Ultrabroadband Spectral Filter on Lithium-Niobate-on-Insulator

Spectral filters with wide bandwidth are required by many critical photonics applications such as communications, nonlinear optics, and spectroscopy. In this study, an adiabatic spectral filter with a flat-top spectral response and wide operational bandwidth is demonstrated on the lithium-niobate-on-insulator platform. By utilizing the spectrally selective waveguide cross-section, we achieve high extinction spectral filtering through a conventional adiabatic asymmetric directional coupler, which is connected to a mode demultiplexer for downstream processing. The fabricated devices exhibit an excess loss as low as 0.4 dB and a sidelobe suppression ratio of 20 dB. A remarkable 1-dB bandwidth of 75 nm is also achieved. By cascading two adiabatic spectral filters, we further demonstrate a wavelength triplexer for the 1577:1490:1342 nm downstream channel separation in next-generation passive optical networks, achieving excess losses of 0.4:0.7:0.4 dB with crosstalk levels of −25:–35:–16 dB. Our adiabatic spectral filter does not include any interference structures, making it inherently suitable for ultrabroadband filtering operations and facilitating future innovations in on-chip ultrabroadband signal processing.