B.S.T.J. Briefs: Reduction of the 1.38-um Water Peak in Optical Fibers by Deuterium-Hydrogen Exchange

Absorption due to the 1.38-/xm overtone of the O H - vibration in silica has long been an obstacle to low-loss propagation in optical fibers at wavelengths near the dispersion minimum. T h e corresponding absorption spectrum of OD~ in silica, however, has a window in this region. A deuterated fiber with OD~ content comparable to that of OH~ in present-day hydrogenated fibers would have a window between 1.0 and 1.65 [xm with essentially no added loss due to water. It is well known that, under certain conditions relatively simple to attain, the hydrogen-deuterium exchange 2 OH" + DzT ~ 2 OD" + H 2 t takes place in silica as an efficient process1"5 and that the necessary exchange activation energy may be obtained by heating 1-4 or by irradiation. 5 We have demonstrated, experimentally, that this exchange can be effected in silica fibers by the simple process of heating the fibers in D2 at atmospheric pressure and elevated temperatures and, as examples, that the high loss peak at 1.38 fim can be reduced by a factor >200 in a 100-ppm O H - silica fiber and >10 in a 2-ppm OH" single-mode fiber. We discuss these experiments briefly and suggest ways in which the exchange might be applied more usefully to the fabrication of preforms from which fibers with extremely low losses in the 1.2- to 1.6-/im wavelength region might be made. 1541