Optical Fiber Sensors for Biomedical Applications

17 Optical Fiber Sensors for Biomedical Applications 675The ratio, R s /R f , can be used to estimate under what conditions a ball-shapedtip loses its focusing effect. However, it should be noted that the theory is only afirst order approximation and ignores the effects of secondary reflections, whichcan lead to significant deviations. In such cases, ray-tracing provides more reliablepredictions of the resulting beam profiles [3].Both tapered and ball-shaped fiber tips provide a flexible approach for focusingincoming and outgoing light and can be optimized for a variety of applications.These include improving the coupling efficiency with the input laser or otheroptical components, improving the detection efficiency with a photodiode or photomultipliertube or, for therapeutic applications, performing high power laserablation.17.3.4.3 Side-Firing TipsSide-firing tips (Fig. 17.9c) redirect the light over a range of angles away fromthe fiber axis. Side-firing fibers are often employed in therapeutic applications suchas photodynamic therapy, thermal therapy, or tissue ablation target scenarios withlimited room or maneuverability. Recently, side-firing tips have also been employedas optical sensors for measuring the radiance distribution in turbid media [5, 6].Template forholding fibersLight InSensorwallTissuea)Fig. 17.9 (a) Lineararrangement of reflectancesensors (after Wang andJacques [9], with permissionfrom Applied Optics)(b) Circular arrangement ofreflectance sensors: (i) sourcefiber (ii) detection fibers(iii) calibration source (afterRef. [10])b)