Polarization-Crosstalk-Tolerant Baud-Rate Timing Recovery for Coherent Optical Interconnects

Baud-rate sampling offers significant reductions in power consumption and implementation complexity for coherent data center interconnects (DCIs), but imposes stricter requirements on timing recovery. In this paper, we propose a polarization- crosstalk-tolerant baud-rate timing recovery scheme, termed nonlinear Mueller and Müller (NL-MM), tailored for coherent DCIs. By introducing a smooth nonlinear function into the timing error detector (TED), the proposed scheme alleviates the performance degradation observed in conventional sign-MM schemes under polarization crosstalk. Simulation results confirm strong robustness across various conditions, including polarization angle variations, phase rotations, amplified spontaneous emission (ASE) noise, and different normalized loop bandwidths (NLBWs). Specifically, the proposed scheme resolves the failure of sign-MM under severe polarization mixing, which causes distorted signal distributions and introduces multiple zero-crossings in the S-curve of the sign-MM, leading to incorrect timing lock positions. At a polarization angle of 45°, it achieves a reduction in timing mean square error (MSE) of up to 37 dB compared to sign-MM. Experimental validation is conducted using a 70 km 118 Gbaud polarization-multiplexed 16QAM system. Under small NLBW, the proposed scheme incurs only a 0.8 dB OSNR penalty relative to an improved Gardner scheme (at 2 SPS). Under large NLBW, it delivers OSNR gains of 3.62 dB and 0.37 dB over the sign-MM and improved Gardner schemes, respectively, due to significantly reduced timing jitter. These results demonstrate the robustness and practical effectiveness of the proposed scheme for polarization- and phase-impaired coherent optical links.