Behavior of alkali impurities and their adverse effect on germania-doped silica fibers

Alkalies, such as sodium and potassium, were found to migrate from over-jacketing natural silica tubes into germania-doped cores forming defects leading to hydrogen-induced loss in the infrared wavelength region. These impurities were found to be removed from the same kind of natural silica tubes during the MCVD process. Results strongly point out that the absence of impurities is essential for 1.55-μm transmission systems based on germania-doped silica fiber.