Sinlge-frequency fiber laser with tunable linewidth by optical self-injection

We experimentally demonstrated a fiber laser with tunable linewidth based on self-injection locking (SIL) and the stimulated Brillouin scattering (SBS) effect. The linewidth-tuning operation is achieved by limiting injection power within the third regime of the feedback effect. SBS can realize single longitudinal mode (SLM) by the ultra-narrow gain range, and generate ultra-narrow laser linewidth because of the high Q value. Additionally, we use Brillouin erbium fiber laser (BEFL) to replace the ordinary Brillouin ring cavity to obtain better SLM property. The lasing with ultra-narrow linewidth is steadily emitted after eliminating the mod-hopping caused by SIL and confirming the stable occurrence of SBS. In addition, The Lorentzian linewidth of DFB (1.7 kHz) and SIL with different injection power are measured by the delayed self-heterodyne interferometry (50 km delay fiber). The Lorentzian linewidth of the BEFL (60 Hz) is accurately measured by the Voigt fitting.