Thin-Film Lithium Niobate Hybrid-Integrated External-Cavity Laser with 90 nm Tuning Range and 20 mA Threshold

Abstract Chip-scale lasers that simultaneously offer low threshold current, wide tuning range, narrow line width, and fast chirp rate are highly desirable for optical communications, sensing, and light detection and ranging (LiDAR) systems. Thin-film lithium niobate (TFLN) external-cavity lasers have attracted considerable interest owing to their linear electro-optic effect and the resulting tunability. However, most chip-scale TFLN external-cavity lasers reported to date provide a tuning range of only 20–45 nm, which is significantly narrower than that of bulk free-space external-cavity lasers. To realize an ultrawide tuning range in a chip-scale external-cavity laser, it is essential to suppress side modes while maintaining a simple cavity architecture. Here, we demonstrate an integrated TFLN external-cavity laser that combines a broadband reflective semiconductor optical amplifier (RSOA) gain chip with a TFLN wavelength-selective reflector chip. By employing a simple double-ring external cavity and a tunable Sagnac loop reflector, the laser achieves a record 90 nm tuning range and an optimal side-mode suppression ratio of 54 dB. The low-loss TFLN external cavity, together with its fast electro-optic effect, enables a threshold current as low as 20 mA, a minimum intrinsic line width of 2.5 kHz, and a chirp rate of 1.6 × 1016 Hz/s