Coherent cavity coupling in O-band silicon photonic sensors for water environment detection

Silicon photonic refractive index sensors based on optical resonators offer high sensitivity and compact integration; however, their performance is fundamentally limited by resonance linewidth and spectral resolution. In this work, we propose and numerically investigate a silicon-on-insulator (SOI) refractive index sensor based on a coherently coupled dual-cavity architecture, designed to achieve linewidth narrowing and enhanced sensing performance. The device consists of two loop resonators interconnected via a directional coupler and side-coupled to a bus waveguide, enabling controlled coherent interference and extended photon lifetime. The novelty of the design lies in using the directional-coupler-mediated interaction between two loop cavities to favor a dominant coupled supermode while suppressing parasitic multi-interference features. A comprehensive parametric optimization of the coupling length, inter-resonator gap, bus-cavity gap, and resonator radius is performed using finite-element simulations. The results show that under optimal coupling conditions, the structure supports a dominant supermode with suppressed multi-interference effects, leading to a significantly reduced resonance linewidth and improved spectral purity. As a result, the proposed sensor achieves a high refractive index sensitivity of 304.02 nm/RIU, a quality factor of approximately 2.2 × 10⁴, and a limit of detection as low as 1.64 × 10^- 4 RIU, corresponding to sub-ppm-level sensing performance. Compared to conventional single-ring and other coupled-resonator configurations, the proposed architecture provides a favorable trade-off between sensitivity, limit of detection, and fabrication complexity relative to slot or SWG-based designs. Moreover, operation in the O-band offers distinct advantages for sensing in aqueous environments. The presented approach demonstrates an effective strategy for engineering narrow-linewidth resonances in silicon photonics and provides a promising route toward high-resolution SOI sensing devices compatible with established CMOS fabrication workflows, for chemical and biological applications.