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On-chip optical filters are fundamental components in optical signalprocessing. While rare-earth ion-doped crystals offer ultra-narrow opticalfiltering via spectral hole burning, their applications have primarily beenlimited to those using bulk crystals, restricting their utility. In this work,we demonstrate cavity-enhanced spectral filtering based on rare-earth ions inan integrated nonlinear optical platform. We incorporate rare-earth ions intohigh quality-factor ring resonators patterned in thin-film lithium niobate. Byspectral hole burning in a critically-coupled resonance mode, we achievebandpass filters ranging from 7 MHz linewidth, with 13.0 dB of extinction, to24 MHz linewidth, with 20.4 dB of extinction. These filters outperform those ofthe highest quality factor ring resonators demonstrated in the thin-filmlithium niobate integrated platform. Moreover, the cavity enablesreconfigurable filtering by varying the cavity coupling rate. For instance, asopposed to the bandpass filter, we demonstrate a bandstop filter utilizing anunder-coupled ring resonator. Such versatile integrated spectral filters withhigh extinction ratio and narrow linewidth could serve as fundamentalcomponents for optical signal processing and optical memories on-a-chip.

Cavity-enhanced narrowband spectral filters using rare-earth ions doped in thin-film lithium niobate