Ultrafast tunable lasers using lithium niobate integrated photonics

Early works(1) and recent advances in thin-film lithium niobate (LiNbO3) on insulator have enabled low-loss photonic integrated circuits(2,3), modulators with improved half-wave voltage(4,5), electro-optic frequency combs(6) and on-chip electro-optic devices, with applications ranging from microwave photonics to microwave-to-optical quantum interfaces(7). Although recent advances have demonstrated tunable integrated lasers based on LiNbO3 (refs. (8,9)), the full potential of this platform to demonstrate frequency-agile, narrow-linewidth integrated lasers has not been achieved. Here we report such a laser with a fast tuning rate based on a hybrid silicon nitride (Si3N4)-LiNbO3 photonic platform and demonstrate its use for coherent laser ranging. Our platform is based on heterogeneous integration of ultralow-loss Si3N4 photonic integrated circuits with thin-film LiNbO3 through direct bonding at the wafer level, in contrast to previously demonstrated chiplet-level integration(10), featuring low propagation loss of 8.5 decibels per metre, enabling narrow-linewidth lasing (intrinsic linewidth of 3 kilohertz) by self-injection locking to a laser diode. The hybrid mode of the resonator allows electro-optic laser frequency tuning at a speed of 12 x 10(15) hertz per second with high linearity and low hysteresis while retaining the narrow linewidth. Using a hybrid integrated laser, we perform a proof-of-concept coherent optical ranging (FMCW LiDAR) experiment. Endowing Si3N4 photonic integrated circuits with LiNbO3 creates a platform that combines the individual advantages of thin-film LiNbO3 with those of Si3N4, which show precise lithographic control, mature manufacturing and ultralow loss(11,12).

Automated summary

Thin-film lithium niobate (LiNbO3) on insulator has enabled various applications in integrated photonics, but the potential of this platform for frequency-agile, narrow-linewidth lasers has not been fully explored. In this study, we present a fast-tuning laser based on a hybrid silicon nitride (Si3N4)-LiNbO3 photonic platform and demonstrate its use in coherent laser ranging. By combining the advantages of LiNbO3 and Si3N4, this platform offers precise lithographic control, mature manufacturing, ultralow loss, and the ability to achieve narrow-linewidth lasing.