Turbulence mitigation of four mode-division-multiplexed QPSK channels in a pilot-assisted self-coherent free-space optical link using a photodetector array and DSP-based channel demultiplexing

In this Letter, we demonstrate turbulence mitigation of four mode -division -multiplexing (MDM) quadrature-phaseshift-keying (QPSK) channels in a pilot -assisted selfcoherent free -space optical (FSO) link using a photodetector (PD) array and digital signal processing (DSP)-based channel demultiplexing. A Gaussian pilot beam is co -transmitted with four 1-Gbaud QPSK channels carried by four orbital angular momentum (OAM) modes. The pilot beam experiences similar turbulence -induced wavefront distortion to the data beams. At the receiver, the turbulence distortion is mitigated by its conjugate during the pilot-data mixing in a PD array. Subsequently, we demultiplex the four channels by applying in DSP a fixed matrix on the signals. Results show that our approach has <3 -dB turbulence -induced power penalty at a 7% forward error correction (FEC) limit under a turbulence strength of 2w0/r0 = similar to 4.4. The same turbulence can cause >18 -dB penalties for a local oscillator (LO) -based coherent MDM system. (c) 2024 Optica Publishing Group