Turbulence-resilient pilot-assisted self-coherent free-space optical communications using a photodetector array for bandwidth enhancement

We experimentally demonstrate a 4-Gbit/s 16-QAM pilot -assisted, self-coherent, and turbulence-resilient free-space optical link using a photodetector (PD) array. The turbu-lence resilience is enabled by the efficient optoelectronic mixing of the data and pilot beams in a free-space -coupled receiver, which can automatically compensate for turbulence-induced modal coupling to recover the data's amplitude and phase. For this approach, a sufficient PD area might be needed to collect the beams while the bandwidth of a single larger PD could be limited. In this work, we use an array of smaller PDs instead of a single larger PD to over-come the beam collection and bandwidth response trade-off. In the PD-array-based receiver, the data and pilot beams are efficiently mixed in the aggregated PD area formed by four PDs, and the four mixing outputs are electrically com-bined for data recovery. The results show that: (i) either with or without turbulence effects (D/r0 =-8.4), the 1-Gbaud 16-QAM signal recovered by the PD array has a lower error vector magnitude than that of a single larger PD; (ii) for 100 turbulence realizations, the pilot-assisted PD-array receiver recovers 1-Gbaud 16-QAM data with a bit-error rate below 7% of the forward error correction limit; and (iii) for 1000 turbulence realizations, the average electrical mixing power loss of a single smaller PD, a single larger PD, and a PD array is-5.5 dB,-1.2 dB, and-1.6 dB, respectively. (c) 2022 Optica Publishing Group