Material Structure of Germamium-Doped Optical Fibers and Preforms

The chemical vapor deposition process, in which oxides are deposited and simultaneously fused on the inner surface of a fused silica tube, has become a valuable technique for fabricating low-loss1,2 and graded-index optical fibers. 3 In modifications and refinements of this technique, higher depositional rates and very low-loss single-mode fibers4 have also been achieved. An important question that arises in utilizing this process concerns the correlation of the deposited material structure in the preform to that in the resulting optical fiber. Can one be confident, for example, that the same distribution of refractive index that is introduced into the preform by changing the material composition of the deposited layers exists in the fiber pulled from this preform? This determination is necessary if one is to reliably fabricate those graded-index profiles required to achieve a minimum of pulse dispersion. 5 This is due to the fact that the shaping of the index profile is quite critical because the reduction-in-pulse-dispersion-vs-profile curves exhibit a singularitylike behavior in the region of the optimum index distribution. 6 Evidence for the preservation of the deposited profile has recently been reported, based on the observation of a linear increase in refractive index in a fiber which was pulled from a preform in which the 1681