Narrow-linewidth single-frequency fiber laser based on whispering gallery mode resonator self-injection locking

Narrow-linewidth fiber lasers are distinguished by their long coherence length, stable output, and low noise. These qualities have led to their widespread use in areas such as coherent optical communication, coherent LiDAR and distributed fiber sensing. Whispering gallery mode resonators (WGMRs) provide a significant filtering effect, and the spectral width of their resonance is determined by the cavity's quality factor (Q). However, inserting a WGM microcavity can cause severe power loss in a laser. Furthermore, the effectiveness of linewidth compression depends on feedback from the transmitted resonant light into the main laser cavity. A WGMR in an over-coupled regime with a Qfactor exceeding 10⁹ was used in an erbium-doped ring fiber laser (EDFL) via self-injection locking to achieve deep linewidth compression. Experimental results demonstrate that the WGM microcavity successfully performs mode selection for the fiber laser. The laser exhibits a linewidth of less than 1 kHz, an output power approaching 4mW and an optical signal-to-noise ratio greater than 50dB. This research presents a novel approach to achieve compact, narrow-linewidth fiber lasers.