The knowledge of temperature effects in rare-earth doped fibers is of great interest in order to improve the performance and stability of monolithic fiber lasers and fiber amplifiers. Fiber Bragg gratings (FBGs) are, in general, well applicable to measure local temperatures within an optical fiber. We report on the inscription of FBGs in rare earth doped optical fibers, the reduction of inherent absorption effects in the FBGs and the FBG-based temperature measurement within the core of actively doped fiber samples during core pumping. Experiments were performed on fibers with standard geometry (125µm cladding, 8-10µm core diameter) and different Yb content but comparable P and Al composition. The FBGs were inscribed using UV-femtosecond pulses and a modified Talbot-Interferometer. A Bragg reflection wavelength at 1550 nm far from the Yb-fluorescence spectrum was chosen for the temperature measurements. Short fiber samples (1cm) with the inscribed FBGs were investigated for different cooling conditions with air or water cooling. Temperature changes have been measured depending on the Yb content under high inversion and depending on the time including the photodarkening kinetics of the fiber. The experiments have shown that FBGs can be a useful tool for the investigation of temperature within actively doped optical fibers and might also be applicable for spatial distributed temperature measurements during fiber laser operation and for inversion detection.