Formation of Tm2+ ions in germanosilicate glass optical fibers

For the first time, we have developed Tm2+–Tm3+ co-doped germanosilicate glass optical fibers by providing a strong reduction environment based on a solution doping technique in a modified chemical vapor deposition process. The incorporation of the Tm2+ ions into the fiber core was identified by a broad optical absorption band appearing from 350 to ∼900 nm, which was due to 4f–5d electronic transitions of the Tm2+ ions. The electron paramagnetic resonance spectrum was also measured to confirm the formation of the Tm2+ ions in the fiber preform by addition of a reducing agent during the preform preparation. The concentration of the Tm2+ ions in the fiber core was found to increase with the increase of heat treatment temperature just after drying the soaked porous core layers.