  Atmospheric turbulence generally limits free-space optical (FSO)
communications, and this problem is severely exacerbated when implementing
highly sensitive and spectrally efficient coherent detection. Specifically,
turbulence induces power coupling from the transmitted Gaussian mode to
higher-order Laguerre-Gaussian (LG) modes, resulting in a significant decrease
of the power that mixes with a single-mode local oscillator (LO). Instead, we
transmit 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 corresponding
pairs of the beams' modes. During mixing, a conjugate of the turbulence
experienced by the pilot tone is automatically generated and compensates the
turbulence experienced by the data, and nearly all orders of the same
corresponding modes efficiently mix. We demonstrate a 12-Gbit/s
16-quadrature-amplitude-modulation (16-QAM) polarization-multiplexed (PolM) FSO
link that exhibits resilience to emulated turbulence. Experimental results for
turbulence D/r_0~5.5 show up to ~20 dB reduction in the mixing power loss over
a conventional coherent receiver. Therefore, our approach automatically
recovers nearly all the captured data power to enable high-performance coherent
FSO systems.
