Hydrogen-induced losses in germanium-doped fibers is significantly reduced by increasing the concentration of oxygen in the core region of the glass. The increase in oxygen functions to "heal" the germanium-deficient defects, thus substantially reducing the sites where hydrogen bonding can intrude. Advantageously, the presence of the excess oxygen does not compromise the ability to create UV-induced gratings in the fiber's core area. Indeed, the stability of the glass has been found to increase even further during UV radiation. Thus, an FBG structure suitable for use in harsh, high temperature environments can be formed.