  We report on detailed in-situ distributed temperature measurements inside a
high power fiber amplifier. The deducted thermal load and the TMI-threshold of
a commercial LMA fiber with 25 micron core and 400 micron cladding was measured
at various seed wavelengths. By matching these results with detailed
simulations we show that photodarkening has a negligible impact on the thermal
load and, therefore, on the TMI threshold in this fiber.
