Preform core diameter measurement by fluorescence.

A new nondestructive method is presented to measure the core diameters of single-mode optical fiber preforms without the necessity to index match the cladding. Ultraviolet light from two miniature mercury-discharge lamps is incident on opposite sides of the preform transverse to its axis. The resulting fluorescent emission, which peaks at the core interface, is detected with a video camera and used to calculate the diameter. The imaging effect of the cladding is treated theoretically. A circular cladding is shown to introduce a magnification just equal to its refractive-index value for small core diameters. An elliptically distorted cladding is found to just shift the image in the paraxial approximation, thus making this technique widely applicable. Experimental results of core-diameter measurements by this and other techniques agree to ~1%-the precision of respective measurements.