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Atmospheric turbulence generally limits free-space optical (FSO)communications, and this problem is severely exacerbated when implementinghighly sensitive and spectrally efficient coherent detection. Specifically,turbulence induces power coupling from the transmitted Gaussian mode tohigher-order Laguerre-Gaussian (LG) modes, resulting in a significant decreaseof the power that mixes with a single-mode local oscillator (LO). Instead, wetransmit a frequency-offset Gaussian pilot tone along with the data signal,such that both experience similar turbulence and modal power coupling.Subsequently, the photodetector (PD) optoelectronically mixes all correspondingpairs of the beams' modes. During mixing, a conjugate of the turbulenceexperienced by the pilot tone is automatically generated and compensates theturbulence experienced by the data, and nearly all orders of the samecorresponding modes efficiently mix. We demonstrate a 12-Gbit/s16-quadrature-amplitude-modulation (16-QAM) polarization-multiplexed (PolM) FSOlink that exhibits resilience to emulated turbulence. Experimental results forturbulence D/r_0~5.5 show up to ~20 dB reduction in the mixing power loss overa conventional coherent receiver. Therefore, our approach automaticallyrecovers nearly all the captured data power to enable high-performance coherentFSO systems.

Turbulence-Resilient Coherent Free-Space Optical Communications using  Automatic Power-Efficient Pilot-Assisted Optoelectronic Beam Mixing of Many  Modes