Rare-earth doped single-mode optical fibers are extensively being used as optical amplifiers and fiber lasers [1,2]. One of the parameters that affects the performance of optical amplifiers is the degree of rare-earth confinement in the fiber core. Theoretical studies [3,4] have shown that varying degrees of erbium confinement lead to a variation in the gain, gain coefficient and optimal operational length of an optical amplifier. A practical evaluation of the erbium profile in an erbium doped fiber is a challenging task because of the small size of the fiber core. In the past, techniques for estimating erbium confinement have included an analysis of partially drawn thick (≈ 1 - 5 mm diameter) preforms, with an implicit assumption that the drawing process does not change the confinement parameter. Most quantitative chemical analysis techniques have been used for preform analysis. Specifically, secondary ion mass spectroscopy (SIMS) and electron probe microanalysis (EPMA) in conjunction with neutron activation analysis (NAA) and x-ray fluorescence (XRF) have been successful in quantitatively evaluating the erbium concentration and confinement in preforms [5]. The final draw of the preform into fibers with core diameters on the order of 4 µm leads to further diffusion of the erbium in the core and the analysis of preforms can tend to be inaccurate. Other methods currently being investigated for determining erbium confinement in fibers are near-field microscopy [6] and cathodoluminescence. In this paper, we present a novel diagnostic method that uses a differential mode launching technique.

© 1992 Optical Society of America

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