FSR-Free Fano Resonance in a Photonic Molecule With Dual Side-Coupled Phase-Shifted Bragg Gratings

Fano-resonant photonic molecules hold great promise for ultra-compact optical filtering and sensing, yet their performance is usually limited by the free-spectral-range (FSR) of the resonators. In this letter, we demonstrate an FSR-free Fano resonance realized in a silicon-on-insulator photonic molecule comprising a bus waveguide side-coupled to two bent, π-phase-shifted sidewall Bragg-grating (BG) cavities. Rigorous coupled-mode theory analysis reveals that the interference between the two gratings generates a single, sharp asymmetric line shape whose slope rate (SR) and extinction ratio (ER) are governed by the bus waveguide width. On the silicon-on-insulator (SOI) platform, we experimentally demonstrate a fabricated device exhibiting an extinction ratio (ER) of 37 dB, an average slope rate (SR) of 162 dB/nm, and an FSR-free response. The proposed structure is expected to find wide applications in optical filtering and sensing that demand single-resonance operation over a broad spectral window.