Formation of laser-active centers in bismuth-doped high-germania silica fibers by thermal treatment.

The effect of thermal annealing on the luminescent and laser properties of high-germania-core silicate fibers doped with bismuth was investigated. We studied the behavior of optical absorption assigned to the bismuth-related active centers associated with germanium as well as the behavior of unsaturable absorption in annealed fibers with respect to the Bi content. The dependence of the increment of the active center content on the Bi concentration in the annealed fibers was obtained. We achieved laser oscillations near a wavelength of 1700 nm with a slope efficiency of 18% using a 8.5 m long Bi-doped fiber. The comparison of the output parameters of the laser based on an annealed Bi-doped fiber with the ones of a pristine Bi-doped fiber laser is given. The performance of the obtained bismuth-doped fiber lasers was modeled using the propagation and rate equations of a homogeneous quasi-two-level laser medium. Theoretical results are compared with experimental ones.