Raman investigation of the rate of OH uptake in stressed and unstressed optical fibers

Raman measurements of the rate of OH uptake in silicone clad pure fused silica optical fibers (530 parts/10⁶initial OH), immersed in water at room temperature, were accomplished with and without tensile stress by determining the integrated intensity of the OH-stretching contour at ∼ 3690 cm^-1. Up to tensile stresses of 1.56 GPa (15.6 kbar), no significant changes in the rate of OH uptake, relative to the uptake rate for an unstressed fiber, were observed. For diffusion through silicone rubber, a 14-percent increase in the overall Raman OH signal from the fused silica occurred in 200 h. This 14- percent increase means that a large increse in the OH concentration ocured within a few micrometers from the surface of the 200-μ-diam fiber, because diffusion of water is very slow in fused silica at room temperature.