Dynamic Linear Spectral Analysis of SiN Microring Resonator Sensors Under Nonlinear Laser Sweeps

Abstract

Integrated silicon nitride (SiN) micro-ring resonators (MRRs) are attractive for high-speed refractive-index sensing, where accurate and rapid interrogation of resonance motion is critical. Conventional approaches based on optical spectral analyzers and tunable lasers suffer from high complexity and an inherent trade-off between speed and accuracy. Frequency-modulated spectroscopy (FMS) using chirped lasers offers high measurement speed but is fundamentally limited by laser frequency-sweep nonlinearity, which leads to severe spectral distortions and frequency errors, particularly under high-speed and dynamic operation, rendering conventional time-to-frequency mapping invalid. In this work, we demonstrate a linear and high-speed FMS system enabled by real-time optical instantaneous frequency (OIF) estimation for dynamic spectral analysis of integrated photonic resonators. By directly tracking the laser's instantaneous frequency during strongly nonlinear sweeps, the proposed approach eliminates chirp-induced distortion and enables direct reconstruction of both static and time-varying spectral responses without relying on sweeping linearity assumptions. Using a nonlinear frequency sweep spanning 6.23 GHz within 1.25 μs, corresponding to an effective sweep speed of 39872 nm/s (4.98 GHz/μs), the system accurately tracks sinusoidal resonance motion up to 80 kHz. The method is experimentally validated using a thermally tunable SiN MRR fabricated on IMEC's iSiPP200 platform, exhibiting a loaded quality factor of 1.65 × 105. The proposed system resolves ultra-small peak-to-peak phase modulations down to approximately 0.15°, corresponding to resonance shifts as small as 79 MHz, and achieves a >50% reduction in dynamic frequency error compared to conventional FMS reconstruction.