“…However, rare-earth ions are usually stable in the trivalent state and it is not easy to obtain the divalent thulium ions in silica glass based optical fibers. The divalent state of some rare-earth ions, such as samarium and europium, in bulk glasses [4,5] or in glass fibers using aerosol delivery technique in modified chemical vapor deposition (MCVD) process [6,7] has been demonstrated. Even though there have been efforts to obtain the divalent thulium ions in alkaline-earth fluoride crystals by irradiating trivalent thulium (Tm 3+ ) ions with c-ray [8,9] or in tetrahydrofuran (THF) solution by reducing TmCl 3 with Na naphthalide [10], no experimental results have been reported for Tm 2+ ions in optical fibers and even in any bulk glasses to the best of our knowledge.…”