Goniometric Characteristics of Sapphire Optical Fibers

Abstract

Oblique tip optical fibers are presently used in biomedical applications for the ablation of unhealthy tissue along organ walls. By exploiting total internal reflection at the angled tip, a laser beam guided down the optical fiber exits at an angle perpendicular to the optical axis. The thermal mapping of a developing Diesel after-treatment system requires an accurate knowledge of the wall temperatures within a ceramic filter. Under this research, the same total internal reflection characteristics are examined with respect to accepting versus expelling radiation. Two sapphire optical fibers, one with a flat tip and the other with a 45 degree oblique tip, were examined. The flat tip fiber was used to validate the experimental configuration while the oblique tip fiber was the focus of this research project. The primary objective was to locate the area from which the oblique tip fiber accepts incident radiation. The flat tip fiber acceptance angle, determined using the angular scan method, was 9 degrees. This was equated to the numerical aperture specifications provided by the manufacturer through the use of an integration method. The oblique fiber accepts a nominal level of radiation from an angle roughly 15 degrees off the central axis with the principal quantity, 10 times greater, accepted from an angle 90 degrees off the central axis through the cylindrical wall. This response is due to the maximum level of total internal reflection off the cut face and minimal wall reflections experienced by the transmitted radiation. The oblique tip fiber experiences an impeded ability to accept radiation when compared to the flat tip optical fiber. In order to receive a similar intensity count on the spectrometer, the integration time was increased from 27 ms to 1200 ms for the flat and oblique tip fibers respectively. This is believed to be a consequence of external reflections due to the cylindrical surface geometry of the fiber. Future research topics are suggested for the oblique tip fiber targeted at isolating the viewing window, increasing the radiation gathering capabilities, and examining the full 360 degree spatial response.