Simplified coherent TDM-PON in upstream burst-mode detection with enhanced DC leakage tolerance

This paper investigates the generation and impact of optical DC leakage in upstream burst-mode coherent TDM-PON systems. In upstream burst transmission, the laser remains on to avoid wavelength drift even when the optical network unit (ONU) is inactive, leading to the generation of residual DC leakage caused by a limited modulator extinction ratio and bias drift from the null point. As the number of ONUs increases, the accumulated optical DC leakage becomes large. In a more extreme case, where the optical signal comes from a distant ONU and all the other inactive ONUs are located close to the optical line terminal, the accumulated DC power can even exceed the signal power, resulting in SNR degradation and increased quantization noise. To address these challenges, we propose a flipped-frequency heterodyne detection scheme combined with SCM signaling. This approach upconverts the DC tones from low-frequency regions to high-frequency regions, allowing DC leakage to be effectively filtered out by the bandwidth-limited receiver before analog-to-digital conversion. As a proof of concept, we analyze the impact of DC leakage in both the time and frequency domains, compare the DC tolerance of PAM and SCM under different detection schemes, and support our findings with both simulation and experimental results. Finally, a 200G upstream simplified coherent TDM-PON system is experimentally demonstrated, showing up to 18 dB improvement in DC leakage tolerance compared to a baseband PAM4 signal at the 29 dB N1 class loss budget.