Demonstration of Turbulence-Resilient Self-Homodyne 12-Gbit/s 16-QAM Free-Space Optical Communications Using a Transmitted Pilot Tone

Free-space optical (FSO) communication links can benefit from recovering both amplitude- and phase-encoded data (e.g., quadrature-amplitude-modulation, QAM) by mixing a local oscillator (LO) with a received data beam in a coherent detector. However, atmospheric turbulence can cause power coupling from a fundamental Gaussian mode of the data beam to higher-order modes, resulting in a significantly degraded mixing efficiency between the data and a fundamental Gaussian LO. Previously, a turbulence-resilient pilot-assisted self-coherent FSO QAM link has been demonstrated by transmitting a frequency-offset Gaussian pilot beam along with the Gaussian data beam. The pilot and data beams experienced similar modal coupling and thus can efficiently mix in a self-heterodyne detector. However, a frequency guard band was used to avoid signal-signal beating interference, thereby reducing the utilization efficiency of the detector's bandwidth. To increase the detector's bandwidth utilization, we experimentally demonstrate in this article a turbulence-resilient 12-Gbit/s 16-QAM FSO link using pilot-assisted self-homodyne detection. The pilot beam is located at the same center optical frequency as the data channel but on an orthogonal polarization. At the receiver, we utilize self-homodyne detection to achieve the efficient mixing of the data and pilot. Compared to self-heterodyne, our approach has ∼2X the utilization efficiency of the detector's bandwidth. Compared to conventional LO-based coherent (homodyne or intradyne) detection, the pilot-assisted self-homodyne link shows up to ∼20-dB improvement of optical-to-electrical mixing efficiency under a turbulence strength of
$2{{\boldsymbol{w}}}_0/ \ {{\boldsymbol{r}}}_0 = \ \sim\! {\boldsymbol{7}}.{\boldsymbol{5}}$
. The turbulence resilience of the link is demonstrated with a bit-error rate below the 7% forward error correction limit for 400 different turbulence realizations.