We study thermal bleaching of photodarkening-induced loss in ytterbium-doped fibers. Post-irradiation heating of a photodarkened fiber is shown to result in further increase the loss which is attributed to both a permanent increase of loss-inducing color centers and a temperature-dependent broadening of the absorption spectrum. The permanent heat-induced increase of loss is believed to indicate presence of an intermediate energy state in the NIR photochemical mechanism for photodarkening. Further, we apply the demarcation energy curve approach to derive the thermal activation energy of the induced defects. For the studied commercial 20-μm-core-diameter LMA fiber, the energy distribution consists of a single peak, located at 1.3 eV with a FWHM of 0.31 eV.
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