AgClBr photonic crystal fibers for the middle infrared

Photonic crystal fibers (PCFs) are normally holey fibers, made of silica glass, which is opaque in the mid- and far- infrared spectral range 3-20μm. We have fabricated novel PCFs by multiple extrusions of silver halide (AgCl _xBr_1-x) crystalline materials, which are highly transparent in this spectral range. These PCFs are composed of two solid materials: the core consists of pure AgBr (n = 2.16), and the cladding includes small diameter fiberoptic elements, made of AgCl (n = 1.98). These AgCl fiberoptic elements are arranged in two concentric hexagonal rings around the core. This structure gives rise to a cladding region of lower refractive index, thus ensuring total internal reflection. Flexible PCFs of outer diameter 1mm and length of about 1m were fabricated, and their optical properties were measured. Measurements of numerical aperture, laser power transmission and evanescent wave spectroscopy indicated that the PCFs behave like a core-clad structure. There was a good agreement between the results and those obtained by theoretical simulations. Silver halide PCFs would be extremely useful for IR laser power transmission, for IR radiometery and for IR spectroscopy.