  A study of the mechanisms responsible for the infra-red to near infra-red
up-conversion in Tm3+-doped silica fibers is presented. Upconversion
luminescence was observed from the 3H4 level of Tm3+ under 1586 nm pumping into
the 3F4 level. The quadratic dependence of the up-conversion luminescence at
800 nm on the 1800 nm luminescence from the 3F4 level confirms that the 3H4
level is populated by a two photon process. Two possible processes are proposed
as mechanisms responsible for the up-conversion: excited state absorption and
energy transfer up-conversion. The decay characteristics of the luminescence
from the 3H4 level were studied under direct and indirect pumping at 786 and
1586 nm, respectively. By comparing the decay waveforms to the solution of a
simple set of rate equations, the energy transfer up-conversion process (3F4,
3F4 ! 3H4, 3H6) was established at Tm2O3 concentrations greater than 200 ppm.
