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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Influence of cross-relaxation parameter on the sensitized photon avalanche

Eugen Osiac, Voicu Lupei

Published Feb 23, 2000• 0 citations
Publisher

Abstract

The paper discuses the influence of cross-relaxation parameter on the characteristics of the sensitized photon avalanche. A system Pr3+ as activator ion and Yb3+ as sensitizer ion were considered for the avalanche process. By numerical modelling of a system of rate equations, time dependencies and pump rate dependencies of populations of energy levels were calculated for three different values of cross-relaxation rate. Correlation between cross-relaxation rate and threshold value of pump intensity was established.

DOI
10.1117/12.378671

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The paper discuses the influence of cross-relaxation parameter on the characteristics of the sensitized photon avalanche. A system Pr3+ as activator ion and Yb3+ as sensitizer ion were considered for the avalanche process. By numerical modelling of a system of rate equations, time dependencies and pump rate dependencies of populations of energy levels were calculated for three different values of cross-relaxation rate. Correlation between cross-relaxation rate and threshold value of pump intensity was established.
The phenomenon of photon avalanche, which involves both an excited state absorption process from a metastable level and a cross relaxation energy transfer process responsible for the large increases in population of this metastable level, has been investigated in LaCl3:Pr3+.{\mathrm{LaCl}}_{3}:{\mathrm{Pr}}^{3+}.LaCl3​:Pr3+. In this paper, we report on the intensity dependence of both the steady-state fluorescence and the temporal evolution of the avalanche signal as a function of temperature (80 K to room temperature). Comparisons are made between experimental results and mathematical models based on a six-level system described by a series of coupled rate equations combined with the propagation of a Gaussian beam. We develop a model with only one adjustable parameter, the excited state absorption cross section, which provides a good description of all of our experimental data. We have estimated values of the excited state absorption cross section for the various ${}^{3}{\stackrel{\ensuremath{\rightarrow}}{{\mathrm{H}}_{5}}}^{3}{\mathrm{P}}_{0}$ pumping transitions at different temperatures.
Here we report and discuss the avalanche-like up-conversion behavior in absence of the avalanche. We experimentally observed significant changes in the slope of the curve for the intensity dependence of up-conversion luminescence of erbium ions in the green band (520-560 nm) on the pump intensity of the diode laser. Such changes are typical for the photon avalanche. However, the concentration of erbium ions is insufficient for an efficient exchange of energy between them, and the excitation of a photon avalanche is not possible. Using a simple three-level approximation of the up-conversion process model, we have shown that the observed avalanche-like luminescence process can also occur in the absence of a photon avalanche due to the nonlinear relation between the efficiency of two pumping channels of erbium ions caused by the intensity dependence of the pump spectrum.
A Theoretical estimation of the threshold of end-pumped laser in three-level system containing the cross relaxation is deduced from the transition rate-equations and the transmission equations of the laser- and pump-light for active ions. The lasing performance of the crystal can be presupposed, and parameters, such as optimum length of the laser rod, optimum concentration of active ions and optimum reflectivity of reflectors in the resonant cavity et al., can be estimated by the use of deduced formula that will help to design the laser.
An analysis is made of some processes that occur under conditions of accumulation of population inversion due to the 4 I 13/2 – 4 I 15/2 resonant transition in Er 3+ ions in ytterbium–erbium (Yb–Er) glasses of various compositions. Methods of measuring the parameters of direct and reverse excitation energy transfer between Er 3+ and Yb 3+ ions are also studied. It is found that, under certain conditions achieved in practice, appreciable bleaching of the medium should be observed at the frequency of the pump radiation, particularly under conditions of high-intensity reverse transfer of excitation energy from Er 3+ to Yb 3+ ions. This is typical of glasses having a low rate of intracenter relaxation of Er 3+ ions. It is also shown that nonsteady-state rate equations that assume a linear relationship between the rate of interionic transfer and the ground state population of the acceptor consistently describe the energy characteristics of the excitation processes of Yb–Er glasses. In this case, it is not necessary to introduce in the calculation model additional excitation losses that increase nonlinearly with the pump radiation.
We have made an unexpected discovery of highly nonlinear optical behavior in certain praseodymium-doped lanthanum halides. When a LaCl3 or LaBr3 crystal containing Pr3+ ions is exposed to green laser radiation slightly in excess of a certain critical intensity, Pr fluorescence increases by orders of magnitude. Infrared output near 4.5-μm wavelength is associated with the large visible output, and pump power is strongly absorbed. We call this new phenomenon a photon avalanche. Critical pump power density is in the 1–15 W/cm2 range. At higher pump rates, a population inversion and lasing action take place between the lowest infrared levels of the Pr ion. Experimental results are presented and compared to predictions of a photon avalanche model based on resonant energy transfer.