Research Progress in Narrow Linewidth Laser Technology (Invited)

Laser linewidth, an important parameter characterizing laser coherence, has attracted considerable attention since the dawn of laser technology. Narrow-linewidth lasers have been widely used in the fields of gravitational wave detection, cold atomic physics, coherent optical communication, optical precision measurement, and microwave photonic signal processing because of their high spectral purity, long coherence length, and low phase noise. With the development of modern information technology, narrow-linewidth lasers, as the core light source for these applications, are expected to have some new properties, such as extreme tuning of parameters, time-frequency ultrastability, wavelength tuning, and wavelength sweeping, while the intrinsic linewidth, noise, and other parameters are further optimized. The intrinsic linewidth of a laser originates from spontaneous radiation noise; to eliminate the spontaneous radiation noise, researchers are focusing on the development of linewidth compression of narrow-linewidth lasers. Throughout the development history of narrow-linewidth lasers, the laser cavity configuration from a simple two-mirror structure, the distributed Bragg reflector (DBR), the distributed feedback (DFB) main cavity configuration to the external cavity configuration and finally to the wavelength adaptive distributed weak feedback configuration, whose core idea is to use the feedback signal on the spontaneous radiation noise suppression. This paper takes the evolutionary development of laser main cavity configuration as the narrative vein, summarizes the research progress of various narrow linewidth laser technologies, and compares the similarities and differences of various laser cavity configurations in the laser linewidth compression and noise suppression. Furthermore, this paper focuses on the newly developed wavelength-adaptive distributed weak feedback narrow-linewidth lasers. Moreover, it analyzes and discusses the physical concept of narrow-linewidth lasers as well as the core devices and system performance based on this type of lasers.