This PDF file contains the front matter associated with SPIE Proceedings Volume 6878, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

In this paper we discuss some recent results concerning beam wander-induced scintillation of a Gaussian-beam wave along a horizontal path with constant C²_n and also from ground to space, the latter based on the Hufnagle-Valley C²_n. We consider cases of tracked beams and untracked beams, both of which involve a certain amount of beam wander. Theoretical models of scintillation and corresponding probability density function (pdf) models are compared with simulation data over a broad range of beam diameters. We include both collimated and focused beams. For the uplink from ground to space path, we also examine some fade statistics.

The evolution of the scintillation and topological charge of coherent vortex beams in turbulence is analyzed. Although such beams typically have inferior scintillation properties when compared with a comparable Gaussian beam, the topological charge of the vortex beam shows significant stability. The possibility of using such beams in optical communications is discussed.

High Resolution WRF (Weather Research and Forecasting)/microscale code simulations are carried to predict and characterize stratospheric Optical Turbulence (OT) layers induced by jet streams and gravity waves under various local atmospheric conditions. This information in turn is used to improve prognostic parameterizations of eddy mixing coefficients and diagnostic parameterizations of optical turbulence for the tropopause and the lower stratosphere regions. Non-homogeneous, anisotropic, non-Kolmogorov patchy shear-stratified stratospheric turbulence requires that a fine mesh be used to resolve stiff velocity and temperature gradient profiles. Our approach is based on vertical nesting and adaptive vertical gridding in nested WRF/microscale codes. We perform effective ensemble forecasting by using initial and boundary conditions from both GFS and high resolution T799L91 ECMWF datasets. This methodology is applied to the analysis of field data from the Hawaii 2002 campaign and TREX Campaign (Terrain-induced Rotor Experiment), Owens Valley, CA, 2006. We obtain local distributions of simulated optical turbulence (C²_n ) in the upper troposphere/lower stratosphere using explicit simulations and parametrization formula that show strongly laminated structures with thin layers of high values of refractive index. These layers are characterized by steep vertical gradients of potential temperature and are located at the edges of relatively well mixed regions produced by shear instabilities and wave breaking.

The Air Force Institute of Technology Center for Directed Energy (AFIT/CDE) has developed a first principles atmospheric propagation and characterization model called the Laser Environmental Effects Definition and Reference or LEEDR. This package enables the creation of profiles of temperature, pressure, water vapor content, optical turbulence, and atmospheric particulates and hydrometeors as they relate to line-by-line layer extinction coefficient magnitude at wavelengths from the UV to the RF. Worldwide seasonal, diurnal, and geographical variability in these parameters is accessed from probability density function (PDF) databases using a variety of recently available resources to include the Extreme and Percentile Environmental Reference Tables (ExPERT), the Master Database for Optical Turbulence Research in Support of the Airborne Laser, and the Global Aerosol Data Set (GADS). GADS provides aerosol constituent number densities on a 5° x 5° grid worldwide. ExPERT mapping software allows the LEEDR operator to choose from specific site or regional upper air data to characterize correlated molecular absorption, aerosol absorption and scattering by percentile. The integration of the Surface Marine Gridded Climatology database, the Advanced Navy Aerosol Model (ANAM), and the Navy Surface Layer Optical Turbulence (NSLOT) model provides worldwide coverage over all ocean regions on a 1° x 1° grid. Molecular scattering is computed based on Rayleigh theory. Molecular absorption effects are computed for the top 13 absorbing species using line strength information from the HITRAN 2004 database in conjunction with a community standard molecular absorption continuum code. Aerosol scattering and absorption are computed with the Wiscombe Mie model. Each atmospheric particulate/hydrometeor is evaluated based on its wavelength-dependent forward and off-axis scattering characteristics and absorption effects on laser energy delivered at any wavelength from 0.355 μm to 8.6 m. LEEDR can also produce correlated optical turbulence profiles in percentile format. In addition, probability of cloud free line of sight (CFLOS) for hundreds of land sites worldwide is available in LEEDR. Effects of layers of fog, several types of rain and several types of water droplet and ice clouds can also be considered. In addition to describing some of the underlying theory to the LEEDR calculations, this paper presents graphical results for several different scenarios. These generic scenarios are meant to exemplify how LEEDR enables the physically realistic data capture of atmospheric effects on electromagnetic propagation.

Three decades of experiment by the authors have shown that the combination of high intensity light emitting diodes, silicon photodiodes, and large aperture moulded Fresnel lens collimators of moderate focal length provide effective and economical long distance atmospheric optical communications. While the use of larger transmitter and receiver lenses increases optical flux at the detector, their greatest advantage is in dramatically reducing the depth of the scintillation or rapid signal fading. This is caused by differential phase distortion, beam steering, focusing/defocusing by air turbulence cells along the transmission path, and the effects of local coherence. It has been observed that scintillation effects diminish rapidly when the transmitter and receiver apertures are larger than the central diffraction peak of the distant aperture, or about 30-cm diameter for red light over a 160-km path.

Practical implementation of the adaptive optic technique to mitigate the negative effects of the atmospheric turbulence on the propagating laser beam requires a minimal size (point-wise) beacon on the target. Existing methods to address this challenging problem apply sequential iteration cycles and wavefront control with adaptive optics mirror. However a large number of iteration cycles may require a time substantially longer than the frozen time of the atmosphere, especially in presence of the aero-optic effects. This report examines an alternative solution to this problem. Generation of the laser hot spot on the image-resolved diffused target can be achieved by using a phase conjugation scheme and realization of an open cavity with the target serving as one of its couplers. We have demonstrated that controlling the beam structure in the target image plane of the laser transmitter enables formation of the localized beacon on the target. As a result, the number of iterations required for an effective system operation can be significantly reduced, making its operation cycle much shorter than frozen time.

Undesirable turbulence effects present during propagation and imaging through the atmosphere are often reduced using adaptive optics. Many current adaptive optics systems use the Shack-Hartmann wavefront sensor requiring measurement and reconstruction of the incoming wavefront at the pupil plane. Indirect wavefront sensing methods, such as image sharpening, are based on data obtained from the image plane. We are developing an image sharpness sensor based on the Fourier spectrum of an image. High spatial frequencies contain information about the edges and fine detail of the image. Our premise is the maximizing of the high spatial frequencies will sharpen the image. In our setup the Fourier transform of the image is generated optically (and essentially instantaneously) and then various spatial-frequency bands are filtered with an opaque mask. The remaining Fourier spectrum is integrated optically and a sharpness signal is measured with a single photodetector. The collected sharpness value is used in a closed-loop to control the deformable mirror until the sharpness is maximized. We have constructed both a simulation and a laboratory experiment to study the sensor and its performance in an adaptive optics system.

A compact, lab-sized dissemination chamber is designed to characterize the fluorescence of aerosols. The chamber, designed according to short-range lidar principles, uses light-induced fluorescence (LIF) with a 355 nm pulsed source. Aerosols concentration inside the chamber can reach hundreds of thousands of ppl. Background noise and irradiance are very low and will allow accurate measurements of spectral signatures. The chamber will serve to study the correlation with spectroscopic data obtained using a long-range lidar system owned by Defence Research and Development Canada (DRDC). Pollens, bacteria, spores, dusts and other atmospheric aerosols will be studied under various environmental conditions. The chamber will be used to create trustworthy libraries for the remote sensing of bio-aerosols.

This report will describe the progress towards modeling the radiance of a mesospheric atomic sodium guidestar pumped with a continuous-wave, narrow-linewidth source. We will model the cases of pumping only the D₂a line and pumping both the D₂a and D₂b lines simultaneously. The simulation is named the sodium guidestar simulation or SGS.

Calculations of the second-order statistical characteristics of a polarized, lowest-order Gaussian beam which propagates through a jet-stream layer, between the tropopause and the lower stratosphere, are carried out. Comparison of the results based on the refractive-index structure parameter C²_n as predicted by a Hafnagen-Valley model and based on data taken at a recent campaign shows that the jet-stream can significantly affect intensity distribution and spreading of the beam, especially at high zenith angles.