Dependence of quantum efficiency and core propagation loss on doping concentration in Tm-doped silica fibers

A set of six thulium-doped aluminosilicate fibers with Tm³⁺-doping level ranging from 0.4 to 4.8 wt.% was fabricated, characterized, and investigated. By maintaining a similar aluminium content across fiber samples, it was possible to study the effect of Tm-doping level on lasing performance, quantum efficiency and core propagation loss in the 2 µm wavelength range. The results show a linear increase in core propagation loss from 0.11 dB/m at 0.4 wt.% Tm³⁺ to 0.29 dB/m at 4.8 wt.% Tm³⁺. Thus, an approximately order of magnitude increase in thulium concentration results in a 2.7 increase in propagation loss in the spectral range of 1900-2050 nm. The quantum efficiency increases with doping concentration, reaching a value of 1.92, beyond which it saturates for concentrations above 3 wt.%. This investigation focused on continuous-wave (CW) lasers shows that the highly doped Tm fibers necessary for high-power laser systems, suffer from significant losses at the signal wavelength. Approximately two orders of magnitude higher compared to active ytterbium doped fibers.