Fluorescence profiling of single-mode optical fiber preforms

A new nondestructive and noncontacting method to measure accurately the refractive-index profile of single-mode optical fiber preforms is presented. The method is based on the fact that a Ge-doped preform core fluoresces strongly when excited by UV radiation. The preform, which need not be index matched, is excited transversely to its axis by a He-Cd laser emitting at a wavelength of 325 nm. The intensity of the resulting fluorescence radiation is detected by a video camera and plotted giving the index distribution directly. The profiles so obtained in <1 min are in excellent agreement with those measured by interferometry and by the focusing method, thus providing a simple yet powerful alternative to those techniques.