The diffusion of water in optical fibers

The diffusion of “water” (OH) into the core of optical fibers can affect the transmission properties. An increase in the water will result in an increased loss of high order modes, a reduction in numerical aperture, and increased microbending losses if there is coupling to these higher order modes. An extra 10ppm OH uniformly distributed through the core would produce 12dB/km loss at 0.95μm and about 1dB/km loss at 0.90μm.Based on the extrapolation of existing diffusion coefficient measurements and solubility data, calculations are made for the diffusion of atmospheric H₂O through the cladding into the core and also for the diffusion of OH from the cladding into the core. Diffusion can occur fairly rapidly during the high temperatures reached when depositing, collapsing, and drawing the fibers, and also slowly during extended period ambient temperature storage and use.