Direct observation of bistability and resonance evolution in silicon microring resonators using a CW pump-broadband probe technique.

Thermal nonlinearities in silicon microring resonators induce resonance shifts and bistability that can impact the stability and performance of optical communication systems. We present a CW pump-broadband probe technique that decouples optical excitation from spectral readout, enabling direct observation of bistability and resonance evolution in low-Q silicon microring resonators under self-heating. Unlike conventional single-laser approaches, where resonance behavior is inferred from transmitted optical power, the proposed method directly measures the resonance spectrum while the cavity is optically excited, providing simultaneous access to the resonance wavelength, linewidth, and resonance depth. Using this approach, we experimentally characterize thermal hysteresis over a wide range of optical powers and evaluate a reduced phenomenological thermal model. Direct observation of the cavity state reveals power-dependent evolution of the resonance linewidth and depth, and incorporating the measured cavity-state evolution significantly improves the predictive accuracy of the reduced model. The resulting model enables reproducible steady-state cavity conditions to be established by balancing optical and electrical heating, achieving equivalent cavity temperature fluctuations of approximately 20 mK (1σ). Building on this capability, we introduce a steady-state operating-space framework that separates the physical cavity state from the external experimental control variables. The proposed method provides a practical framework for quantitative characterization of thermal nonlinearities and nonlinear optical processes in resonant silicon photonic devices.