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Journal of Lightwave Technology

Characterization of clusters in rare earth-doped fibers by transmission measurements

Monica K. Davis, Michel J. F. Digonnet, R. H. Pantell

Published Jan 1, 1995• 27 citations
Publisher

Abstract

We show theoretically that the absorption saturation behavior of clustered rare earth-doped fibers depends strongly on the percentage and/or the size of the clusters, which can form the basis for a simple diagnostic tool. When applied to published transmission measurements in Er-Al-P-doped fibers, this technique indicates low percentages of clusters in fibers co-doped with P and/or Al. In Nd-doped fibers with no co-dopants, it shows the presence of a high percentage of large clusters. Estimates of the time constant and strength of the cross relaxation process in Nd and Er clusters are inferred from these measurements.>

DOI
10.1109/50.365196

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We show theoretically that the absorption saturation behavior of clustered rare earth-doped fibers depends strongly on the percentage and/or the size of the clusters, which can form the basis for a simple diagnostic tool. When applied to published transmission measurements in Er-Al-P-doped fibers, this technique indicates low percentages of clusters in fibers co-doped with P and/or Al. In Nd-doped fibers with no co-dopants, it shows the presence of a high percentage of large clusters. Estimates of the time constant and strength of the cross relaxation process in Nd and Er clusters are inferred from these measurements.>
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 basic understanding of the susceptibility of fiber amplifiers (SOFA) is required to undertake device optimization, e.g., for uses in optical communications systems. In recent work Desurvire1 has developed a semiclassical treatment of the SOFA for erbium-doped fiber amplifiers (EDFAs) taking account of the Stark splitting of the energy levels in the Er-doped glass. That work, which utilized the standard Kramers-Kronig relationships, was applicable to the unsaturated regime of operation. In this contribution, generalized Kramers-Kronig relations2 are utilized to obtain analytical expressions for the SOFA valid even in the saturation regime. It is then shown how, given experimentally measurable absorption cross-sections, the SOFA may be calculated in any operating conditions. In this way gain and refractive-index changes occurring in EDFAs may be deduced, again having universal validity.
A detailed set of measurements is presented on determining the emission and absorption cross sections of Er/sup 3+/ doped fibers for the /sup 4/I/sub 13/2/ to /sup 4/I/sub 15/2/ transition. Two techniques are employed: the Fuchtbauer-Ladenberg analysis, based on spectroscopic data, and a more direct technique involving optical saturation of the transition. The cross sections, and in particular their ratio, are significantly different for the two techniques. Possible reasons for this are discussed, and it is concluded that the Fuchtbauer-Ladenberg approach is inappropriate in this situation.>
To explain the high third-order nonlinearity observed in rare earth doped fibers, we examine the role of the various transitions of the dopants. We present experimental evidence that in fibers doped with Er3+, Nd3+, or Yb3+, most of the nonlinearity is caused by the dopant's UV transitions, i.e., it is non-resonant in nature. The magnitude of the nonlinearity is shown to be in excellent quantitatively agreement with a model. Fiber modulators and switches based on this widespread mechanisms are predicted to operate over a large range of signal wavelengths and with negligible signal loss.