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23 March 2005 Numerical aperture in microstructured polymer optical fibers
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Microstructured or 'air-clad' fibers, with air holes surrounding a large core, have demonstrated much wider light acceptance angles than conventional fibers. Recently, a new method employing leaky modes has been used to determine the numerical aperture in highly multimode air-clad microstructured fibers. It shows that an exceptionally high NA can only be achieved when bridge thickness is much smaller than the wavelength. The physical basis of these key results is understood in terms of the conditions for efficient excitation of bridge local modes, which radially propagate power into the outer jacket. A number of microstructured polymer optical fibers (mPOF) have been fabricated with a large core suspended by thin supporting bridges. Such mPOFs provide freedom in microstructure geometry, combined with greater mechanical flexibility. This makes them particularly suited to applications demanding high light capture efficiency from broad-beam sources with irregular shapes and in situations involving tight bends. A demonstration of the technology is presented and the measured numerical apertures of these fibres show robust agreement with theoretical calculations over a broad wavelength range.
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Nader A. Issa, Clemens von Korff Schmising, Martijn A. van Eijkelenborg, and Whayne E. Padden "Numerical aperture in microstructured polymer optical fibers", Proc. SPIE 5691, Optical Fibers and Sensors for Medical Applications V, (23 March 2005);

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