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Quantum Electronics

Population kinetics of laser levels of neodymium ions in POCl3—SnCl4—235UO22+—Nd3+excited by fission fragments

Ekaterina A. Seregina, A F Dobrovol'skii, D. V. Kabakov, А.А. Серегин, G. V. Tikhonov

Published Aug 31, 2009• 6 citations
Publisher

Abstract

The population kinetics of laser levels of neodymium ions in the uranium-containing POCl 3 —SnCl 4 — 235 UO 2 2+ —Nd 3+ laser liquid irradiated in a BARS-6 pulsed reactor is studied experimentally and theoretically. The relations between the populations of the upper and lower laser levels are studied at different time moments with respect to the pump pulse. An inverse population of laser levels is observed and found to be directly proportional to the pump power. The efficiency of pumping of the upper laser level of neodymium ions in the laser medium by uranium fission fragments is determined, and quantitative information is obtained on the inverse population, gain, and their dependences on the pump power.

DOI
10.1070/qe2009v039n08abeh014037

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The population kinetics of laser levels of neodymium ions in the uranium-containing POCl 3 —SnCl 4 — 235 UO 2 2+ —Nd 3+ laser liquid irradiated in a BARS-6 pulsed reactor is studied experimentally and theoretically. The relations between the populations of the upper and lower laser levels are studied at different time moments with respect to the pump pulse. An inverse population of laser levels is observed and found to be directly proportional to the pump power. The efficiency of pumping of the upper laser level of neodymium ions in the laser medium by uranium fission fragments is determined, and quantitative information is obtained on the inverse population, gain, and their dependences on the pump power.
Characteristics of ytterbium to erbium energy transfer in Yb:Er:phosphate glasses have been studied for a case when Nd phosphate glass laser is used as a pumping source. A theoretical model of the energy transfer was developed to reproduce the experimentally observed population dynamics of the Er/sup 3+/ upper and lower laser levels. The results show that saturation and frequency hole burning of the inhomogeneously broadened Yb/sup 3+/ absorption line are of primary importance in determining the dynamics and efficiency of the Yb/sup 3+/-Er/sup 3+/ energy transfer under Nd laser pumping. The influence of other loss channels that have been identified in previous studies is found to be relatively small. The model also yields the lifetime of the frequency hole, as well as the effective rates of energy transfer between groups of Yb/sup 3+/ and Er/sup 3+/ ions in phosphate glasses.>
An experimental method was developed for the determination of the rate of relaxation of the 4I11/2 state and of the absorption cross section of the 4I11/2–4F3/2 transition. These parameters were deduced from the dependences of the intensity of the λ = 0.88 μ luminescence, excited as a result of the 4I11/2–4F3/2 transition by laser pulses of λ = 1.06 μ wavelength, on the energy density and duration of the laser pulses. Estimates of the relaxation time obtained for the investigated silicate (GLS-1) and phosphate (LGS-40, LGS-I) glasses, and also for a liquid based on POCl3-SnCl4, gave τ21 < 2 nsec. A considerable increase in the population of the 4I11/2 level occurred as a result of the Raman scattering of high-power coherent radiation by vibrations of the PO4 group.
An improved method was developed for determining the probability W21 of relaxation of the population of the 4I11/2 level of neodymium ions. The relationship between W21 degree of dumping, and parameters of the dumping pulses was obtained by solving the rate equations for the populations subject to an allowance for the Stark structure of the energy levels 4F3/2 and 4I11/2. At T=300°K the probability values were W21=108 sec−1 for Y3Al5O12:Nd3+ and W21=1.5×108 sec−1 for La2O2S:Nd3+. In the case of a BaGd2-xNdx(MoO4)4 crystal at 77°K the probability was W21=2.5×108 sec−1. The influence of Wn on the efficiency of utilization of the stored energy was found to depend on the energy gap between the lower Stark sublevel of the 4I11/2 level and the final sublevel in the stimulated transitions.
An analysis is made of the problem of kinetics of the superradiance decay in the case of nonlinear quenching of excited centers in four-level active media. A method is proposed for estimating the initial axial gain and of the stimulated emission cross sections of active media: it is based on a simultaneous analysis of the normalized superradiant flux and integrated population of the active medium. The proposed method is not sensitive to nonlinear quenching. A qualitative analysis is made of the decay of excitation in neodymium pentaphosphate crystals after pulsed excitation with laser radiation of the 0.538 μ wavelength and stimulated emission cross sections are estimated for the 4 F 3/2 → 4 I 11/2 transition in Nd 3+ resulting in lasing at 1.051 μ and at some other wavelengths of this transition.
We report the measurement of the amplification cross section σ21of the Nd3+ion in the POCl3-SnCl4-(H2O) system by three different methods. The first is based on the simultaneous measurement of the variation of fluorescence andQ-spoiled laser emission. The second is purely spectroscopic. The third is based on the measurement of k12at several temperatures. The two last methods give similar results. The most probable value is\sigma_{21} = 8.5 \times 10^{-20}cm2. An explanation for the lowest value\sim 6 \times 10^{-20}cm2found with the first method is given.