Million-Q Dual-Polarization Micro-Fabry-Perot Resonators in Silicon Nitride Photonic Integrated Circuits

Miniaturized Fabry-Perot standing-wave resonators and whispering-gallery travelling wave resonators constitute foundational building blocks for photonic integrated circuits. While both architectures offer transformative potential through high quality factors and dual-polarization operation, integrated Fabry-Perot resonators face significant challenges in simultaneously achieving ultra-high Q-factors and broadband thermal tunability for fundamental transverse magnetic (TM0) and transverse electric (TE0) modes within a compact footprint-primarily due to polarization-dependent losses in conventional chip-scale reflectors. Here, we overcome this limitation by demonstrating an integrated silicon nitride dual-polarization micro-Fabry-Perot resonator with polarization-insensitive Sagnac loop reflectors and multimode waveguides to effectively suppress losses and enable high-performances for both fundamental transverse magnetic (TM0) and transverse electric (TE0) modes. The device achieves record loaded quality factors of 2.38*106 (TM0) and 3.48*105 (TE0) respectively and intrinsic quality factors will be even higher. Moreover, both two modes are tuned over the whole free spectral range of around 0.111 nm (TM0) and 0.112 nm (TE0) with the thermal tuning efficiencies of approximately 1.04 pm/mW (TM0) and 1.24 pm/mW (TE0). These advances establish a new benchmark for compact, high-performance dual-polarization resonators in optical sensors, nonlinear and integrated quantum photonics.