MODTRAN4, the newly released version of the U.S. Air Force atmospheric transmission, radiance and flux model is being developed jointly by the Air Force Research Laboratory/Space Vehicles Directorate and Spectral Sciences, Inc. It is expected to provide the accuracy required for analyzing spectral data for both atmospheric and surface characterization. These two quantities are the subject of satellite and aircraft campaigns currently being developed and pursued by, for instance: NASA (Earth Observing System), NPOESS (National Polar Orbiting Environmental Satellite System), and the European Space Agency (GOME--Global Ozone Monitoring Experiment). Accuracy improvements in MODTRAN relate primarily to two major developments: (1) the multiple scattering algorithms have been made compatible with the spectroscopy by adopting a corrected-k approach to describe the statistically expected transmittance properties for each spectral bin and atmospheric layer, and (2) radiative transfer calculations can be conducted with a Beer-Lambert formulation that improves the treatment of path inhomogeneities. Other code enhancements include the incorporation of solar azimuth dependence in the DISORT- based multiple scattering model, the introduction of surface BRDF (Bi-directional Radiance Distribution Functions) models and 15 cm^-1 band model for improved computational speed.

Radiance measurements conducted from a high-altitude platform to retrieve surface properties will potentially involve long, near-horizontal viewing geometries. The computer code MODTRAN is widely used for the prediction of the propagation of infrared radiation through the lower atmosphere. Consequently, we have undertaken to test the predictions of MODTRAN for the 3 - 5 and 8 - 12 micron spectral regions under mid-Eastern desert conditions. This paper compares experimental measurements in geometries of interest with calculations using the latest version of MODTRAN. Results indicate a strong dependence of the remotely sensed radiation on both the aerosol and water vapor content.

In applications of adaptive optics, understanding the statistical nature of the upper atmospheric turbulence is critical. Today there is a lack of detailed knowledge of upper atmospheric turbulence in the region of the tropopause. Tropopause measurements have been made with simplifications in experimental designs and the potential for experimentally induced artifacts. Recent measurements by various groups indicate the presence of non-Kolmogorov behavior, asymmetric turbulence structure and finite outer scale sizes. Analysis of current balloon borne data collection techniques suggests that signal processing and the balloons own wake may have influenced the sensor. To address some of these concerns, a new measurement platform has been designed that carries 15 constant current probes that are simultaneously sampled at 16 bits at over 1500 samples per second during a controlled descent. All of the signals are then telemetered to the ground without on board processing. The data are then merged with atmospheric data and payload orientation to product key turbulence parameters, including turbulence strength, the inner and outer scales, the temperature structure function, power spectral density of the turbulence, and turbulence isotropic behavior.

An application of Doppler SODAR technique has been made in order to evaluate the main atmospheric variables affecting the boundary layer structure in a plain terrain. Besides directly monitoring such meteorological variables as wind profiles, the application of a number of methods and algorithms enabled the estimation of features such as atmospheric turbulence, Monin-Obukhov length, friction velocity and PBL depth, which are all crucial for both straightforward meteorological applications and as an input to atmospheric pollutant dispersion models. Such a study has been developed within a SODAR measurement campaign carried out by the Laboratorio per la Meteorologia e la Modellistica Ambientale in cooperation with the research center of ENEL/CRAM during the 1997-98 wintertime in the industrial area of Campi Bisenzio (near Florence), Italy.

During the past several months, we have operated a laser- illuminated imaging system at North Oscura Peak. This paper describes modifications to this system as well as a series of experiments to demonstrate the utility of polarization measurements for material discrimination. The largest site illuminated was at a range of 10 km.

The authors present a wavefront reconstruction technique for beams forward scattered and back scattered through cirrus clouds. The technique uses ray distributions from the Coherent Illumination and Ray Trace Imaging Software for Cirrus which traces the propagation and E field vectors through a 3D volume of ice crystals in the shape of columns, plates, bullets, and bullet rosettes with random positions and polydisperse sizes and orientations. The wavefronts are then propagated to a telescope receiver on the ground and imaged in the receiver focal plate. A modification transfer function for each of these images is calculated and compared to the MTF for a diffraction-limited system.

During March 1999 FGAN-FOM performed an experiment at the Baltic Sea. Image sequences of shipborne point like IR- sources were taken in seven wavebands from the short to the long wave IR (2 micrometers - 12 micrometers ). Detection range performance for multi-spectral sensors in the marine boundary layer were analyzed for different meteorological conditions, especially for low visibilities (sea fog).

We apply out previously-developed turbid-media backpropagation algorithm to imaging extended objects imbedded in turbid media such as clouds. Although the backpropagation algorithm was developed initially for biomedical applications, the underlying development is general enough to encompass imaging objects imbedded in any sort of turbid media whose scattering properties dominate their absorption properties. For non-biomedical applications, imaging data is usually obtained only for a limited number of view angles. As a result, we look at the potential of the backpropagation algorithm to reconstruct an image of an object, imbedded in a cloud, from a single view. Using both computer-simulated data and laboratory data, we show that the backpropagation algorithm successfully increases resolution in these types of images. Because the backpropagation algorithm incorporates a depth-dependent deconvolution filter, it turns out that the optimal image quality obtained in the reconstruction occurs for the deconvolution filter which corresponds to the location of the object in the medium. This surprising result permits object localization in the range dimension even when the illuminating radiation is continuous-wave illumination, such as sunlight.

Nonlinear amplitude and phase distortions of the laser beams propagating through the water aerosol studied experimentally are presented in this paper. Pulses of CO₂- and YAG- lasers effect small water drops. As a result of this action, the drops were evaporated and fragmented into many microparticles. Deformations of wave front of a sensing beam with the Gaussian form have been studied. Measurements of the phase were carried out with the use of the Mach-Tsender interferometer. The time of explosion as a function of absorbed energy and pulse duration have been obtained in these experiments. The spatial and temporal inhomogeneities of a sensing wave front as function of the incident radiation (their wavelength, energy, and pulse width) have been studied. It was shown that the fast heating of particles and vapor bubbles generation leads to defocus of sensing beam, but intensive vapor fluxes lead to the focus of sensing beam. The time of beginning the distortion process was measured as function wavelength and absorbed energy of incident pulses. These experimental results may be useful for development of the adaptive system operating under nonlinear condition.

High resolution imaging with adaptive optics and numerical image restoration processing is now an operational reality for ground-based telescopes. It provides a real-time compensation for turbulence degraded images. However, these correction techniques are only effective in a limited field of view due to anisoplanatism. For endoatmospheric applications, strong intensity fluctuations may also limit the degree of correction. In this paper, we estimate the isoplanatic domain as a function of the degree of correction of high resolution imaging systems and the transition between weak and strong intensity fluctuation regimes. We show that the saturation onset of scintillation corresponds to an isoplanatic field which becomes smaller than the resolution of adaptive optics systems.

By using a recently developed theory of scintillation that is valid under fluctuation conditions that include the focusing and saturation regimes, we develop a general model for predicting power fluctuations (or aperture averaging) over a finite-size collecting aperture. In addition, we calculate the covariance function and implied temporal spectrum of such power fluctuations. Increasing the size of the collecting aperture decreases the high frequency content of the irradiance spectrum.

An extensive set of measurements of scintillation over a 17.55 km path have been made using point sources at wavelengths of 633 nm and 10.6 micrometers , and using an extended thermal source at 3 - 5 micrometers and 8 - 12 micrometers . The basic data consists of normalized variances, probability histograms and normalized autocorrelation functions of intensity. The main aim was to product a set of data that might be used as inputs to models for scintillation. The measurements, as expected, showed a very large range of observed fluctuations, with a highest recorded normalized variance at 633 nm of approximately equals 34 and an average value of 4.8 (averaged over 130 data sets), with a standard deviation of 4.1. The probability histograms have been fitted using log- normal, exponential, log-normally modulated exponential and K distributions. As a general rule, the log-normal model gives a good fit in a large number of cases. Power spectra and correlations functions were measured and show the expected trends with wavelength, with average correlation times (defined in the text) in the range 10 msec (visible) to 68 msec (CO₂).

Use of fractional moments of low order is here proposed for processing data of intensity fluctuations from optical atmospheric propagation measurements. In this paper we check the accuracy of low order moment estimation and their ability to discriminate which one, among a number of candidate theoretical distributions, better represents the experimental histograms of intensity. The comparison method is tested by sampling sets of data from three popular distributions, that is Ln, LnME and K distribution. Applications to experimental sets of data are also presented.

We propose regularized versions of Maximum Likelihood algorithms for Poisson process with non-negativity constraint. For such process, the best-known (non- regularized) algorithm is that of Richardson-Lucy, extensively used for astronomical applications. Regularization is necessary to prevent an amplification of the noise during the iterative reconstruction; this can be done either by limiting the iteration number or by introducing a penalty term. In this Communication, we focus our attention on the explicit regularization using Tikhonov (Identity and Laplacian operator) or entropy terms (Kullback-Leibler and Csiszar divergences). The algorithms are established from the Kuhn-Tucker first order optimality conditions for the minimization of the Lagrange function and from the method of successive substitutions. The algorithms may be written in a `product form'. Numerical illustrations are given for simulated images corrupted by photon noise. The effects of the regularization are shown in the Fourier plane. The tests we have made indicate that a noticeable improvement of the results may be obtained for some of these explicitly regularized algorithms. We also show that a comparison with a Wiener filter can give the optimal regularizing conditions (operator and strength).

The Generalized Seeing Monitor is a new instrument for the monitoring of the atmospheric optical parameters (AOP) like the spatial-coherence outer scale L₀, the seeing (epsilon) ₀ and the isoplanatic angle (theta)₀. This instrument, equipped with 4 identical modules, works like a Shack-Hartman and sensor and allows the analysis of the perturbed wavefront (observation of the angle-of-arrival fluctuations at four points. Several measurement campaigns have been done at different sites over the world: Cerro Paranal, La Silla, Cerro Pachon (Chile), Oukaimeden (Morocco) and Maydanak (Uzbekistan). The main results obtained for the AOP (L₀, (epsilon) ₀, (theta) ₀) during these missions are presented and discussed. The whole of these measurements provides an important data bank allowing the analysis of the temporal variability of these parameters.

Random aberrations due to atmospheric turbulence determine the angular resolution of ground-based telescopes. Adaptive optics systems compensate the wavefront degradation before detection. Although systems with a large number of subapertures in the wavefront sensor and of actuators in the deformable mirror provide the best results, they are complicated and expensive. In contrast, simpler adaptive optics systems, (less than one actuator per atmospheric coherence diameter), compensate partially the wavefront distortions, having great potential application. The statistics of the image plane light intensity in partial compensation have been described using the Rician distribution. In order to achieve a more complete description of the phenomena, we describe the photon statistics in the whole image plane using the Poisson transform of the Rician distribution. When there is no compensation, the photon statistics follow a Bose-Einstein distribution. In partial compensation the PSF is composed by a bright core, where the photon statistics follow a Laguerre distribution, surrounded by a speckled halo with Bose- Einstein statistics. Some special cases have been studied and useful approximations have been derived. Theoretical results fit well with simulated values. This description of the light statistics as a function of the compensation may be used to extract more information about the object.

We present a hybrid VLSI/Optical system for real-time adaptive phase distortion compensation. On-chip CMOS circuitry performs parallel perturbative stochastic gradient descent/ascent of an externally supplied optimization metric, e.g. a direct measure of image or laser beam quality. Our custom mixed-mode analog-digital VLSI system can directly control several adaptive optics elements such as micro-electromechanical mirrors, liquid crystal spatial light modulators and tilt mirrors. Here we use a tilt mirror and a MEMS mirror with 37 control parameters each of which is adjusted independently and in parallel to manipulate the wavefront phase profile. We present experimental results demonstrating successful operation for an adaptive laser- beam transmitter system. The parallel VLSI architecture is extendable to higher resolutions (N equals 10³ to 10⁶ parameters).

We have conducted laboratory experiments and measurements to characterize an OKO Technologies membrane mirror. In this paper, we describe a closed-loop demonstration system built around the OKO mirror.

Adaptive optical systems used to date for laser communication applications have been based on adaptive optics technology specifically developed for astronomical imaging, where intensity fluctuations are considerably smaller than those encountered by ground-to-ground (or ground-to-space) lasercom systems. These adaptive optical systems are based on direct wavefront measurements (Hartmann-Shack sensor, shearing interferometers, etc.) and the wavefront conjugation principle. In the presence of strong intensity fluctuations these types of systems perform poorly. Here we propose an optical communication link with an adaptive transmitter based on optimization of a beam quality metric that can be measured using a corner-cube interferometer. This beam quality metric is examined to determine its suitability for use in an adaptive laser communication system.

Two-wavelength holography, when the hologram is recorded at one wavelength and reconstructed at some shifted wavelength, is an efficient tool for many applications. Optically addressed liquid crystal spatial light modulators are very convenient for recording thin dynamic holograms and, in particular, for the purposes of the dynamic two-wavelength holography. On such a basis one can realize the dynamic interferometer, providing the arbitrary scaling of the wave front distortions. Such an interferometer can be of use for solution of some of the tasks of the adaptive optics, namely, for simplification of the procedure of measuring of the robust wavefront distortions, for recording of the dynamic holographic correctors, working in spectral ranges, where the direct holographic record is impossible, in particular, in mid-IR range of spectrum, and for extension of the range of distortions, which can be corrected by means of the phase valve, mounted in the negative optical feedback loop. We report the experimental realization of such an interferometer.

Some problems, connected with development of ground-based adaptive telescope, particularly, with its fitting additional optical system for laser guide star formation, are treated in the paper. The point of the work is determination of the type of the laser guide star being formed. Here, the calculational results are presented for scheme for laser guide star formation, when arbitrary magnitudes of the correlation between random angular displacements of the image of scattering volume stipulated by the laser beam fluctuations over direct and back paths can be obtained. Expressions for the monostatic and bistatic schemes are obtained as limiting cases.

An effect of anisoplanarity is caused by difference of aberrations on paths of the direct and reference beams. These differences are due to variances of path geometric characteristics (different angles or divergence) or due to the presence of temporal lag. In many cases the causes of these factors are similar, moreover, methods of their mathematical representations are similar too. Next factor limiting the possibilities to use adaptive correction for turbulence aberrations is fast motion of an object. This imposes strict conditions on the rate of adaptive control and on the power of a Rayleigh beacon.

While developing deformable mirrors for adaptive optics, and while studying scintillations, we also tested the method of curvature sensing and different variants of it. By very carefully adjusting the optics we were able to discern variations on the scale of one nanometer. The limited dynamic range of the detectors and various optical artifacts caused systematic and random errors of 5 - 8 percent. Calibration of the measurements turned out to be a difficult issue as well. One of the main problems with non-atmospheric measurements was vignetting. We suspect that strong atmospheric scintillation might cause similar problems in curvature sensing, because of light scattered outside the measurement aperture, leading to errors in the estimated wave front phase. We looked into measuring turbulence along the optical path, by comparing field data of both Hartmann- Shack and intensity sensors collected under similar conditions. It seems that some of the turbulence can be tracked back to its range, but this is still being tested. If so, it might be possible to correct it using multi- conjugate optics and reduce significantly scintillation effects. Scintillation can also be removed artificially by correcting a scaled-up version of the turbulence at a scaled-down conjugate distance and vice versa.

Phase-diversity wavefront sensing has been implemented for the measurement of turbulence-distorted atmospheric wavefronts in applications of adaptive optics for essentially-horizontal propagation paths. The selected implementation of phase-diversity provides a wavefront sensor capable of estimating atmospheric distortions when observing extended scenes and provides a range-weighted sensing of the atmospheric distortions dependent on the angular region of the scene used for measurement. The data inversion, based on a Green's function analysis, is fast and robust enough for real-time implementation. For measurements of the atmospheric properties this wavefront sensor is being used with bright, compact sources to give high signal to noise measurements for integrated atmospheric effects along defined optical paths. The implementation used facilitates measurements of the atmospheric distortions along separate propagation paths. By simultaneous measurements along 3 separate paths a library of spatio-temporal atmospheric distortions and information about the isoplanicity of the distortions will be compiled for use in assessing applications of adaptive optics in horizontal propagation conditions. The principles of measurement, the details of implementation and some preliminary results will be described.

The CO2 laser DIAL concept has been applied successfully to the detection of common industrial byproducts and a variety of toxins and simulants (see Figures 1 and 2). Work to date has focused almost exclusively on detection of these compounds at low concentrations and at relatively short standoff ranges. The current techniques use large optics, multimode lasers and direct detection. The large optics partially offset the reduced sensitivity of the direct detection receiver. The standoff range can be enhanced by increasing the pulse energy but will still be severely limited by the inherently high noise equivalent power (NEP) oflong wavelength infrared (LWIR) detectors. It has long been recognized that range performance can be enhanced considerably in a photon counting sense by using a fully coherent system (see Figure 3 for state-of-the-art NEP estimates for direct and coherent detectors); good speckle averaging, however, should be retained in coherent sensors to achieve high-sensitivity and this will have a profound influence on the measurement technique. Standoff ranges of 100-200 km can be achieved with high altitude platforms (ER-2 or balloons) and coherent transmit/receive (shared) 30 cm aperture systems with pulse energies of< 1 .0 Joules/pulse (J/p) and repetition rates of 30-50 Hz. A key feature of the high sensitivity measurement approach is to use the coherent mode-locked pulse burst waveform as a wide bandwidth source to provide both speckle averaging (within each pulse) and coherent detection. There is also considerable synergism between the systems required for long-range imaging laser radar and remote sensing; both sensors will use coherent CO2 lasers and similar pulse formats. The laser radar/remote sensing functions can be combined into a single sensor suite. The key functional upgrade is to incorporate tunability in the mode-locked transmitter to produce, on demand, a selection of different CO2 laser wavelengths within the 9-1 1 pm atmosphere window and thus accommodate the remote sensing capability; a matched frequency agile local oscillator is also required for the shot noise limited coherent receiver, to provide 'photon counting' capabilities at all wavelengths of interest.