This PDF file contains the front matter associated with SPIE Proceedings Volume 7588, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

We introduce a realistic model for the impact of atmospheric phase and amplitude fluctuations on free-space links using either synchronous or nonsynchronous detection. We compare options for atmospheric compensation, including active modal methods and diversity combining techniques. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise. We study the effect of various parameters, including the ratio of receiver aperture diameter to wavefront coherence diameter, the scintillation index, the number of modes compensated, and the number of independent diversity branches combined at the receiver. We analyze outage Shannon capacity, placing upper bounds on the achievable spectral efficiency and enabling the performance of specific system designs to be predicted.

Free-space optical communications systems provide the opportunity to take advantage of higher data transfer rates and lower probability of intercept compared to radio-frequency communications. However, propagation through atmospheric turbulence, such as for airborne laser communication over long paths, results in intensity variations at the receiver and a corresponding degradation in bit error rate (BER) performance. Previous literature has shown that two transmitters, when separated sufficiently, can effectively average out the intensity varying effects of the atmospheric turbulence at the receiver. This research explores the impacts of adding more transmitters and the marginal reduction in the probability of signal fades while minimizing the overall transmitter footprint, an important design factor when considering an airborne communications system. Analytical results for the cumulative distribution function are obtained for tilt-only results, while wave-optics simulations are used to simulate the effects of scintillation. These models show that the probability of signal fade is reduced as the number of transmitters is increased.

We consider theoretically and numerically the suppression of fluctuations (scintillations) of a laser beam propagating through turbulent atmospheres by applying a phase modulator. Both spatial and temporal phase variations introduced by this phase modulator are analyzed. The explicit dependences of the scintillation index on the initial correlation length and finite-time phase variations for long propagation paths are obtained. Results of modeling and numerical simulations are presented. We demonstrate that an appropriately chosen phase modulator can significantly suppress the scintillations of the laser beam caused by turbulent atmospheres.

The dynamic features of the adaptive optical systems are researched. It is executed development of the traditional adaptive systems, having in its composition wave-front sensor, active mirrors and guide source. The efficiency of the using algorithms of phase and amplitude-phase corrections are analyzed. The schemes full and partial correcting of the phase distortion are considered. It is entered in analysis the new class adaptive systems, including in itself, the "system constant delay", as well as "speed" system, "forecasting" adaptive system and systems, using idea of "frozen" fluctuations in optical wave. For development of this ideas I offer to use the differential optical measurements in channel of guide source.

A hybrid centroiding technique involving Iteratively Weighted Center of Gravity (IWCoG) algorithm and correlation technique for a Laser Guide Star (LGS) based Shack Hartmann wavefront sensor is proposed. A simple method for simulating LGS elongated spots with photon noise and read out noise is demonstrated. The problems associated with IWCoG are addressed (a) Error saturation is minimized by adding random numbers iteratively to centroid positions, (b) non uniform convergence of Centroid Estimation Error (CEE) is reduced by using the hypothesis that the iteration number with maximum correlation between the weighting function and the actual spot image function is the iteration with minimum error, (c) convergence rate is improved by shifting the weighting function to the point of maximum intensity in first iteration. The novelty of the algorithm is tested by comparing with other centroiding algorithms.

In this paper we report the results of the analysis and experimental modeling of the target-in-the-loop (TIL) approach that is used to form a localized beacon for a laser beam propagating through turbulent atmosphere. The analogy between the TIL system and the laser cavity has been used here to simulate the process shaping the laser beacon on a remote image-resolved target with rough surface. The TIL breadboard was integrated and used for laboratory modeling of the proposed approach. This breadboard allowed to simulate the TIL arrangement with a rough-surface target and laser beam propagation through the turbulent atmospheric layer. Here we present the initial results of the performed studies.

At present, system design usually assumes the Kolmogorov model of refractive index fluctuation spectra in the atmosphere. However, experimental data indicates that in the atmospheric boundary layer and at higher altitudes the turbulence can be different from Kolmogorov's type. In optical communications, analytical models of mean irradiance and scintillation index have been developed for a traditional Kolmogorov spectrum and must be revised for non-Kolmogorov turbulence. The image quality (resolution, MTF, etc.) is essentially dependent on the properties of turbulent media. Turbulence MTF must be generalized to include non-Kolmogorov statistics. The change in fluctuation correlations of the refractive index can lead to a considerable change in both the MTF form and the resolution value. In this work, on the basis of experimental observations and modeling, generalized atmospheric turbulence statistics including both Kolmogorov and non-Kolmogorov path components are discussed, and their influence on imaging and communications through the atmosphere estimated for different scenarios of vertical and slant-path propagation. The atmospheric model of an arbitrary (non-Kolmogorov) spectrum is applied to estimate the statistical quantities associated with optical communication links (e.g., scintillation and fading statistics) and imaging systems. Implications can be significant for optical communication, imaging through the atmosphere, and remote sensing.

The DARPA Optical RF Communications Adjunct (ORCA) program was created to bring high data rate networking to the warfighter via airborne platforms. Recent testing of the ORCA system was conducted by the Northrop Grumman Corporation (NGC) at the Nevada Test and Training Range (NTTR) at the Nellis Air Force Range near Tonopah, NV. The University of Central Florida (UCF) conducted a parallel test to measure path-averaged values of the refractiveindex structure parameter, the inner scale of turbulence, and the outer scale of turbulence along the ORCA propagation path from an airborne platform to the ground at Antelope Peak. In addition, weather instrumentation was set up at ground level on Antelope Peak to measure local conditions on the mountain top. This paper presents background information on expected atmospheric conditions for the channel, models that were used by UCF for the measurements, path-averaged values of the three atmospheric parameters, and a C_n² profile model as a function of altitude.

Real-time measurement of effective wind speed helps in continuous monitoring of the temporal bandwidth in adaptive optics systems. In this paper, we propose a simple and efficient method for estimating wind speed from wavefront sensor data. A small portion from the center of a wavefront arriving at time 't' is mapped at all possible positions on phase screens coming later than 't' by a time n × Δt_int (Δt_int-integration time, n-integer) to form correlation maps. Wind speed and direction is estimated statistically via the calculation of the vectors formed by joining the central portion and the position of maximum correlation on the correlation maps. Number of such realizations to be considered to arrive at an accurate estimate of wind speed was optimized to reduce computing time and increase accuracy.

Electromagnetic and electro-optical (EM/EO) propagation and scattering in the ocean is of interest for a wide range of science problems. For example, the biological productivity of ocean waters through photochemical processes is governed by the vertical attenuation of solar radiation. Also, EO scattering theory is the primary basis for determining biogeochemical parameters (e.g. phytoplankton, suspended sediments, and dissolved matter) from the water leaving optical radiance. In addition, EO scattering from suspended sediments and bubbles is the limiting factor for active lidar systems used to map the sea bottom. This work will review specific applications of EO/EM scattering theory with regard to the influence of bubbles and droplets on remote sensing in the nearshore ocean. The current state of understanding concerning models and applications for optical scattering from bubbles in the water column as well as microwave scattering from water droplets produced by breaking waves at the ocean surface will be discussed as well as future research directions.

Under real flow conditions, the critical-angle scattering of a spherical bubble is too noisy to obtain directly the bubble diameter and refractive index. To solve this problem and to limit the drawback of any counting technique, the critical angle refractometry and sizing (CARS) technique has been extended as a collective ensemble technique. As an inverse method it allows to get the size distribution and composition of a cloud of bubbles. In this paper we review the principle, the advantages and limits of this new optical particle characterization method.

Characterization of aerosol scattering and absorption properties is essential to accurate radiative transfer calculations in the atmosphere. Applications of this work include remote sensing of aerosols, corrections for aerosol distortions in satellite imagery of the surface, global climate models, and atmospheric beam propagation. Here we demonstrate successful instrument development at the Laboratory for Aerosols, Clouds and Optics at UMBC that better characterizes aerosol scattering phase matrix using an imaging polar nephelometer (LACO-I-Neph) and enables measurement of spectral aerosol absorption from 200 nm to 2500 nm. The LACO-I-Neph measures the scattering phase function from 1.5° to 178.5° scattering angle with sufficient sensitivity to match theoretical expectations of Rayleigh scattering of various gases. Previous measurements either lack a sufficiently wide range of measured scattering angles or their sensitivity is too low and therefore the required sample amount is prohibitively high for in situ measurements. The LACO-I-Neph also returns expected characterization of the linear polarization signal of Rayleigh scattering. Previous work demonstrated the ability of measuring spectral absorption of aerosol particles using a reflectance technique characterization of aerosol samples collected on Nuclepore filters. This first generation methodology yielded absorption measurements from 350 nm to 2500 nm. Here we demonstrate the possibility of extending this wavelength range into the deep UV, to 200 nm. This extended UV region holds much promise in identifying and characterizing aerosol types and species. The second generation, deep UV, procedure requires careful choice of filter substrates. Here the choice of substrates is explored and preliminary results are provided.

Ghost imaging in the presence of turbulent atmosphere in both arms of the arrangement is predicted to show the improvement if the propagation distances from the source are sufficiently large. For intermediate distances from the source ghost images are very weak or unavailable.

Intensity fluctuations of incoherently superposed Gaussian beams are formulated in weak turbulence by employing the extended Huygens-Fresnel principle. Each individual beam superposed is taken to be fully incoherent. The scintillation index evaluated for different number of beams indicates that as the number of beams increase, scintillations decrease. Incoherent superposition of smaller sized Gaussian sources exhibits smaller fluctuations. Comparing the scintillation index arising from incoherently superposed Gaussian beams to the scintillation index of coherently superposed Gaussian beams of the same structure shows that incoherent superposition yields lower intensity fluctuations, thus can be advantageous in atmospheric optical communication links.

Propagation of elegant higher-order Gaussian beams in turbulent atmosphere is studied in detail. Analytical propagation formulae of elegant higher-order Gaussian beams in turbulent atmosphere are derived based on extended Huygens-Fresnel integral. The intensity and spreading properties of elegant higher-order Gaussian beams and standard higher-order Gaussian beams in turbulent atmosphere are studied numerically and comparatively. It is found that the propagation properties of elegant higher-order Gaussian beams and standard higher-order Gaussian beams are much different from their properties in free space The standard higher-order Gaussian beams spread more rapidly than the elegant higher-order Gaussian beams in turbulent atmosphere.

The interest for free space laser beam propagation has recently increased due to several experiments. These experiments have shown that optical links through the atmosphere can be effectively established, regardless of the several deleterious negative effects of refractive index fluctuations. The most deleterious effect is the scintillation which, consequently, has been widely investigated. However, it has not been reported yet what influence a ground profile has on scintillation values. In this paper we show theoretical results of the ground impact on scintillation, for a laser link between aircraft and mountain top; Antelope peak Nevada. The link was investigated for a specific mountainous profile of the ground for path lengths of 50 km and 200 km. The theoretical analysis shows that, if a low value is assumed for the Rytov variance (weak turbulence condition), then the presence of high peaks or mountains along the propagation path could have a remarkable impact on scintillation because the scintillation saturation effects do not occur; if the Rytov variance assumes medium-high values (moderate-to-strong turbulence conditions), then the scintillation index will be close to the saturation regime. Therefore the scintillation will be slightly affected by the mountains along the path, both for the plane wave model and the Gaussian beam wave model.

In this paper we review our work done in the evaluations of the root mean square (rms) beam wander characteristics of the flat-topped, dark hollow, cos-and cosh Gaussian, J₀-Bessel Gaussian and the I₀-Bessel Gaussian beams in atmospheric turbulence. Our formulation is based on the wave-treatment approach, where not only the beam sizes but the source beam profiles are taken into account as well. In this approach the first and the second statistical moments are obtained from the Rytov series under weak atmospheric turbulence conditions and the beam size are determined as a function of the propagation distance. It is found that after propagating in atmospheric turbulence, under certain conditions, the collimated flat-topped, dark hollow, cos- and cosh Gaussian, J₀-Bessel Gaussian and the I₀-Bessel Gaussian beams have smaller rms beam wander compared to that of the Gaussian beam. The beam wander of these beams are analyzed against the propagation distance, source spot sizes, and against specific beam parameters related to the individual beam such as the relative amplitude factors of the constituent beams, the flatness parameters, the beam orders, the displacement parameters, the width parameters, and are compared against the corresponding Gaussian beam.

Analytical propagation formula for the propagation factor of a phase locked radial laser array beam is derived based on the extended Huygens-Fresnel integral and the Wigner distribution function. Evolution properties of the propagation factor in turbulent atmosphere are studied numerically. It is found that unlike its propagation invariant properties in free space, the propagation factor of a radial laser array beam increases on propagation in turbulent atmosphere, and is closely related to the parameters of initial beam and the atmosphere.

The propagation of partially coherent vortex beams through atmospheric turbulence in weak-to-strong fluctuation regimes is investigated. Irradiance profiles from wave optics simulations and analytical theory compare favorably for a variety of link parameters. Simulation results indicate that partially coherent vortex beams can reduce scintillation index values relative to comparable classic Gaussian Schell model beams when turbulence conditions are mediate to strong. However, the overall propagation performance of partially coherent vortex beams, as measured by the metric Δ, tends to be poorer than classic Gaussian Schell model beams because of larger inherent beam spread.

We find that on propagation in turbulent ocean with power spectrum being a combination of power spectra of temperature and salinity fluctuations a stochastic beam can exhibit polarization changes which are qualitatively very similar to those which occur on propagation in atmospheric turbulence. However, pronounced quantitative difference between polarization changes in atmosphere and ocean should be noted. The analytical development is supported by numerical examples.