Energy transfer up-conversion in Tm3+-doped silica fiber

https://doi.org/10.1016/j.jnoncrysol.2005.11.019Get rights and content

Abstract

A study of the mechanisms responsible for the infra-red to near infra-red up-conversion in Tm3+-doped silica fibers is presented. Up-conversion 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.

Introduction

The broadband emission from the 3H4  3F4 transition in Tm3+ has been identified as one of the more promising candidates for optical amplification in the telecommunication S-band (1470–1530 nm). Amplification in the S-band region has been observed in many Tm3+-doped fluoride glasses and crystals [1], [2], [3], [4]. The low phonon energies associated with fluoride glass and crystals allow many of the Tm3+ transitions to operate with quantum efficiencies near 100%. When doped into glasses with higher phonon energies such as silica, the quantum efficiencies of these transitions are reduced, in some cases, to a few percent. Although recent work in Tm3+-doped silica fibers showed a fourfold increase in the quantum efficiency of the 3H4  3F4 amplifying transition through the incorporation of aluminum [5], this is still considerably less than the almost 100% efficiency observed in Tm3+ doped fluoride glasses. If further improvements in the efficiency of the Tm3+ transitions are to be realized in silica glass, an understanding of the spectroscopic processes involved in the related levels is required.

Extensive studies have been carried out on the cross-relaxation process (3H4, 3H6  3F4, 3F4) originating from the 3H4 level in various Tm3+-doped crystals and oxide glasses [6], [7], [8]. However, little attention has been given to the energy transfer up-conversion process originating from the 3F4 level (3F4, 3F4  3H4, 3H6). This energy transfer up-conversion process is advantageous to the operation of an S-band amplifier as it acts to increase the population inversion between the 3H4 and 3F4 energy levels. On the other hand this process may be seen as a loss mechanism for the operation of a 2 μm laser which would result in higher lasing thresholds and lower slope efficiencies. The (3F4, 3F4  3H4, 3H6) up-conversion process has been observed in various Tm3+-doped crystals [9], [10], [11]. However, in some cases, the sharp spectral lines associated with these materials results in a lack of spectral overlap between the 3F4  3H4 absorption and the 3F4  3H6 emission cross-section, which reduces the efficiency of the energy transfer process. In silica glasses, where the energy levels are inhomogeneously broadened, the spectral overlap between the absorption and emission cross-sections is expected to be much stronger and therefore the effect of energy transfer up-conversion is expected to be much greater. Jackson, in studies of heavily Tm3+-doped silica fibers, attributed quenching of the 3F4 level’s population to the (3F4, 3F4  3H4, 3H6) energy transfer process and another possible process involving the 3H5 level (3F4, 3F4  3H5, 3H6) [12]. In this work the energy transfer up-conversion process (3F4, 3F4  3H4, 3H6) is established and studied in Tm3+-doped silica fibers at low Tm2O3 concentrations.

Room temperature up-conversion luminescence has been observed from the 3H4 level of Tm3+ under 1586 nm pumping into the 3F4 level. Spectroscopic methods were used to investigate the mechanisms responsible for this up-conversion in fiber samples with three different Tm2O3 concentrations, namely 200, 550 and 2900 ppm.

Section snippets

Experimental details

The samples used in this investigation were fabricated using the modified chemical vapor deposition and solution doping techniques [13]. The fabricated optical preforms were drawn into fiber. The three alumino-silicate fiber samples had Tm2O3 concentrations of 200, 550 and 2900 ppm and contained 4.8, 4.5 and 4.6 mol% of Al2O3, respectively. The Tm2O3 concentration was estimated by measuring the absorption peak at 790 nm using the multiple cut back technique and comparing the value to that obtained

Results

The absorption spectra of the 3H6  3F4 and 3H6  3H4 transitions for the 550 ppm sample are shown in Fig. 1. The 3H6  3H4 transition is characterized by a strong, relatively narrow absorption peak centered at 790 nm, whilst the 3H6  3F4 transition consists of an extremely broad absorption peak spanning ∼400 nm, centered at 1640 nm. Under excitation at 1586 nm into the 3F4 level, luminescence at 800 nm was observed. The transverse up-conversion emission spectrum could not be obtained due to insufficient

Discussion

The behavior of the up-conversion luminescence when the pump excitation has been removed can be described by considering a simple set of rate equations relating the populations of the 3F4 and 3H4 energy levels, namelydn1dt=-n1τ1-WETUn12c,dn2dt=-n2τ2+12WETUn12c,where n1 and n2 represent the normalized population of the 3F4 and 3H4 levels, respectively, c is the Tm2O3 concentration, WETU is the energy transfer up-conversion co-efficient and τ1 and τ2 are the lifetimes of the 3F4 and 3H4 levels,

Conclusion

Up-conversion luminescence from the 3H4 level in Tm3+-doped silica fibers when excited at 1586 nm has been identified. The up-conversion luminescence from the 3H4  3H6 transition showed a quadratic dependence on the luminescence from the 3F4  3H6 transition, indicating a two photon up-conversion process. The double exponential decay of the up-conversion luminescence confirmed that ETU was operating at Tm2O3 concentrations as low as 200 ppm. The fitted value of τ1 obtained from the rate equation

Acknowledgments

The authors gratefully acknowledge Visho Zeqaj from Victoria University, Australia for obtaining the absorption spectra of the fiber samples and Dr Pavel Peterka from the Institute of Radio Engineering and Electronics in Prague, Czech Republic for many helpful discussions. This work was supported by the Australian Research Council, and Centre National de la Recherche Scientifique, in France.

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