RESUMEN
With appropriate processing techniques, sonic anemometry can provide useful insights into the strength and spectral shape of optical turbulence. Closed form propagation models and simulations require stationary optical turbulence statistics to make a meaningful comparison with experimental results. This work presents a new approach for examining the stationarity of data provided by sonic anemometry and by fitting optical turbulence parameters. Von Kármán, Greenwood-Tarazano, and one minus exponential spectral models are fitted to experimental data. Each model fit is evaluated using information criteria. The Greenwood-Tarazano model is shown to provide the best fit to experimental data. Optical turbulence parameters from the Greenwood-Tarazano model are compared with results from instruments at varying heights above the ground.
RESUMEN
Optical Kerr effects induced by the propagation of high peak-power laser beams through real atmospheres have been a topic of interest to the nonlinear optics community for several decades. This paper proposes a new analytical model for predicting the filamentation/light channel onset distance in real atmospheres based on modulation instability model considerations. The normalized intensity increases exponentially as the beam propagates through the medium. It is hypothesized that this growth can be modeled as a weighted ratio of the Gaussian beam diameter at range to the lateral coherence radius and can be used to set the power ratio for an absorbing, turbulent, nonlinear media to estimate the beam collapse distance. Comparison of onset distance predictions with those found from computer simulation and deduced from field experiments will be presented. In addition, this model will be used with an analytical approach to quantify the expected radius of light channels resulting from self-focusing both with and without the production of a plasma filament. Finally, this paper will describe a set of 1.5-micron, variable focal length USPL field experiments. Comparisons of theoretical radius calculations to measurements from field experiments will be presented.
RESUMEN
Experimental data are presented that demonstrate the evolution of the anisotropy/isotropy of atmospheric statistics throughout the course of four days (two winter, two summer) near the ground over a concrete runway in Florida. In late January and early February of 2017, a 532 nm near-plane-wave beam was propagated 1 and 2 km at a height of 2 m above the runway, and irradiance fluctuations were captured on a CCD array. In August of 2017, a 532 nm Gaussian beam was propagated 100 m at a height of near 2 m, and fluctuation data were captured on a CCD array. Winter data were processed to calculate the covariance of intensity and summer data processed to calculate the scintillation index. The resulting contours indicated a consistent pattern of anisotropy early in the day, evolving into isotropy midday, and returning to anisotropy in late afternoon. Accompanying atmospheric and wind data are presented throughout the measurement days.
RESUMEN
Experimental measurements were recently made which displayed characteristics of plane wave propagation through anisotropic optical turbulence. A near-plane wave beam was propagated a distance of 1 and 2 km at a height of 2 m above the concrete runway at the Shuttle Landing Facility, Kennedy Space Center, Florida, during January and February of 2017. The spatial-temporal fluctuations of the beam were recorded, and the covariance of intensity was calculated. These data sets were compared to a theoretical calculation of covariance of intensity for a plane wave.
RESUMEN
Image distortions caused by atmospheric turbulence are often treated as unwanted noise or errors in many image processing studies. Our study, however, shows that in certain scenarios the turbulence distortion can be very helpful in enhancing image processing results. This paper describes a novel approach that uses the scintillation traits recorded on a video clip to perform object ranging with reasonable accuracy from a single camera viewpoint. Conventionally, a single camera would be confused by the perspective viewing problem, where a large object far away looks the same as a small object close by. When the atmospheric turbulence phenomenon is considered, the edge or texture pixels of an object tend to scintillate and vary more with increased distance. This turbulence induced signature can be quantitatively analyzed to achieve object ranging with reasonable accuracy. Despite the inevitable fact that turbulence will cause random blurring and deformation of imaging results, it also offers convenient solutions to some remote sensing and machine vision problems, which would otherwise be difficult.
RESUMEN
We present the theory, design, simulation, and experimental evaluations of a new laser transmissometer system for aerosol extinction rate measurement over long paths. The transmitter emits an ON/OFF modulated Gaussian beam that does not require strict collimation. The receiver uses multiple point detectors to sample the sub-aperture irradiance of the arriving beam. The sparse detector arrangement makes our transmissometer system immune to turbulence-induced beam distortion and beam wander caused by the atmospheric channel. Turbulence effects often cause spatial discrepancies in beam propagation and lead to miscalculation of true power loss when using the conventional approach of measuring the total beam power directly with a large-aperture optical concentrator. Our transmissometer system, on the other hand, combines the readouts from distributed detectors to rule out turbulence-induced temporal power fluctuations. As a result, we show through both simulation and field experiments that our transmissometer system works accurately with turbulence strength Cn2 up to 10-12 m-2/3 over a typical 1-km atmospheric channel. In application, our turbulence- and weather-resistant laser transmissometer system has significant advantages for the measurement and study of aerosol concentration, absorption, and scattering properties, which are crucial for directed energy systems, ground-level free-space optical communication systems, environmental monitoring, and weather forecasting.