Mode diversity coherent receiver with few-mode fiber-coupling for high-speed free-space optical communication under atmospheric turbulence

摘要

We report investigations on the mode diversity receiver applied in optical links from low-earth-orbital satellites to a ground terminal. We numerically simulated temporal power variations coupled to each mode of a FMF under atmospheric turbulence at several elevation angles. For weak influence of turbulence, the optical beam mainly coupled to the fundamental mode of the FMF; for strong influence of turbulence, it coupled significantly to higher order modes of the FMF and mode diversity reception was more effective. Emulating the fast-changing coupled power to each mode of the FMF with variable optical attenuators and arbitrary waveform generators, we experimentally evaluated the performance of three-mode diversity reception of 10 Gb/s QPSK. Digital signal processing was performed on 20-ms-long successively acquired data bursts, which were long enough to contain typical features of fading. We confirmed that digital combining could track the severe power variations regardless of elevation angles. We extended our investigation to the possibility of more severe conditions and we showed through simulation further improvement using multiple mode diversity receivers combined with aperture diversity.