Tm3+ Doped Multi-Ring Profile Single-Mode Fiber Laser for Application in the Eye-Safe Spectral Range

New construction of a silica-based multi-ring profile of Tm³⁺ doped fiber laser is presented. Optimisation of both multi-ring refractive index profile and active ions distribution towards high beam quality mode propagation is a new approach in developing active fiber constructions. Using a system of layers forming the core allows controlling the dopant profile and, consequently, the emission properties of fiber lasers (power density, beam profile shape etc.). The preform fabricated using MCVD technology is presented. The refractive index profile (Δn = 9·10⁻³), EPMA-EDS analysis (Tm₂O₃ and Al₂O₃ concentration 0.56 and 5.2 wt% respectively), and excited state (³F₄) decay time are shown. The double-clad fiber laser based on the Fabry-Perot resonator construction (17/240 μm core/cladding) was used for single-mode laser beam generation at a wavelength of 1938.8 nm (exc. @ 790 nm) with an FWHM bandwidth of 0.17 nm (1.68 W output power, M²_x = 1.10, M²_y = 1.23). For the optimized gain fiber (25/330 μm core/cladding diameters) length, the laser provided an output power of >5 W with a slope efficiency of ∼26%, determined to the absorbed pump power.