Low-cost 100 G PON based on a residual optical carrier for carrier recovery

We experimentally demonstrate a cost-effective dual-polarization quadrature phase shift keying (DP-QPSK) coherent passive optical network (PON) system that operates at 100 Gbits/s/λ. This system utilizes distributed feedback lasers (DFBs) and a carrier recovery algorithm facilitated by a bifunctional frequency-domain pilot tone (FPT). To reduce costs in coherent PON implementations, low-cost DFBs are employed as the sole light sources, replacing the more expensive external cavity lasers (ECLs) at both the optical line terminal (OLT) and the optical network units (ONUs). To mitigate the increased phase noise (PN) associated with the broader linewidth of DFB lasers, we introduce the straightforward residual optical carrier (ROC) method. This technique involves slight adjustments to the optical modulator bias to insert an FPT at the direct current (DC) component in the optical domain, thereby assisting in both the frequency offset equalization (FOE) and the carrier phase recovery (CPR). Additionally, digital subcarrier multiplexing (DScM), widely recognized in optical access networks for delivering diverse services, is integrated with the ROC method, incorporating an appropriate inter-subband guard interval to prevent interference between the FPT and the data signal. Experimental results demonstrate successful transmission performance for baud rates of 28 Gbaud and 32 Gbaud, respectively, achieving power budgets of 33.90 dB and 35.65 dB at the 7% hard-decision forward error correction (HD-FEC) threshold of (3.8 × 10^-3) and the 20% soft-decision forward error correction (SD-FEC) threshold of (2 × 10^-2), with both power budgets exceeding the required threshold of 29 dB. Furthermore, for complexity analysis of phase compensation, a comparison of the DScM-QPSK signal with and without FPT-aided CPR is conducted, demonstrating the superiority.