Bit-error-rate-driven system-level optimization of microring resonator round-trip length for four-level pulse amplitude modulation wavelength division multiplexing optical receivers

Add–drop microring resonators (MRRs) are widely used as compact wavelength-selective filters in silicon-photonic wavelength division multiplexing (WDM) receivers. However, the MRR round-trip length simultaneously governs adjacent channel suppression and filtering-induced waveform distortion, making length selection a nontrivial system-level design problem. We present a bit-error-rate (BER)-driven optimization of the MRR round-trip length for four-level pulse-amplitude-modulation (PAM-4) WDM optical interconnect receivers and evaluate its robustness to practical impairments. An end-to-end receiver model is implemented in ANSYS Lumerical INTERCONNECT, and the round-trip length is swept as a single design parameter under otherwise identical transmitter and receiver conditions. The results show that BER is minimized at an intermediate round-trip length. At shorter lengths, BER degradation is dominated by inter-channel interference arising from adjacent channel leakage, whereas at longer lengths, resonance linewidth narrowing increases filtering-induced waveform distortion and inter-symbol interference. Robustness is assessed under target channel optical frequency detuning and increased injected electrical noise power spectral density. Although the low-BER operating window narrows as impairments increase, the overall U-shaped BER-length characteristic and an intermediate-length optimum are preserved, with only modest shifts in the BER-minimizing length under detuning. These results indicate that BER-based round-trip length selection provides a practical system-level design criterion for MRR-based receiver filters in PAM-4 WDM optical interconnect links.