Mitigating optical multipath interference in IM-DD systems via efficient joint notch coding and nonlinear equalization.

Optical multipath interference (MPI) is a major performance-limiting impairment in IM-DD links, inducing strong signal-dependent intensity fluctuations, particularly when large-linewidth DFB lasers and high-order PAM formats are used. In the frequency domain, MPI manifests as low-frequency and narrowband noise. In this Letter, we propose a low-complexity joint mitigation scheme that combines minimum block digital sum (MBDS) notch coding at the transmitter with a 2nd-order polynomial nonlinear filter (PNLF) equalizer at the receiver. The MBDS scheme enables a significantly deeper and wider low-frequency notch with lower overhead, while the PNLF further suppresses MPI-induced level fluctuations with minimal computational cost. Experimental results of 40 Gbaud and 56 Gbaud MPI-impaired PAM-4 O-band IM-DD systems demonstrate the SIR tolerance improvements of 3.25 dB and 5.23 dB by the proposed joint scheme, respectively, outperforming existing schemes.