Terminal reflections in fiber-optic image guides

Fibered image guides for confocal reflectance endomicroscopy suffer from Fresnel reflections at the fiber terminals, which can limit signal-to-noise ratio in these systems. A model that describes these terminal reflections is presented to better understand how they can be managed most effectively. An expression for the refractive index of termination that minimizes the reflection as a function of the fiber's normalized frequency is derived for step-index fibers, while a graphical solution is presented for graded-index fibers. The model predicts that terminal reflections from graded-index fibers are more sensitive to variations in fiber size and changes in wavelength than step-index fibers. A method is also presented to measure the refractive index that allows one to minimize the terminal reflections in an image guide. The technique uses the inherent mode coupling of the fibers in the image guide, allowing the isolation and measurement of reflections from only one end of the fiber. An achievable minimum backreflection of -36 dB was measured at 635 nm in a commercial image guide with 30,000 fibers. (C) 2009 Optical Society of America