Measurements of OH Diffusion in Optical-Fiber Cores

The diffusion of water (H2O) into optical fiber cores can affect both fiber loss1 and bandwidth. 2 Water contamination of the fiber core could occur during three different periods--preform manufacture, fiber drawing, and long-term environmental exposure. The first two have previously been studied; 3 5 however, environmental data on fused silica require longer exposure times 6 because of slow diffusion rates. It would be ideal to have an activation energy which would allow the calculation of concentration changes as a function of temperature. Thus, the effect of water diffusion in a fiber could be calculated based on the expected operating conditions for that fiber. Reported here is the first measurement of the activation energy for OH diffusion in a fiber core. In the transmission wavelength region of current interest, 0.8 to 1.5 /mi, two major OH absorption bands appear. These bands are at 1.38 and 0.95 /mi and are, respectively, the first and second overtone bands of the fundamental stretching vibration at 2.7 /mi. Two candidates for system wavelengths are 0.87 and 1.3 /im--very close to the OH absorption bands. A 10-ppm increase of OH would increase the loss by about 12 dB/km at 0.95 /mi, 1.0 dB/km at 0.90 /mi,7 and 7 dB/km at 1.3 /mi.8 Thus, knowledge of the OH diffusion rate in optical fiber waveguides 283