Measurement of erbium confinement in optical fibers: a differential mode-launching technique

We present a selective mode-launching technique to determine the confinement of erbium in optical fibers. The loss per meter, alpha(lm), of the individual LP(lm) modes (LP(01) and LP(11) at 980 nm) is measured, and the ratio gamma = alpha(01)/alpha(11) is related to the concentration of erbium in the core. We assume a Gaussian diffusion profile for erbium and relate the dependence of gamma on erbium confinement by using exact mode profiles. This technique uses only two sets of measurements and is independent of erbium concentration.