Lasing properties of thermally treated ${\text{GeO}}_2$–${\text{SiO}}_2$ glass fibers doped with bismuth

We present results on the lasing properties of the Bi-doped high-germania glass fibers thermally treated at different heating and cooling conditions. The absorption and luminescence spectra, the luminescence lifetime of bismuth-related active centers (BACs) formed in the Bi-doped fibers before and after treatment were measured. Analyzing the results, it was shown that the concentration of the BACs could be increased by approximately two times after treatment at certain conditions. A series of experiments regarding laser action at 1730 nm using pristine and treated Bi-doped fibers was performed. From the dependencies of the slope efficiencies of the Bi-doped fiber lasers on the length of the active fibers obtained at various cooling conditions, it was found that the optimal length $L\approx 20$ m of the treated active fibers required for the realization of the Bi-doped fiber lasers is two times shorter than that of the pristine fibers. In addition, the efficiency of the developed lasers being $\approx 18\%$ with respect to the absorbed pump power is greater than that of the lasers based on the pristine fibers ($\approx 10\%$ at $L=20$ m), but it is lower than their maximum efficiency ($\approx 25\%$ at the optimum lengths of 45 m). From the numerical simulation of the Bi-doped fiber laser, a number of parameters needed for estimation of the native and thermally induced BAC concentrations were determined.