Experimental investigation on the effect of wavelength on aperture averaging in FSO communications.

Atmospheric turbulence is a major impairment in free-space optical (FSO) communication systems. Based on the fact that the size of receiver aperture is much larger than operation wavelength, aperture averaging extracts inherent receiver diversity gains and can be used as an effective fading mitigation technique. In this Letter, we consider the three most common wavelengths used in telecommunications, namely 1550, 1310, and 850 nm, and present a comparative performance evaluation of aperture averaging under the same emulated atmospheric conditions. Our emulator is in the form of an atmospheric chamber equipped with adjustable heaters, coolers, and fans to create the desired level of turbulence. Our results demonstrate that by changing the wavelength and/or aperture size, a strong turbulence condition with Gamma-Gamma statistics can turn into a weak turbulence condition with log-normal statistics. Such a phenomenon can be observed even for relatively small aperture sizes if the wavelength is sufficiently high.