We experimentally demonstrate a single sensor virtual ghost imaging (VGI) configuration that uses the physics of nearly diffraction free light sources to penetrate partially obscuring media or turbulent media and generate images of remote opaque objects. Randomly displaced nearly diffraction free Bessel beams provided improved illumination patterns for VGI and resolving small distant targets. VGI recovered the image of objects even when the coarse illuminating Bessel beam was transmitted through obscuring and turbulent media or through a small transversely displaced aperture. Bessel beam experiments are compared with Gaussian beam experiments under similar conditions.

Optical turbulence research contributes to improved laser communications, adaptive optics, and long-range imaging systems. This paper presents experimental measurements of scintillation and focal spot displacement to obtain optical turbulence information along a near-horizontal 2.33 km free-space laser propagation path. Calculated values for the refractive index structure constant (C(n)(2)) and Fried parameter (r0) are compared to scintillometer-based measurements for several cases in winter and spring. Optical measurements were investigated using two different laser sources for the first and second parts of the experiment. Scintillation index estimates from recorded signal intensities were corrected to account for aperture averaging. As a result, we found that an earlier calculation algorithm based on analysis of log-amplitude intensity variance was the best estimator of optical turbulence parameters over the propagation path considered.

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Experimental research is conducted to determine the characteristic behavior of high frequency laser signal intensity data collected over a 2.33 km optical path. Results focus mainly on calculated power spectra and frequency distributions. In addition, a model is developed to calculate optical turbulence intensity (C(n)/2) as a function of receiving and transmitting aperture diameter, log-amplitude variance, and path length. Initial comparisons of calculated to measured C(n)/2 data are favorable. It is anticipated that this kind of signal data analysis will benefit laser communication systems development and testing at the U.S. Army Research Laboratory (ARL) and elsewhere.

Refractive index and microclimate fluctuations can significantly affect free-space laser communications. To better understand these physics relationships, optical scintillometer data were collected over a near-horizontal propagation path along with in-situ rooftop measurements of temperature variance. Regression analysis of time-averaged data revealed that fairly high correlation values (i.e., R >/= 0.80) occurred in 8 of 21 cases studied. Analysis suggests that point sensors can provide valuable information on optical turbulence for extended paths. Additional research is recommended to further explore point measurements and their relation to integrated values of optical turbulence over inhomogeneous paths.

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A key element in determining point-to-point acoustic transmission within and above forests is modeling the variation (with height above ground) of the effective speed of sound. Effective speed of sound is readily derived from estimates of air temperature, relative humidity, and wind velocity. However, meteorological models for the forest canopy vary from comparatively simple to academically complex, requiring different amounts and numbers of inputs and computer capabilities. In addition, not all canopy profile models are suitable for acoustic applications. In this paper, a meteorological computer model for the forest canopy is developed to derive continuous profiles of effective sound speed from the ground to 3 h, where h is the height of the canopy. In turn, these profiles are used to make some initial approximations of short-range acoustic transmission loss through a uniform forest stand for typical clear sky, midday atmospheric conditions. Also, a radiative transfer and energy budget algorithm is incorporated into the model to obtain the appropriate heat source profile for any time of day. Thus, physics-based micrometeorology is coupled to acoustics for future applications of acoustic information in forest environments.

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