Mitigation of relative intensity noise of quantum dash mode-locked lasers for PAM4 based optical interconnects using encoding techniques

Quantum dash (Q-Dash) passively mode-locked lasers (PMLLs) exhibit significant low frequency relative intensity noise (RIN), due to the high mode partition noise (MPN), which prevents the implementation of multilevel amplitude modulation formats such as PAM4. The authors demonstrate low frequency RIN mitigation by employing 8B/10B and Manchester encoding with PAM4 modulation format. These encoding techniques reduce the overlap between the modulation spectral content and the low-frequency RIN of the Q-dash devices, at the expense of increased overhead. The RIN of the 33.6 GHz free spectral range Q-dash PMLL was characterized, and the results obtained show very high levels of RIN from DC to 4 GHz, but low levels for higher frequencies. The performance improvement for 28 GBaud 8B/10B and Manchester encoded PAM4 signal has been demonstrated compared to the case when no encoding is used. Finally, the effect of RIN on the system performance was demonstrated by comparing the bit error rate (BER) performance of the PAM4 signaling obtained with an external cavity laser (ECL) to those obtained with Q-dash PMLL.