Parallel random fiber Bragg gratings in cladding for a narrow linewidth random fiber laser

Using the fs-IR laser point-by-point inscription method, parallel-cladding random fiber Bragg gratings (ParallelCLRFBGs) were fabricated in a single-mode fiber (SMF) as a novel, to the best of our knowledge, filtering device for random fiber lasers (RFLs). The Parallel-CLRFBGs effectively control light scattering and interference, enhancing system feedback through increased backward Rayleigh scattering. Compared to conventional axially cascaded random gratings, gratings aligned in parallel within sub-centimeter cladding improve the integration of a laser system. The results show that RFLs using Parallel-CLRFBGs in a hybrid cavity achieve a narrow linewidth output of 138 Hz, almost half that of RFLs using conventional core-random fiber Bragg gratings (Core-RFBGs).