Rate equations describing the dynamic processes in erbium (Er3+)-doped pentaphosphate crystals were developed to obtain the population processes of all levels of Er3+ ions in pentaphosphate ErP5O14 crystals under 650 nm laser excitation. The effects of pump power on the population (ρ) dynamic process were analyzed. The spontaneous radiative processes, multiphonon nonradiative processes, absorption of the pump photons, corresponding stimulated radiation processes, and all other possible energy transfer processes between Er3+ ions were included in the rate equations. The pump power was assumed to be constant with time (t). Considering that the equation ρ(t0+Δt)=ρ(t0)+(dρ/dt)Δt would be held if Δt was small enough and using the initial condition ρ1=1.0, ρi=0(i≠1), the time-dependent populations of all states could be obtained from the rate equations, from which the steady-state solution of the rate equation was deduced. The comprehensive study of the population dynamic process of all energy-level Er3+ ions in ErP5O14 crystal when excited by 100 mW–107 W has resulted in the conclusion that the strong upconversion luminescence would behave excellently when 650 nm pump laser power reaches the level of 1000–105 W. The representative population dynamic process when excited by 105 W laser power indicates I15/24 and F9/24 levels have a kind of great takeover co-contribution in up-converting population to higher states. The steady-state population probability of I15/24 is as small as 0.0617. The population of the F9/24 state increases rapidly to about a 0.362 high level and sequentially decreases smoothly to a stable value of about a 0.0374 low level, which implies a population already having been upconverted to higher level. Excellent results clearly exhibit that the main up-conversion luminescent level S3/24 has a great population probability of about 0.212, which results in a strong up-conversion luminescence of 543 nm, and it is so exciting that a very strong 379 nm blue up-conversion luminescence could be achieved when 650 nm pump laser power is enhanced to the level of 1000–105 W. The results of this article are significant both to science and applications.
