Channel fading-robust pre-coders for low complexity few-mode IM/DD short reach fiber communications

In this paper, an autoencoder (AE) is used to optimize linear and nonlinear digital pre-coders for short reach few-mode fiber transmission. Mach-Zehnder modulation is used to modulate the intensity of the optical carriers, and direct detection is employed at the receiver. The modes are (de)multiplexed using photonic lanterns with air cladding. The AEs are optimized to be robust against cross-talk (XT) and distance variations, which result in time varying power fading across the different modes. The system is exemplified using a simulation of a graded index fiber with the mode groups $LP_{01}$ and $LP_{11}$ used for data transmission. The channel and (de)multiplexer XT parameters and fading distributions are obtained from experimental measurements. In the case of unknown distance, the distance-robust pre-coding allows for on-off keying transmission with an error rate below the chosen forward error correction threshold even at severe XT expectations and variations. In the case of a fixed-length system, pre-coding enables high order modulation such as 4PAM and 8PAM. In this case, nonlinear pre-coding achieves signal-to-noise ratio gains of up to 1.5 dB w.r.t. linear pre-coding and up to 4 dB w.r.t. simple power pre-emphasis.