Single-frequency fiber laser implemented via ultra-narrowband transient grating formed by a fiber loop mirror and flexible perovskite

Single frequency fiber laser (SFFL) can generate stable single longitudinal mode oscillation output and has broad application prospects in high-precision measurement, laser radar and other fields. Usually, SFFLs with short straight cavity structures (<3 cm) cannot effectively absorb pump power due to their shorter gain, resulting in lower output power. The complex doping structure and multi-mode oscillation of ring cavity lasers are issues. This article combines Cs_0.15FA_0.85PbI_2.85Br_0.15 material with Sagnac loop reflector and applies it to erbium-doped fiber laser to construct a linear cavity structure with a meter level cavity length. Among them, Sagnac, as a broadband reflector, provides strong spectral constraint capability with its annular interference structure. When a perovskite material with nonlinear effects is placed inside a loop, it will interact with the light field propagating in the opposite direction inside the loop, forming a periodic absorption grating related to the light intensity distribution. The bandwidth of this dynamic grating is very narrow, which can effectively suppress multi longitudinal mode oscillations and achieve single frequency operation. Based on this, we obtained a single frequency fiber laser with an output spectral bandwidth of approximately 1.2 kHz and an output power of 13.9 mW in the 1550 nm band. The laser implemented based on this structure can reduce the laser linewidth and improve output efficiency while avoiding complex structures. The research results indicate that Cs_0.15FA_0.85PbI_2.85Br_0.15 metal halide perovskite is an excellent saturated absorber, and the kHz linewidth laser based on it has important application value in high-precision measurement and spectral analysis.