This PDF file contains the front matter associated with SPIE Proceedings Volume 7200, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing

When different incidences are launched in atmospheric turbulence, it is known that the intensity fluctuations exhibit different characteristics. In this paper we review our work done in the evaluations of the scintillation index of general beam types when such optical beams propagate in horizontal atmospheric links in the weak fluctuations regime. Variation of scintillation indices versus the source and medium parameters are examined for flat-topped-Gaussian, cosh- Gaussian, cos-Gaussian, annular, elliptical Gaussian, circular (i.e., stigmatic) and elliptical (i.e., astigmatic) dark hollow, lowest order Bessel-Gaussian and laser array beams. For flat-topped-Gaussian beam, scintillation is larger than the single Gaussian beam scintillation, when the source sizes are much less than the Fresnel zone but becomes smaller for source sizes much larger than the Fresnel zone. Cosh-Gaussian beam has lower on-axis scintillations at smaller source sizes and longer propagation distances as compared to Gaussian beams where focusing imposes more reduction on the cosh- Gaussian beam scintillations than that of the Gaussian beam. Intensity fluctuations of a cos-Gaussian beam show favorable behaviour against a Gaussian beam at lower propagation lengths. At longer propagation lengths, annular beam becomes advantageous. In focused cases, the scintillation index of annular beam is lower than the scintillation index of Gaussian and cos-Gaussian beams starting at earlier propagation distances. Cos-Gaussian beams are advantages at relatively large source sizes while the reverse is valid for annular beams. Scintillations of a stigmatic or astigmatic dark hollow beam can be smaller when compared to stigmatic or astigmatic Gaussian, annular and flat-topped beams under conditions that are closely related to the beam parameters. Intensity fluctuation of an elliptical Gaussian beam can also be smaller than a circular Gaussian beam depending on the propagation length and the ratio of the beam waist size along the long axis to that along the short axis (i.e., astigmatism). Comparing against the fundamental Gaussian beam on equal source size and equal power basis, it is observed that the scintillation index of the lowest order Bessel-Gaussian beam is lower at large source sizes and large width parameters. However, for excessively large width parameters and beyond certain propagation lengths, the advantage of the lowest order Bessel-Gaussian beam seems to be lost. Compared to Gaussian beam, laser array beam exhibits less scintillations at long propagation ranges and at some midrange radial displacement parameters. When compared among themselves, laser array beams tend to have reduced scintillations for larger number of beamlets, longer wavelengths, midrange radial displacement parameters, intermediate Gaussian source sizes, larger inner scales and smaller outer scales of turbulence. The number of beamlets used does not seem to be so effective in this improvement of the scintillations.

In this paper, we study and compare the propagation properties of both scalar and vector vortex beams through turbulent atmosphere. The atmosphere turbulence effect is simulated with von Kármán power spectrum model using index of refraction structure parameter, C_n², values ranging from 10^-14 m^-2/3 to 10^-12 m^-2/3. For different propagation distances, the irradiance pattern and the scintillation index are computed for a fundamental Gaussian beam, a scalar vortex with topological charge of +1 and a radially polarized beam under the same turbulence condition. The results demonstrate the advantages of using vector vortex to mitigate atmospheric effects. In addition, it is found by analyzing the polarization pattern of the transmitted vector vortex beams, that they may find applications in free space communications and remote sensing.

Fluctuations in Stokes parameters of stochastic electromagnetic beams on propagation in turbulent atmosphere are studied. The expressions are derived for the scintillation indexes (contrasts) of the Stokes parameters and their versions normalized by the instantaneous intensity in the case when the beam is generated by the electromagnetic Gaussian Schell-model source with uniform polarization. We illustrate our analytical results by numerical examples.

Employing our previously obtained formulation of the average received intensity for arbitrary optical beam in turbulent atmosphere, intensity patterns of already known and new source profiles are obtained. Arbitrary beam is defined as an incidence having arbitrary source field distribution which is produced by decomposing the source into pixels and assigning the related field to each pixel. For each source field originating from each pixel, incremental received field is found and the total received field is obtained by superposing the contributions from all the incremental received fields. Using the mentioned formula governing the arbitrary beam excitation in turbulence, average received intensity patterns for various types of beams such as cos-Gaussian, cosh-Gaussian, higher-order annular, flat-topped, general type and arbitrary beams are obtained. Our results can be applied in atmospheric optics communication links, reflection from rough surfaces, optical cryptography, optical imaging systems and propagation of partially coherent light.

The radiative transfer (RT) approach is widely used for studying scattering from layered random media with rough interfaces. Although it has been successful in several applications such as remote sensing of atmosphere it is well known that this approach involves certain approximations. In this paper these assumptions and approximations are reexamined and explained. To enable this a statistical approach is employed to this problem and the governing equations for the first and second moments of the wave fields are derived. A transition is hence made to arrive at a system of equations corresponding to that of the RT approach. It is thus found that more conditions are implicitly involved in the RT approach than generally believed to be necessary.

Cirrus clouds in the upper troposphere and lower stratosphere (UTLS) can impact the efficiency and effectiveness of infrared directed energy (laser) applications, including laser communications systems, due to attenuation (absorption and scattering) of energy. The accurate prediction of cirrus clouds, including subvisual cirrus, is often difficult for operational numerical weather prediction (NWP) models because the models require high resolution and advanced cloud microphysics schemes. We solved the fully three-dimensional, moist, compressible, non-hydrostatic Navier-Stokes equations using a vertically-stretched adaptive grid nested within the Weather Research and Forecasting (WRF) model over a geographical region of interest. We used an adaptive time-split integration scheme for the temporal discretization. We used the Thompson cloud microphysical parameterization scheme for the cirrus cloud development. The initial conditions and boundary conditions for the WRF simulations were extracted from the European Centre for Medium Range Weather Forecasting (ECMWF) T799L91 global analyses. We ran the simulation for a domain centered on the coast of Southern California and the results are compared to meteorological satellite and radiosonde observations for selected locations.

Modern laser tracking systems use digital video cameras for imaging the tracked objects in atmosphere. Using images from camera for measurement lead to a measurement error of object coordinates and all other related parameters due to atmospheric turbulence and thermal blooming of laser beam which track the object. In our investigation the turbulent and thermal blooming features of object coordinates measurement error are considered. The estimations in the conditions of mutual action of distorting atmospheric effects mentioned above are generated. The calculation of error using our estimations in most cases became two orders faster. Modeling is performed for the case of strong turbulent fluctuations as well as for the case of weak turbulence.

A brief review is provided of the current state of the concepts, analytic theory and simulation capabilities related to the propagation of spatially partially coherent beams through turbulence. A spatially, partially coherent beam can reduce scintillation at the receiver and provide better free space optical link performance.

A fiber bundle arrangement has been proposed in the literature to produce a partially spatially coherent beam. The bundle contains fibers of random lengths and the source has limited temporal coherence. The analytical expression for the autocorrelation function of the bundle pupil is derived, which defines the behavior of the beam as it propagates. Simulation results are used to verify the analytic theory. The far-field irradiance pattern for the bundle arrangement is addressed analytically and through wave optics simulation. Averaging the irradiance pattern for a number of simulation realizations was shown to converge to the analytic result. The process of averaging over a small band of source wavelengths is developed and is shown to result in a spatially partially coherent beam. Simulation results again compared favorably with the analytic theory.

The propagation of vortex beams as an information carrier for free-space laser communications has been proposed as a method for propagating through weak and strong atmospheric turbulence. This paper shows simulations of vortex charge conservation through turbulence and an analysis of the results. We also show an experimental demonstration of the generation of optical vortices with both integral and fractional charge using a segmented deformable mirror.

A new concept of a free-space, high-speed optical communication (FSOC) system based on spectral encoding of radiation from a broadband pulsed laser is developed. It is known that the intensity fluctuations of a partially coherent beam in combination with a time-averaging photodetector leads to a significant scintillation reduction with the corresponding improvement of the bit error rate by several orders of magnitude. Unfortunately, the time-averaging method cannot be applied directly to gigabit data rate communication. The main limitation of this method is related to the requirement that the correlation time between different spatially coherent spots be shorter than the response time of the photodetector. We propose to extend the technique of scintillation suppression, based on time averaging of a partially coherent beam, to gigabit data rate FSOC. In our approach, information is encoded in the form of amplitude modulation of the spectral components of the laser pulse which has a broad spectrum. To examine the intensity fluctuations of a partially coherent beam under the conditions of strong turbulence, we developed an asymptotic method for solution of the kinetic equation for the photon distribution function. We show that, for long distances, scintillations and beam wandering can be significantly suppressed.

Lasers offer tremendous advantages over RF communication systems in bandwidth and security. Atmospheric turbulence causes severe received power variations and high bit error rates (BERs) in airborne laser communication. If two or more laser beams are separated sufficiently, they can effectively "average out" the effects of the turbulence. This requisite separation distance is derived for multiple geometries, turbulence conditions, and turbulence effects. Integrating multiple techniques into a system alleviates the deleterious effects of turbulence without bulky adaptive optics systems. Wave optics simulations show multiple transmitters, receiver and transmitter trackers, and adaptive thresholding significantly reduce BER (by over 10,000 times).

Data-rates of long-range free-space optical communication links are deteriorated by atmospheric turbulence which causes power in the bucket fluctuations. In order to compensate for those effects the use of adaptive optics is envisioned. Different solutions have been proposed for the correction. We study here the performances of several compensation methods, encompassing both amplitude and phase and phase-only precompensation. In the case of phase-only precompensation we studied two system designs, one which is dedicated to symmetrical communication systems and the other to dissymmetric systems. In the dissymmetric case we studied two ways of driving the deformable mirror: the use of a Shack-Hartmann wavefront sensor and a model-free phase modulation. For each compensation architecture simulation results covered weak, moderate and strong turbulence conditions.

The use of multiple, mutually incoherent beams for a laser link through turbulence can reduce the effects of scintillation at the receiver. A recent approach involving an array of parallel propagating beams in a circle pattern has been presented in the literature and studied to determine the best beam spacing for the lowest scintillation effects. This approach has the disadvantage of reduced irradiance at the receiver as the beam separation increases. To remove this constraint, we present a modified approach where the multiple beams are individually aimed at the receiver. A wave (physical) optics simulation was applied to study the performances of 2-, 3-, 4-, and 5-beam transmitter sources. The mean irradiances I and scintillation indices σ_I² are examined for some specific link geometries and fluctuation strengths and the results are compared with those of the original method. For evaluation, we also employ a recently proposed performance metric that incorporates both I and σ_I² to indicate the near-optimal beam separation for the cases studied. Finally, we examine the probability of fade for our selected cases the modified approach has improved performance.

We desire to simulate in the laboratory the speckle fields that would be produced by laser light scattered off of rough surfaces. We are investigating the ability of liquid crystal spatial light modulators to simulate under electronic control the statistical properties of speckle fields. Characteristics of the liquid crystal spatial light modulator, including temporal response, spatial correlation, and discreteness in phase modulation will affect the ability to accurately simulate arbitrary speckle fields. In the work reported here we describe that status of characterization of two types of liquid crystal modulators. The method for accomplishing this characterization is described.