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Optics Letters

Experimental evidence of differences in the absorption spectra of clustered and isolated ions in erbium-doped fibers

B.N. Samson, Wei H. Loh, J.P. de Sandro

Published Dec 1, 1997• 16 citations

Abstract

The absorption spectra of clustered and isolated ions in erbium-doped germanosilicate fibers have been experimentally studied. The ground-state absorption spectra broaden as the degree of erbium-ion clustering increases, indicating that the absorption spectrum of clustered ions is significantly different from that of the homogeneous ions. This fact is confirmed by comparison of the broadened absorption spectra with the fiber's unbleachable loss spectrum, a direct measurement of the clustered ions. This is the first experimental evidence to the authors' knowledge that indicates different absorption cross sections for the two species of ions in germanosilicate glass, an assumption that is used in the theoretical description of self-pulsing in erbium-doped fiber lasers but is in direct contradiction to the pair-induced quenching model widely used to characterize erbium-doped fiber amplifiers.

DOI: 10.1364/ol.22.001763

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The absorption spectra of clustered and isolated ions in erbium-doped germanosilicate fibers have been experimentally studied. The ground-state absorption spectra broaden as the degree of erbium-ion clustering increases, indicating that the absorption spectrum of clustered ions is significantly different from that of the homogeneous ions. This fact is confirmed by comparison of the broadened absorption spectra with the fiber's unbleachable loss spectrum, a direct measurement of the clustered ions. This is the first experimental evidence to the authors' knowledge that indicates different absorption cross sections for the two species of ions in germanosilicate glass, an assumption that is used in the theoretical description of self-pulsing in erbium-doped fiber lasers but is in direct contradiction to the pair-induced quenching model widely used to characterize erbium-doped fiber amplifiers.

Strong interest in heavily doped erbium-doped waveguides1 and in the applications of heavily doped erbium-doped fibers (EDFs) in lasers has resulted in several recent studies of collective processes in erbium ions. Rapid cross relaxation within a fraction of clustered ions, called pair-induced quenching (PIQ), has been identified as the main cause of EDF performance degradation.2-4 To date, studies on the dependence of EDF performance on ion concentration have been focused on pair/cluster-induced nonsaturable absorption of EDFs,2 which leads to the reduced quantum efficiency (QE) of EDF lasers.4 Recently, detailed theoretical analysis of the effect of PIQ on the performance of erbium-doped fiber amplifiers (EDFAs) has also been reported.4

Ion clustering is one of the major dissipative processes in the operation of Er(3+)-doped planar waveguide amplifiers and heavily doped fiber amplifiers. We propose and demonstrate a nondestructive method for measuring the fraction of erbium ions in doped planar waveguides and fibers that are quenched by ultrafast parasitic energy transfer in ion pairs or clusters. The method is based on unsaturable absorption measurements in the 980-nm absorption band. By combining transmission data at resonance with the corresponding data for radiation outside the absorption band, the influence of background and coupling losses can be eliminated.

The dependence of erbium-doped fiber amplifier (EDFA) performance on the erbium ion concentration is studied experimentally and theoretically. The quantum efficiency of the amplifier is found to he strongly dependent on the erbium ion concentration, the signal wavelength, and the relative propagation direction of the pump and signal beams. This dependence is fully explained by the presence of an upconversion mechanism between ions residing in pairs or larger clusters and suggests that other sources of amplifier performance degradation (back-ground loss, excited state absorption, homogenous upconversion) are negligible. The experimental data show that in the present EDFA designs with over 80% quantum/conversion efficiency, the aluminum co-doped fibers with erbium ion concentration less than 20/spl times/10/sup 24/ m/sup -3/ (900 molar ppm Er/sup 3+/) are most suitable.

A nonsaturable component to the 980 nm excited state absorption (ESA) is found in Er3+-doped silica glass fibers. This anomalous behavior of the ESA provides a way of quantitatively characterizing the degree of clustering in these glasses. It is found that the degree of clustering is greater for higher Er3+ concentrations, and is significantly reduced with the addition of aluminum to the glass. With this method it is also found to be possible to distinguish between true clustering and the occurrence of closely spaced Er3+ pairs in a random distribution.

"Theoretical evaluations of polarization degree of the fluorescence excited in erbium doped fibers (EDF) dependence on the excitation light power are presented. In calculations we use the model of randomly oriented and not interacting Er3+ ions. Assumption of the reduced optical anisotropy of the individual ions (with the absorption/emission cross-sections described with the ellipsoids) is shown to be necessary to get a reasonable agreement with the existing experimental data. Similar values of the anisotropy parameters (0.6-0.7) are also utilized to explain the experimental data on polarization hole burning in EDF earlier."