Narrow-Band and FSR-Free Add-Drop Filter on SOI Based on Side-Coupled π-Phase-Shifted Antisymmetric Bragg Grating Resonator

Silicon-based add-drop filters (ADF) with narrow bandwidth are key components in photonic integrated circuits (PICs), which have been widely used in optical communication systems, spectroscopy, and sensing. Microring resonator (MRR) is the most common used ADF, due to its compact size, narrow linewidth, and high flexibility. However, the multiple longitudinal modes of MRR lead to a limited FSR, and restrict the operating range of the entire system. Achieving the narrow-band FSR-free filters with high-performance remains a significant challenge. This paper proposes and experimentally demonstrates a side-coupled Fabry-Pérot (F-P) resonator ADF based on antisymmetric Bragg gratings (ASBGs) on the SOI platform. By carefully designing the grating structure and coupling regions, the ASBG enables resonances from forward first order transverse electric (TE₁) and backward fundamental transverse electric (TE₀) modes, achieving an FSR-free operation with an insertion loss (IL) of 0.7 dB, a side lobe suppression ratio (SLSR) of 25 dB, a 3-dB bandwidth of 0.22 nm, a Q factor of 6821, and a crosstalk (CT) at add port below −20 dB. In addition, an ADF with Fano lineshape has also been implemented. We believe the proposed ADF opens a new avenue for practical applications in integrated optical filtering.