Advanced Digital Signal Processing for High-Capacity Mode-Division Multiplexed Free-Space Optical Communications

Spatial modes provide a potential dimension to boost the capacity of free-space optical (FSO) communication systems. Various modal basis sets can be used. For a given aperture size, complete orthogonal modal basis sets can provide higher capacity compared to incomplete modal basis sets, but are more sensitive to FSO channel impairments, such as atmospheric turbulence. In this invited paper, we review our recent progress in using advanced digital signal processing algorithms for the implementation of high-capacity mode-division multiplexed FSO communication systems when employing complete modal basis sets. Besides turbulence, the relatively high inter-mode crosstalk from a commercial multiplexer/demultiplexer has been taken into account. By employing adaptive loading at the transmitter side and/or advanced multiple-input multiple-output detection algorithms at the receiver side, record-high single-wavelength transmission data rates and spectral efficiency have been achieved over both turbulence-free and turbulent FSO links, where all key devices are commercially available.