On-Chip Reconfigurable High-Order Fabry-Perot Filter Using Fully Tunable Sagnac Loop Mirrors

Reconfigurable optical filters are pivotal components in modern optical systems, providing unparalleled flexibility to adapt to a wide range of application scenarios while significantly reducing the need for iterative design and fabrication processes. In this paper, we propose an on-chip reconfigurable high-order Fabry-Perot (FP) filter based on fully tunable Sagnac loop mirrors (SLMs), offering multi-parameter reconfigurability, particularly in enabling tunability of the FSR. Each SLM incorporates a tunable Mach-Zehnder interferometer (MZI) coupler realized using two variable-width multi-mode interferences (MMIs), which are optimized for a 3 dB power splitting ratio with minimal excess loss, enabling full reflectivity tuning. By thermally tuning the SLM reflectivity and the effective index of the connecting waveguide with the use of micro-heaters placed on top of the waveguides, the resonance wavelength, bandwidth, free spectral range (FSR), and filter order of the proposed high-order FP filter can be flexibly tuned. As a proof of concept, a reconfigurable third-order FP filter is designed, fabricated and experimentally demonstrated. The experimental results show that the proposed reconfigurable FP filter has a tunable order from first to third, a tunable FSR from 0.18 nm to 0.06 nm, a tunable bandwidth spanning from 2.5 GHz to 18.5 GHz, a resonance wavelength tuning range exceeding 0.19 nm, and a maximum extinction ratio of 42.1 dB. With its versatile reconfigurability and exceptional performance, the proposed high-order FP filter shows significant potential for applications in elastic optical networks and adaptive microwave photonic systems.