Bottom shape control of optical fiber soot preform by modified VAD method

Control of the bottom shape of an optical fiber soot preform fabricated by a modified vapor-phase axial deposition (VAD) method is proposed as a way to realize indirect control of the refractive index profile exponent during soot deposition. The technique is based on the close correlation between the profile and the bottom shape. Diameter and refractive index profile fluctuations of the preform with and without control are analyzed. In the case of no control, the fluctuations are larger, and unstable growth of the preform is observed. By controlling the bottom shape, axial fluctuations in the refractive index profile of a transparent preform are reduced to half the magnitude for preforms with no control, and axial diameter fluctuations of the soot and transparent preform are reduced to below ±0.5%. As a result, fibers with a bandwidth of about 1 GHz km at a wavelength of 0.85 μm can be stably fabricated