown that at heights of 100-200 m, effects of fading, caused by turbulence, occur much more frequently than those at the heights of 1-2 km. Data stream parameters such as channel capacity, SNR, and spectral efficiency become stronger at higher altitudes, while at the same time the BER becomes relatively negligible.

Registration of motion-distorted interlaced images captured by a scanning vector imaging sensor.

We present an algorithm to realign images distorted by motion and vibrations captured in cameras that use a scanning vector sensor with an interlaced scheme. In particular, the method is developed for images captured by a staggered time delay and integration camera distorted by motion. The algorithm improves the motion-distorted image by adjusting its fields irrespective of the type of motion that occurs during the exposure. The algorithm performs two tasks: estimation of the field relative motion during the exposure by a normal least-squares estimation technique and improvement of the degraded image from such motion distortion. The algorithm uses matrix computations; therefore it has a computation advantage over algorithms based on the technique of searching for a match. The algorithm is successfully demonstrated on both simulated and real images.

Medical image restoration of dynamic lungs using optical transfer function of lung motion.

When carrying out medical imaging based on detection of isotopic radiation levels of internal organs such a lungs or heart, distortions, and blur arise as a result of the organ motion during breathing and blood supply. Consequently, image quality declines, despite the use of expensive high resolution devices and, such devices are not exploited fully. A method with which to overcome the problem is image restoration. Previously, we suggested and developed a method for calculating numerically the optical transfer function (OTF) for any type of image motion. The purpose of this research is restoration of original isotope images (of the lungs) by restoration methods that depend on the OTF of the real time relative motion between the object and the imaging system. This research uses different algorithms for the restoration of an image, according to the OTF of the lung motion, which is in several directions simultaneously. One way of handling the three-dimensional movement is to decompose the image into several portions, to restore each portion according to its motion characteristics, and then to combine all the image portions back into a single image. An additional complication is that the image was recorded at different angles. The application of this research is in medical systems requiring high resolution imaging. The main advantage of this approach is its low cost versus conventional approaches.

Enhanced-resolution image restoration from a sequence of low-frequency vibrated images by use of convex projections.

An algorithm to increase the spatial resolution of digital video sequences captured with a camera that is subject to mechanical vibration is developed. The blur caused by vibration of the camera is often the primary cause for image degradation. We address the degradation caused by low-frequency vibrations (vibrations for which the exposure time is less than the vibration period). The blur caused by low-frequency vibrations differs from other types by having a random shape and displacement. The different displacement of each frame makes the approach used in superresolution (SR) algorithms suitable for resolution enhancement. However, SR algorithms that were developed for general types of blur should be adapted to the specific characteristics of low-frequency vibration blur. We use the method of projection onto convex sets together with a motion estimation method specially adapted to low-frequency vibration blur characteristics. We also show that the random blur characterizing low-frequency vibration requires selection of the frames prior to processing. The restoration performance as well as the frame selection criteria is dependent mainly on the motion estimation precision.

Myopic Deconvolution of Adaptive Optics Images by use of Object and Point-Spread Function Power Spectra: Comment.

It is suggested here that the lack of total image correction that is typical in adaptive optics (AO) imaging can be attributed in part to blur derived from small-angle scatter of light by aerosols, known also as the adjacency effect, especially as it is a well-established fact that such atmospheric blur is dominant in satellite imagery and the shape of the modulation transfer function after AO correction is strikingly similar to the unique shape of the aerosol modulation transfer function. Further investigation of AO systems to confirm this would aid in and improve image restoration.

Motion-distorted composite-frame restoration.

Many imaging systems produce pictures by the superimposition of two fields of frames of interlaced sequences. Pictures obtained in this way, which are termed composite frames, are severely degraded if relative motion between the camera and the scene occurs. In the presence of motion the composite frame is affected by two types of distortion: the edge staircase effect that is due to the fact that objects appear at different positions in successive fields and motion blur that is due to scene motion during each field exposure. Motion-deinterlacing methods previously proposed to recover the staircase effect neglect motion blur. However, motion blur may be significant, especially in systems designed for low-intensity radiometric imaging that use long exposures or even in short-exposure systems that happen to be in moving vehicles such as tanks, planes, ships, etc. We introduce an algorithm for the restoration of the two types of distortion in a composite frame degraded by linear uniform motion.

Comparison of direct blind deconvolution methods for motion-blurred images.

Direct methods for restoration of images blurred by motion are analyzed and compared. The term direct means that the considered methods are performed in a one-step fashion without any iterative technique. The blurring point-spread function is assumed to be unknown, and therefore the image restoration process is called blind deconvolution. What is believed to be a new direct method, here called the whitening method, was recently developed. This method and other existing direct methods such as the homomorphic and the cepstral techniques are studied and compared for a variety of motion types. Various criteria such as quality of restoration, sensitivity to noise, and computation requirements are considered. It appears that the recently developed method shows some improvements over other older methods. The research presented here clarifies the differences among the direct methods and offers an experimental basis for choosing which blind deconvolution method to use. In addition, some improvements on the methods are suggested.

Performance limitations of a free-space optical communication satellite network owing to vibrations: heterodyne detection.

Free-space optical communication between satellites in a distributed network can permit high data rates of communication between different places on Earth. To establish optical communication between any two satellites requires that the line of sight of their optics be aligned during the entire communication time. Because of the large distance between the satellites and the alignment accuracy required, the pointing from one satellite to another is complicated because of vibrations of the pointing system caused by two fundamental stochastic mechanisms: tracking noise created by the electro-optic tracker and vibrations derived from mechanical components. Vibration of the transmitter beam in the receiver plane causes a decrease in the received optical power. Vibrations of the receiver telescope relative to the received beam decrease the heterodyne mixing efficiency. These two factors increase the bit-error rate of a coherent detection network. We derive simple mathematical models of the network bit-error rate versus the system parameters and the transmitter and receiver vibration statistics. An example of a practical optical heterodyne free-space satellite optical communication network is presented. From this research it is clear that even low-amplitude vibration of the satellite-pointing systems dramatically decreases network performance.

Experimental investigation of the influence of the relative position of the scattering layer on image quality: the shower curtain effect.

The imaging quality of optical systems in a turbid environment is influenced not only by the content of the turbid layer between the object and the optical receiver but also by the inhomogeneity of that medium. This is important, particularly when imaging is performed through clouds, nonhomogeneous layers of dust, or over vertical or slant paths through the atmosphere. Forward small-angle scattering influences image quality and blur more severely when the scattering layer is closer to the receiver. In this study it is the influence of the relative position of the scattering layer on the image quality and modulation transfer function (MTF) that is investigated. The scattering layer in controlled laboratory experiments consists of calibrated polystyrene particles of known size and quantity in a small cuvette. A point source was imaged by a computerized imaging system through a layer containing polystyrene particles, and the point-spread function (PSF) was recorded. The aerosol MTF was calculated using the measured PSF. The MTF was measured as a function of changing relative distance of the scattering layer from the receiver, whereas the object-plane-to-receiver distance was constant. The experimental results were compared to theoretical shower curtain effect models based on the solution from radiative transfer theory under the small-angle approximation. Although the general trend of the experimental results certainly agrees with the theoretical models, it could be that the small-angle approximation method might be of limited validity at such low spatial frequencies. Aggregation also causes some disagreement with predictions from theory.

General restoration filter for vibrated-image restoration.

Mechanical vibrations are often the principal cause of image degradation. Low temporal-frequency mechanical vibrations involve random image degradation that depends on the instant of exposure. Exact restoration requires the calculation of a specific filter unique to each vibrated image. To calculate the restoration filter for each image, one needs the specific optical transfer function unique to the motion in the image. Therefore the instant of exposure and the motion function have to be measured or estimated by some other means. We develop a restoration filter for individual images blurred randomly by low-frequency mechanical vibrations. The filter is independent of the instant of exposure. The filter is designed to give its best performance averaged over a complete ensemble of vibrated images. Although when applying the new filter to any vibrated image the restoration achieved is slightly poorer than that achieved with an exact filter unique to the specific motion function, the new filter has the advantage of simplicity.

Beam width and transmitter power adaptive to tracking system performance for free-space optical communication.

The basic free-space optical communication system includes at least two satellites. To communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. Optical tracking and pointing systems for free space suffer during tracking from high-amplitude vibration because of background radiation from interstellar objects such as the Sun, Moon, Earth, and stars in the tracking field of view or the mechanical impact from satellite internal and external sources. The vibrations of beam pointing increase the bit error rate and jam communication between the two satellites. One way to overcome this problem is to increase the satellite receiver beacon power. However, this solution requires increased power consumption and weight, both of which are disadvantageous in satellite development. Considering these facts, we derive a mathematical model of a communication system that adapts optimally the transmitter beam width and the transmitted power to the tracking system performance. Based on this model, we investigate the performance of a communication system with discrete element optical phased array transmitter telescope gain. An example for a practical communication system between a Low Earth Orbit Satellite and a Geostationary Earth Orbit Satellite is presented. From the results of this research it can be seen that a four-element adaptive transmitter telescope is sufficient to compensate for vibration amplitude doubling. The benefits of the proposed model are less required transmitter power and improved communication system performance.

Adaptive optical transmitter and receiver for space communication through thin clouds.

Optical space communication from satellite to ground or air to air consists of clouds as part of communication channels. Propagation of optical pulses through clouds causes widening and deformation in the time domain and attenuation of the pulse radiant power. These effects decrease the received signal and limit the information bandwidth of the communication system. Having dealt with the other effects previously, here we concentrate on pulse broadening in the time domain. We derive a mathematical model of an adaptive optical communication system with a multiscattering channel (atmospheric cloud). We use knowledge about the impulse response function of the cloud to adapt the communication parameters to the transfer function of the cloud. The communication system includes a receiver and a transmitter. We adapted the transmitter to atmospheric conditions by changing the bit error rate. One can adapt the receiver to the atmospheric condition by changing the parameters of the detector and the filter. An example for a practical communication system between a low Earth orbit satellite and a ground station cover by cloud is given. Comparison and analysis of an adaptive and semiadaptive system with cloud channels are presented. Our conclusion is that in some cases only by such adaptive methods is optical communication possible.

Probing and monitoring aerosol and atmospheric clouds with an electro-optic oscillator.

Monitoring, probing, and sensing characteristics of aerosol clouds is difficult and complicated. Probing the characteristics of aerosols is most useful in the chemical and microelectronic industry for processing control of aerosols and emulsion, decreasing bit error rate in adaptive optical communication systems, and in acquiring data for atmospheric science and environment quality. We present a new mathematical and optical engineering model for monitoring characteristics of aerosol clouds. The model includes the temporal transfer function of aerosol clouds as a variable parameter in an electro-optic oscillator. The frequency of the oscillator changes according to changes in the characteristics of the clouds (density, size distribution, physical thickness, the medium and the particulate refractive indices, and spatial distribution). It is possible to measure only one free characteristic at a given time. An example of a practical system for monitoring the density of aerosol clouds is given. The frequency of the oscillator changes from 1.25 to 0.43 MHz for changes in aerosol density from 2000 to 3000 particulates cm(-3). The advantages of this new method compared with the transmissometer methods are (a) no necessity for line-of-sight measurement geometry, (b) accurate measurement of high optical thickness media is possible, (c) under certain conditions measurements can include characteristics of aerosol clouds related to light scatter that cannot be or are difficult to measure with a transmissometer, and (d) the cloud bandwidth for free space optical communication is directly measurable.

Effect of particulates on performance of optical communication in space and an adaptive method to minimize such effects.

Decreased signal-to-noise ratio and maximum bit rate as well as increased in error probability in optical digital communication are caused by particulate light scatter in the atmosphere and in space. Two effects on propagation of laser pulses are described: spatial widening of the transmitted beam and attenuation of pulse radiant power. Based on these results a model for reliability of digital optical communication in a particulate-scattering environment is presented. Examples for practical communication systems are given. An adaptive method to improve and in some cases to make possible communication is suggested. Comparison and analysis of two models of communication systems for the particulate-scattering channel are presented: a transmitter with a high bit rate and a receiver with an avalanche photodiode and a transmitter with a variable bit rate and a new model for an adaptive circuit in the receiver. An improvement of more than 7 orders of magnitude in error probability under certain conditions is possible with the new adaptive system model.

Effects of absorption on image quality through a particulate medium.

A new approach to studying the effects of absorption by aerosols and molecular particulates of electromagnetic radiation is presented. In contradiction to the conventional concept that absorption gives rise to constant attenuation, it is shown here that the particulate-absorbed irradiance is spatial frequency dependent. An analytically corrected model of the aerosol modulation transfer function and the aerosol mutual coherence function is presented. An important application of this model is in thermal imaging, in which particulate-absorption effects are very significant.

Prediction of effects of weather on image quality: preliminary results of model validation.

Using the atmospheric modulation transfer function area (MTFA) as a single-valued numerical criterion for image quality horizontally near the ground propagated through the atmosphere, a statistical study of atmospheric imaging data accumulated over a three-year period has led to the determination of regression coefficients with which to quantitatively predict image quality as a function of wavelength, over the 400-1000- nm wavelength region, according to weather forecast. Utilization of this procedure is simple: one plugs in expected values for wind speed, air temperature, and relative humidity in the regression coefficient expression for MTFA. The larger the expected MTFA, the better the expected image quality. Two sets of regression coefficient data have been obtained, one each for desert and nondesert climates, corresponding to summer and winter data here. Preliminary experimentation over a different line of sight indicates that the accuracy of the prediction is fairly reliable.

Heterodyne detection through rain, snow, and turbid media: effective receiver size at optical through millimeter wavelengths.

Both scattering and turbulence can effect the spatial coherence of short wavelength signals propagating through the open atmosphere. In this paper, the influence of forward scattering on heterodyne receiver performance is investigated, taking into account turbulence. It is shown that the effect of forward scattering is to reduce the effective heterodyne receiver area through spatial coherence degradation. A common approach to scattering as an attenuation phenomenon is not always valid. Generally, this approach underestimates the SNR. The accuracy of the attenuation approach depends on the ratio R of the actual receiver diameter to the scattering particle diameter. If R >100, scattering is essentially large angle and the typical treatment of scattering as an attenuation effect is indeed justified. However, for small R, forward scattering is primarily small angle, field coherence is noticeably affected by forward scattering, and the attenuation approach is not valid. Further, it is shown that the SNR is improved when the ratio of the scattering particulate size to turbulence coherence diameter decreases. From the practical point of view, the most important result of this study is that small receivers use their area more effectively than large receivers. Thus, an array of several small receivers may perform better than one large receiver with the same total area. The treatment here is particularly relevant for coherent detection through clouds, fog, precipitation, and turbid media in general, including liquid media.