Single-wavelength converged IM-DD/coherent PON for heterogeneous ONU coexistence using a broadband MZM and bias optimization

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The evolution of passive optical networks (PONs) toward 100 Gb/s/λ and beyond demands a cost-effective migration path that can simultaneously support legacy intensity-modulation and direct-detection (IM-DD) users and emerging high-capacity services. In this work, we propose and experimentally demonstrate a single-wavelength hybrid IM-DD/coherent PON architecture that enables the coexistence of heterogeneous optical network units (ONUs). A broadband thin-film lithium niobate (TFLN) Mach–Zehnder modulator (MZM) is employed at the optical line terminal (OLT) to enable the integrated generation of both a baseband IM-DD signal and an up-converted IQ signal via frequency-division multiplexing (FDM). Furthermore, bias optimization of the MZM is introduced as a key system parameter to balance modulation linearity and receiver sensitivity, thereby enabling flexible performance trade-offs between low-cost DD ONUs and high-capacity coherent ONUs. We experimentally demonstrate downlink transmission of a 50 Gb/s on-off keying (OOK) signal and a 200 Gb/s 16-ary quadrature amplitude modulation (16-QAM) signal over a 20 km standard single-mode fiber (SSMF) simultaneously, achieving a power budget of 31/29 dB, respectively. The impacts of dispersion-induced power fading in DD ONUs and crosstalk in coherent ONUs are mitigated through tailored digital signal processing for each receiver type. The proposed architecture provides a spectrally efficient and scalable solution for heterogeneous optical access networks, reducing wavelength resource consumption while enabling smooth capacity upgrades over existing optical distribution networks (ODNs). By jointly leveraging broadband modulation and bias-controlled signal shaping, this work offers a practical pathway toward the convergence of IM-DD and coherent technologies in next-generation optical access networks.