Dr. Joseph D. Harris Profile

Dr. Joseph D. Harris

at Univ of Maryland College Park

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Publications (2)

Proceedings Article | 12 August 2009 Paper

A new technique for probing length scales in clear air

Christopher Davis, Joseph Harris, Robert Gammon

Proceedings Volume 7463, 746306 (2009) https://doi.org/10.1117/12.828126

KEYWORDS: Sensors, Turbulence, Correlation function, Light scattering, Scattering, Atmospheric propagation, Scintillation, Fluid dynamics, Multiple scattering, Signal detection

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Proceedings Article | 19 August 2008 Paper

Aperture averaging and correlation function measurements in strong atmospheric turbulence for optical wireless applications

Heba Yuksel, Joseph Harris, Yunxin Tang, Robert Gammon, Christopher Davis

Proceedings Volume 7091, 70910N (2008) https://doi.org/10.1117/12.795054

KEYWORDS: Turbulence, Receivers, Correlation function, Sensors, Free space optics, Atmospheric turbulence, Atmospheric optics, Scintillation, Telescopes, LabVIEW

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The performance of free space optical (FSO) links in a clear atmosphere is affected by the non-ideal characteristics of the communication channel. Atmospheric turbulence causes fluctuations in the received signal level, which increase the bit errors in a digital communication link. In order to quantify performance limitations, a better understanding of the effect of the intensity fluctuations on the received signal at all turbulence levels is needed. Theory reliably describes the behavior in the weak turbulence regime, but theoretical descriptions in the intermediate and strong turbulence regimes are less well developed. We have developed a flexible empirical approach for characterizing link performance in strong turbulence conditions through image analysis of intensity scintillation patterns coupled with frame aperture averaging on an FSO communication link. These measurements are complemented with direct measurements of temporal and spatial correlation functions. A He-Ne laser beam propagates 106 meters in free-space over flat terrain about a meter above the ground to provide strong atmospheric turbulence conditions. A high performance digital camera with a frame-grabbing computer interface is used to capture received laser intensity distributions at rates up to 30 frames per second and various short shutter speeds, down to 1/16,000s per frame. A scintillometer is used for accurate measurements of the turbulence parameter C_n². Laboratory measurements use a local strong turbulence generator, which mimics a strong phase screen. Spatial correlation functions are measured using laterally separated point detectors placed in the receiver plane. Correlations and captured image frames are analyzed in Labview to evaluate correlation functions, C_n², and the aperture averaging factor. The aperture averaging results demonstrate the expected reduction in intensity fluctuations with increasing aperture diameter, and show quantitatively the differences in behavior between various strengths of turbulence. This paper will present accurate empirical data in the strong turbulence regime. Such results can help build upon existing empirical data and lead to the development of new theories.

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