Optical Multi-Path Interference Mitigation for PAM4-IMDD Systems Using Balanced Coding

The optical multipath interference (MPI) effect is detrimental to the widely deployed intensity modulation direct detection (IMDD) systems. Here, we develop a balanced coding scheme-minimum running digital sum (MRDS), applied at the transmitter, shaping the signal's spectrum to suppress the low-frequency carrier-carrier beat noise. Thus, it helps mitigate MPI with a much smaller power penalty. Compared to other reported methods, it requires only simple operations with low computational complexity at the expense of small overhead. Simulation and experimental results confirm the coding scheme can effectively improve the bit error rate (BER) performance of a high baudrate four-level pulse amplitude modulation (PAM4) IMDD system under various signal-to-interference ratio (SIR), received optical power (ROP), and laser linewidth. Assisted with the cascade of feed-forward equalization (FFE), post filtering (PF), and maximum likelihood sequence estimation (MLSE), the MRDS coding performs the best among all the compared MPI mitigation methods in an experimental 30 Gbaud PAM4-IMDD system.