Measurement of Incident Intensity of UV Radiation Delivered by Optical Fibers

Abstract

Fiber optics are being used more and more frequently to deliver light to a sample for remote spectroscopic analysis, or to induce photochemistry or photoablation on an otherwise inaccessible sample, e.g., living tissue. For accurate quantitative results, the photon flux at the sample must be known. This paper presents two techniques for simple and accurate measurement of the intensity of ultraviolet light in the range 260 to 369 nm delivered to a distant sample by a fused-silica optical fiber. One technique uses a home-built evanescent wave coupler near the sample to couple excitation light into a monitoring fiber; the other uses a twin-fiber system, with equal amounts of source radiation launched into an excitation and a monitoring fiber. In each method, two Rhodamine B quantum-counter photomultiplier tube detectors are used simultaneously to assess the effectiveness with which the incident intensity is monitored. A critical evaluation of the results is presented. The twin fiber method is, theoretically, very simple; in practice, however, it is difficult to obtain a good wavelength-independent match between the two outputs. The evanescent wave coupling is also wavelength-dependent, but the observed results are in agreement with the predictions of simple evanescent field theory.