Multi‐channel and free‐spectral‐range‐free tunable add‐drop filters on thin‐film lithium niobate

Thin-film lithium niobate (TFLN) has emerged as an attractive platform for integrated tunable photonic filters owing to its strong electro-optic response and low optical loss. However, conventional resonant filters, such as micro-rings, are intrinsically constrained by a limited free spectral range (FSR), which hinders their use in broadband and multi-channel operations. Here we present a TFLN-based add-drop filter that overcomes this limitation by employing a side-coupled travelling-wave Fabry-Pérot (FP) cavity formed with asymmetric multimode waveguide gratings (AMWGs). By engineering the inter-modal coupling through waveguide width tailoring and the reflection bandwidth of AMWGs, we realize a cavity with an intrinsic quality factor of 2.5 × 10⁵. The add-drop filter device also exhibits an FSR-free response across 1,500-1,630 nm wavelength band. A single resonance with a through-port extinction ratio of 20.23 dB and a drop-port insertion loss of 1.81 dB. Wavelength tuning by thermo-optic and electro-optic effect is demonstrated with efficiencies of 34.82 pm/K and 6.9 pm/V, respectively. Furthermore, a four-channel add-drop filter array with 3.2 nm channel spacing and 1.35 dB total through-port insertion loss validates the scalability of the present device. This work demonstrates an efficient approach to overcome the FSR constraint of sharp wavelength filters on TFLN. It can be potentially adopted in dense-wavelength-division-multiplexing communication systems, narrow-bandwidth microwave photonic filters, or high-resolution spectrometers.