Heat-induced Active Centers in Bismuth-doped Optical Fibers for 1.7-µm-region Laser Applications

We report a recently discovered effect of enhancement of the luminescence intensity in bismuth-doped high-germania-core glass fibers as a consequence of heat treatment. Analyzing the experimental results, it was shown that an increase of the luminescence intensity is caused by a thermally induced growth of the bismuth-related active centers (BACs) concentration. The activation energies of formation of the thermally induced bismuth-related active centers and other complexes responsible for unsaturable loss were estimated. It was found that there are major parameters, such as total Bi content, heating temperature, which affect significantly the increment of the BACs concentration. In addition, the gain and lasing characteristics of the bismuth-doped GeO 2 -SiO 2 glass fibers in which a part of the BACs was induced by thermal treatment were studied.