Aperture averaging for studies of atmospheric turbulence and optimization of free space optical communication links

Even after several decades of study, inconsistencies remain in the application of atmospheric turbulence theories to experimental systems, and the demonstration of acceptable correlations with experimental results. We have developed a flexible empirical approach for improving link performance through image analysis of intensity scintillation patterns coupled with frame aperture averaging on a free space optical (FSO) communication link. Aperture averaging calculations are invaluable in receiver design. A receiver must be large enough to collect sufficient power and reduce scintillation effects at a given range, but must also be of practical size. We have constructed an imaging system for measuring the effects of atmospheric turbulence and obscuration on FSO links. A He-Ne laser beam propagates over a range of 863 meters in atmospheric turbulence conditions that vary diurnally and seasonally from weak to strong. 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. The captured image frames are analyzed in Labview to evaluate the turbulence parameter Cn2, temporal and spatial intensity variances, and aperture averaging. 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. The reduction in scintillation with aperture size guides the selection of optimum receiver aperture. Spatial and temporal variance analyses within single frames and between frames are compared and show good agreement.