Femtosecond Laser Structuring of Multicore and Multimode Fibers for Laser Applications

Localized refractive index modification in optical fibers by femtosecond (fs) laser becomes a widely used technology for fabrication of in-fiber gratings or other components for laser and sensor systems. In particular, a point-by-point fs inscription of arbitrary shaped tracks allows one to form 3D refractive index structures in various fiber types with unique features. Here we review our recent results on point-by-point fs inscription of regular and random 3D structures in active and passive multicore (MC) and multimode (MM) fibers and implementation of such structures for the controlling spectral and spatial characteristics of output laser beam. Namely, fiber Bragg gratings (FBGs) fs-inscribed individually in each core of the double-clad 4-core and 7-core Yb-doped fibers were tested as a complex highly-reflective (HR) mirror of MC fiber laser. In the 4-core Yb-doped fiber laser with HR FBG array, the effect of output beam localization in one core is observed at strong fiber bending, which is explained by the influence of the bending-induced stress on pump absorption and core coupling. In the 7-core Yb-doped fiber laser with HR FBG array, individual spectral lines resulted from each in-core FBG reflection are measured to merge into a single narrowband line (~ 0.1 nm at 33 W output) at the induced core coupling, explained by the formation of special supermodes. Two HR FBG arrays have been fs-inscribed in peripherical cores at both ends of the coupled 7-core passive fiber. When pumped into the central core at one end, the MC fiber exhibits singlemode Raman lasing from the central core at the opposite end with narrow bandwidth (defined by large mode area inside the cavity). The laser linewidth is additionally narrowed when random longitudinal shift of ~ 0.5 cm is formed between the FBGs within one array, due to the interference in Michelson-type interferometers formed between the shifted FBGs. Finally, similar random fs-inscribed 3D FBG array with low reflection (R ~ 1%) has been formed at the output end of MM graded-index fiber. When pumped by highly-multimode
$(M^{2}\sim 34)$
broadband (~ 10 nm) laser diodes at 940 nm, a random Raman lasing is observed with power up to ~30 W and greatly improved spatial
$(M^{2} < 2)$
and spectral
$(< 0.2\ \text{nm})$
characteristics of Stokes radiation at 976 nm, after optimization of the structure. Potential of multicore and multimode fiber lasers with regular and random FBG arrays for applications are discussed.