Ring-shaped random structures inscription in multimode fibers assisted by a spatial light modulator

We present the fs laser inscription of ring-shaped random structures using Spatial Light Modulator (SLM) in multimode GRIN fiber. The use of SLM allows one to modulate the phase of the fs radiation incident on it and to write various structures with complex geometries inside the static fiber core. We optimized the fs laser inscription parameters: pulse energy, SLM frame rate, overall length and distances along the fiber of the structures to enhance Rayleigh backscattering level at minimal insertion losses. In particular, scattering structures with random distances along the fiber were written using the Line-by-Line method in single mode fiber at the optimal inscription parameters (pulse energy of 3 μJ, SLM frame rate of 5 Hz, the overall of 2 mm and random distances along the fiber in the range of 5 μm). Further, we created the ring-shaped random structures in 100/140 μm GRIN multimode fiber with enhanced Rayleigh backscattering level by +66 dB/mm relative to the intrinsic fiber level. Owing to the variation of random distances along the fiber and ring’s diameters of structures in range of 0.5 μm and 20 μm, respectively, allows one to obtain a broadband reflection spectrum within 88 nm with a reflection coefficient of 0.01%. The low threshold generation with ring-shaped output beam of the Raman fiber laser with random distributed feedback based on the SLM-inscribed random structures in the multimode fiber is demonstrated for the first time.