With quite broad ultraviolet (UV) absorption and emission spectra

the 4f

N-1

5d energy levels of trivalent rare earth ions doped in hosts are possible and perfect upper laser levels for tunable UV laser.In order to obtain upconversion UV laser based on 5d levels

it is necessary and interesting to investigate the excitation behaviors of 5d levels by upconversion pump.In this paper

a theoretical study for the kinetics of frequency upconversion excitation of Pr

3+

4f5d level in Yb

3+

:Pr

3+

:ZBLAN fiber is performed by using the population rate equations and related transition theories.Yb

3+

ions pumped by the first pump beam (

1

=980 nm) sensitize the excitation of Pr

3+

3

P

0

level through the energy transfer processes and the second pump beam (

2

=329 nm) realizes Pr

3+

4f5d excitation from

3

P

0

.The distributions of the pump beam intensities and the normalized population densities along the fiber

the effective fiber lengths under different pump intensities and Pr

3+

concentrations are calculated.The dependence of 4f5d fluorescence intensity on the incident intensity

fiber length and Pr

3+

concentration are discussed.Pumping efficiency as a function of the pump intensity and Pr

3+

concentration is also given.The calculated results show that the intensity of both

1

and

2

linearly decrease along the fiber and this decrease becomes fast with increasing Pr

3+

concentration.Under the pump intensities assumed here the 4f5d population density does not change in the whole pumped region (i.e.

the effective fiber length) where both pump beams exist.This means the 4f5d excitation under the used pump intensities is saturated.The effective fiber length is in proportion to the pump intensity and in inverse proportion to dopant concentration.It is found that higher dopant concentration is favorable to get large 4f5d fluorescent intensity.The pump efficiency increases with the concentration and pump intensities

but the saturation effect will occur when the pump intensities are too large.The results indicate that the upconversion excitation of Pr

3+

4f5d in Yb

3+

:Pr

3+

:ZBLAN fiber can be effectively achieved by pump of the two beams.The calculated results may be helpful to further theoretical or experimental study of upconversion UV laser based on 5d levels in rare earth doped optical fibers or bulk hosts.