A narrow-linewidth laser refers to a special type of laser whose spectral linewidth is compressed to the order of kHz or even Hz, possessing extremely high spectral purity and frequency stability. It mainly employs single-longitudinal-mode selection and active frequency stabilization techniques to suppress environmental perturbations, thereby reducing frequency drift to an extremely low level. Compared with ordinary lasers, its most prominent advantage is its ultra‑long coherence length (which can reach hundreds of kilometers), making it a core light source in cutting‑edge fields such as gravitational wave detection, optical atomic clocks, high‑precision fiber‑optic sensing, and coherent optical communications.