Tunable flat-top optical filters based on cascaded mode-conversion nanobeam cavities in lithium niobate

Flat-top optical filters play a crucial role in high-selectivity photonic integrated circuits. Here, we propose and experimentally realize compact flat-top optical filters with bandwidths ranging from several to hundreds of GHz, implemented using cascaded mode-conversion nanobeam cavities in lithium niobate on insulator (LNOI). An electro-optic phase shifter is utilized to adjust the resonant wavelength detuning between cavities, enabling continuous bandwidth tuning. In the experiment, we achieve a broadband flat-top filter with a 3-dB bandwidth of 148.5 GHz and a low insertion loss of 0.4 dB within a compact footprint of ∼2.55 × 164 µm². For narrowband operation, a filter with a 3-dB bandwidth of 9.1 GHz and a high extinction ratio of 39.93 dB is demonstrated, whose bandwidth can be further electro-optically tuned to 16.4 GHz. These high-performance and compact tunable filters offer promising potential for advanced applications in LNOI-based photonic integrated circuits.