Fiber coupling efficiency for free-space optical communication through atmospheric turbulence

High-speed free-space optical communication systems have recently utilized components that have been developed for fiber-optic communication systems. The received laser beam in such a system must be coupled into a single-mode fiber at the input of a commercially available receiver module or a wavelength division demultiplexer. However, propagation through atmospheric turbulence destroys the spatial coherence of a laser beam and limits how much of the available power can be coupled into the single-mode fiber. In this paper, we numerically evaluate fiber coupling efficiency for laser light distorted by atmospheric turbulence. This involves the overlap integral of the optical field and the fiber mode and the calculation makes use of the mutual coherence function of the incident light. The results for weak fluctuation conditions indicate what level of coupling efficiency can be expected for a given turbulence strength. In addition, the results provide an estimate of how long the link distance can be before the coupling efficiency degrades to an unacceptable level.