Frequency Division Multiplexing in Cascaded Microring Resonator Sensors

The increasing demand for rapid and multi-target detection in integrated photonic sensing systems necessitates scalable and low-complexity multiplexing strategies. Conventional multiplexing approaches, such as wavelength-division and time-division multiplexing, often suffer from inherent trade-offs between system scalability, performance, and implementation complexity. In this paper, a frequency division multiplexing (FDM) scheme based on cascaded microring resonators (CMRRs) is proposed for multi-channel optical sensing. Distinct sinusoidal voltage signals with different modulation frequencies are applied to the thermal electrodes of reference rings in each sensing channel, enabling the encoding of spectral responses into separable frequency components that can be simultaneously detected by a single photodetector. A numerical demodulation method based on joint least-squares reconstruction of the 2f and 4f modulation frequency components is developed to suppress distortions induced by higher-order spectral terms. Simulations and experiments on a silicon-on-insulator (SOI) platform are conducted to validate the proposed approach. The results show that the system achieves a sensitivity exceeding 300 nm/RIU and a limit of detection on the order of 10−5 RIU, while enabling reliable multi-channel signal separation. These results demonstrate that the proposed FDM scheme provides an effective route to enhance multiplexing scalability without increasing system complexity or hardware cost. The method is compatible with existing multiplexing techniques and offers a practical solution for high-performance multi-channel integrated photonic biosensing applications.