We report on the in-core fiber temperature measurements in Yb2O3-doped aluminum silicate fibers during optical pumping using UV femtosecond-induced fiber Bragg gratings (FBG). The FBG containing fiber samples show a different temperature behavior during pumping before and after thermal annealing, caused by absorption centers generated during grating inscription. These absorption centers can be removed completely by thermal annealing. In this way, a remaining temperature rise during pumping due to the quantum defect of the Yb3+ ions is determined in dependence on the Yb concentration in the range of 0.14 to 0.46 mol% and the pump wavelength.