Infrared silver halide fibers were studied concerning the optical degradation induced by photodarkening. Infrared transmittance decreased notably in the shorter wavelength (near-infrared) region by the exposure to ultraviolet (UV) or visible light below a wavelength of ∼600 nm. Black paint was tentatively coated on the unclad fiber for photodarkening protection, but the infrared transmittance of the fiber decreased heavily due to the evanescent wave absorption by the paint. To reduce this absorption loss, a fluorocarbon polymer was coated between the fiber core and the black paint. Consequently, efficient transmission was realized in the infrared region except for the 7-10-µm wavelength range, where the fluorocarbon polymer has a strong absorption band. In the 2-7-µm wavelength range, the fiber that was heated at 100 °C in the polymer coating process exhibited higher transmittance than the original uncoated fiber. No optical degradation of the coated fiber was observed, even after irradiation with UV light for a period of 70 h.


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