Single-source multiplexed readout of cascaded microring resonators enabled by optical frequency combs

Multiplexed optical sensing using integrated photonic platforms demands scalable interrogation architectures that do not compound hardware complexity as the number of sensing channels increases. Here, we demonstrate simultaneous readout of two cascaded silicon nitride (Si₃N₄) racetrack microring resonators (MRR) with different free spectral ranges (FSR), using a single electro-optic optical frequency comb (EO-OFC) generated from one continuous-wave laser source. By tuning the radio-frequency (RF) modulation frequency to match the spectral separation between selected resonances of the two rings, individual comb lines are aligned to each resonator, enabling independent and simultaneous resonance tracking without wide-range wavelength scanning. A tuneable filter demultiplexes the carrier and first-order sideband channels to dedicated photodetectors, allowing real-time monitoring of both rings in parallel. The system was validated through bulk refractive index (RI) sensing using glycerol solutions at five concentrations (0.039–0.6% v/v). Both rings achieved sensitivities in the range of 26.8–38.5 nm per refractive index unit (nm/RIU) and limits of detection (LoD) on the order of 10 -5 RIU (2.15–2.96 × 10 -5 RIU), with OFC-multiplexed performance matching single-wavelength baseline measurements on the same chip. The carrier and sideband channel assignments were interchangeable, confirming architectural flexibility. These results establish EO-OFC interrogation as a viable, scalable route to multiplexed microring resonator sensing, with direct implications for parallel biosensing, environmental monitoring, and food safety applications.