Сladded and uncladed infrared polycrystalline fibers based on photo- and radiation-resistant single crystals of the AgCl0.25Br0.75 – TlCl0.74Br0.26 and AgCl0.25Br0.75 – TlBr0.54I0.46 systems are proposed. The widest transmission range 3.5-28 μm among currently existing optical fibers is demonstrated. Optical losses in fibers reach 0.2 dB/m at a wavelength of 11-17 μm, which is an excellent value for polycrystalline fibers. The developed IR fibers open up opportunities for creating fiber-optic equipment of the mid- and far IR up to 28 μm, including those operating at high-level of radiation background.