Wavelength-Interleaved 4D-PAM4-TCM in a 4-λ × 100 Gb/s CWDM System

In a practical 4-λ coarse wavelength division multiplexing (CWDM) intensity modulation direct detection (IM/DD) system, different wavelength channels achieve different transmission performances due to 1) inter-channel component differences and 2) chromatic dispersion (CD) - chirp interaction. Large channel performance discrepancies could significantly degrade the average pre-forward error correction (pre-FEC) bit error rate (BER) performance of the system. To improve the system's robustness against channel performance discrepancies, we propose to employ a symbol-level wavelength-interleaver in conjunction with four-dimensional - 4-level pulse amplitude modulation - trellis coded modulation (4D-PAM4-TCM). We experimentally compare the performances of the proposed wavelength-interleaved 4D-PAM4-TCM system and a conventional 4D-PAM4-TCM system in a 4-λ × 100 Gb/s CWDM IM/DD link over 0-20 km. We observe that the wavelength-interleaved 4D-PAM4-TCM system outperforms the conventional 4D-PAM4-TCM system in terms of receiver sensitivity required at the KP4 FEC threshold when sizable channel performance discrepancies exist in such systems.