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Benefiting from the rapid development of fiber-optic devices, high-speed free-space optical communication systems have recently used fiber-optic components. The received laser beam in such a system couples into single-mode fiber (SMF) at the input of the receiver module. This work is oriented to common problems in actual free-space optical coupling systems, such as atmospheric turbulence, optical system aberration, and fiber positioning error. We derive the statistical expectation models of SMF coupling efficiency with optical system aberration in the presence of atmospheric turbulence and the statistical expectation models of SMF coupling efficiency with fiber positioning error in the presence of atmospheric turbulence. The influences of optical system aberration and fiber positioning error on the coupling efficiency under different turbulence strengths are also analyzed in this paper.

Few-mode fiber (FMF) based receiver emerges as a promising solution for free-space optical (FSO) communication due to its excellent performance in the presence of turbulence. We propose a theoretical model that uses the coupling efficiency of FMF to evaluate the performance of FMF-based FSO system in the presence of turbulence. The series solutions and asymptotic solutions to bit-error rate (BER) of such system are derived for maximal-ratio combining (MRC) scheme and equal gain combining (EGC) scheme over the Gamma-Gamma turbulence channels. Simulation results show that for the FMF-based FSO system, the asymptotic BER of MRC and EGC are highly accurate in the large transmitted optical power regimes.

Thanks to the rapid development of coherent optical fiber communication, the fiber-based coupling structure of the free-space optical (FSO) communication systems becomes a typical structure. In such systems, the received light field is coupled to a single-mode fiber or a few-mode fiber (FMF). In this study, we respectively establish the expectation model of FMF coupling efficiency under conditions of optical system aberration (OSA) and fiber positioning error (FPE) based on the Kolmogorov power-law spectrum and plane wave. The performances of FMF with different coupling geometry parameters are analyzed. To the best of our knowledge, it is the first time to analyze the impacts of OSA and FPE on the coupling efficiency of the FMF under different turbulence strengths and coupling geometry parameters are analyzed.

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