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

Abstract

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 Cn 2 . 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, Cn 2 , and the aperture averaging factor. Our correlation measurement technique represents a probe into the length scales of different flows and make for a “fingerprint” of the air conditions at the time. 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.