Tailoring optical properties of bismuth-doped germanosilicate fibers for E/S band amplification

Bismuth-doped germanosilicate optical fibers can operate in the E-,S-,C-,L- and U- telecommunication bands, depending on the pumping wavelength. However, the unclear origin of bismuth NIR luminescence limits the field progress. Herein, we focus on E- and S-band optical properties and demonstrate, for the first time, that number density of bismuth active centers (BACs) with a characteristic absorption at -1400 nm and, consequently, luminescence and gain in the E- and S-band, can be considerably tailored and increased through glass composition, preform and fiber fabrication conditions. Contrary to previous studies that pointed to low GeO2 content, -5 mol%, to present luminescence in this spectral region, we found that, increasing the Ge content in the glass matrix actually enhances the luminescence and the gain. Our study provides new insights into the nature of these BACs and prospects for tailoring spectroscopic properties in Bi-doped germanosilicate optical fibers for E- and Sband fiber amplification.