Power-law dependence of the photodarkening rate constant on the inversion in Yb doped fibers

The description of the photodarkening (PD) kinetics of Yb-doped silica fibers with a stretched exponential function is analyzed in more detail, revealing a dependence of the determined PD parameter values on the measurement time. Thereby, serious uncertainties may occur in advanced interpretations of the results, e.g., the discussion of the rate constant as a function of the Yb inversion, if the measurement time is too short. A method to circumvent the problems of the fitting procedure is suggested, taking advantage of the self-similarity of the PD curves for different inversions. An exponent of 4.3 +/- 0.5 in the power-law dependence of the PD rate constant tau(-1) on the Yb inversion was found for a typical aluminium-codoped fiber.