Turbulence-resilient differential-phase-shift-keying free-space optical communications using automatic multi-mode optoelectronic mixing

Free-space-optical (FSO) communications can benefit from power-efficient modulation formats and detection methods. In general, differential-phase-shift-keying (DPSK) tends to be more optical-power efficient when detecting both output arms in the receiver as compared to intensity modulation. Unfortunately, turbulence can induce power coupling from a Gaussian mode to higher-order spatial modes, which can produce high power loss in the differential receiver. Here we demonstrate a 2.25-Gbit/s turbulence-resilient DPSK FSO link under emulated turbulence effects with a strength of 2 omega(0)/r(0) similar to 5.5. The turbulence-induced modal coupling loss is automatically compensated by optoelectronic (O/E) mixing of many higher-order spatial modes. The experimental results, based on 200 random turbulence realization measurements, indicate that compared with a single-mode coherent receiver, the O/E multi-mode mixing can reduce the average turbulence-induced mixing loss by similar to 14.6 dB.