  The spectroscopic properties of Tm3+/Yb3+ co-doped silica fibers under
excitation at 980 nm are reported. Three distinct up-conversion fluorescence
bands were observed in the visible to near infra-red regions. The blue and red
fluorescence bands at 475 and 650 nm, respectively, were found to originate
from the 1G4 level of Tm3+. A three step up-conversion process was established
as the populating mechanism for these fluorescence bands. The fluorescence band
at 800 nm was found to originate from two possible transitions in Tm3+; one
being the transition from the 3H4 to 3H6 manifold which was found to dominate
at low pump powers; the other being the transition from the 1G4 to 3H6 level
which dominates at higher pump powers. The fluorescence lifetime of the 3H4 and
3F4 levels of Tm3+ and 2F5/2 level of Yb3+ were studied as a function of Yb3+
concentration, with no significant energy back transfer from Tm3+ to Yb3+
observed.
