Ultraviolet-excited fluorescence in optical fibers and preforms.

Germanium-doped optical fibers and preforms are found to fluoresce at a wavelength near 420 nm when excited by UV radiation at a wavelength below 350 nm. The UV absorption spectrum of a preform sample was measured and found to exhibit a strong and relatively broad peak in the vicinity of 240 nm. Experimental results of the application of this effect to the refractive-index profiling of multimode optical fiber preforms is presented. In this application the preform is excited transversely to its axis by a He-Cd laser emitting at a wavelength of 325 nm. The resulting fluorescent radiation is detected by a video camera to give the concentration profile directly. The effects of boron and phosphorous dopants on the profile are also shown.