A systematic spectroscopic study of 22 rate-earth-ion doped ZBLAN glass samples was conducted to investigate the feasibility of sensitizing Tm:ZBLAN with Yb to facilitate the development of an efficient and conveniently pumped blue upconversion fiber laser. It was determined that, under conditions of single-color pumping, 480 nm emission from Tm3+ is strongest when Yb, Tm:ZBLAN is excited at a wavelength of approximately 975 nm. In this case, the strongest blue emission was obtained from a ZBLAN glass sample with a nominal dopant concentration of approximately 2.0 wt percent Yb + 0.3 wt percent Tm. Additionally, it was demonstrated that for weak 975 nm pump intensities, the strength of the blue upconversion emission can be greatly enhanced by simultaneously pumping at approximately 785 nm. This increase in upconversion efficiency is due to a reduction in the number of energy transfer steps needed to populate the Tm3+ 1G4 energy level. Measurements of fluorescence lifetimes as a function of dopant concentration wee also made for Yb3+, and Pr3+ transitions in ZBLAN in order to better characterize concentration quenching effects. Energy transfer between Tm3+ and Pr3+ in ZBLAN is also described.
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