The capability of Tm-doped silica fibers pumped at 790nm to efficiently produce high power emission in the 1.9~2.1μm region has been well documented to date but little has been presented on the reliability of this technology. Early experiments highlighted that photodarkening can be a significant concern when Tm-doped silica fibers are exposed to high intensity blue light. We present a discussion of the processes responsible for the production of blue light in Tmdoped fibers pumped at 790nm and how fiber composition influences these processes. Through optimization of fiber composition we have demonstrated highly efficient lasers exhibiting less than 1% output power degradation per thousand hours.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.