Mitigation of optical multipath interference impact for directly detected PAMn system

Area of Science:

• Optical communications engineering
• Signal processing for optical systems

Background:

• Multipath interference (MPI) significantly degrades performance in intensity modulation direct detection (IMDD) systems.
• Pulse amplitude modulation (PAM4) systems are particularly susceptible to MPI-induced carrier-carrier beat noise.

Purpose of the Study:

• To propose and validate a novel scheme for mitigating MPI impact in IMDD systems, specifically targeting PAM4 applications.
• To enhance the receiver's tolerance to MPI by suppressing associated noise.

Main Methods:

• Theoretical analysis of the proposed high-pass filter (HPF) scheme.
• Numerical simulations to model MPI mitigation.
• Experimental verification using optical transmitters like Mach-Zehnder modulator (MZM), electro-absorption modulated laser (EML), and directly modulated laser (DML).

Main Results:

• The proposed HPF scheme improves MPI tolerance by over 5dB for 106.4Gb/s PAM4 systems using MZM transmitters without chirp.
• For EML transmitters, a 2dB MPI tolerance gain is achievable with a chirp factor below 1.5, though effectiveness is reduced by transient chirping.
• The scheme is ineffective for typical DML transmitters due to significant adiabatic chirp broadening the laser linewidth.

Conclusions:

• A receiver-based HPF is an effective solution for mitigating MPI in PAM4 systems, particularly for chirp-free transmitters.
• The effectiveness of the HPF scheme is dependent on transmitter characteristics, with chirping in EML and DML impacting performance.
• This approach offers a practical method to enhance the robustness of optical communication systems against MPI.