Refractive index decrement was discovered in a fiber made from photo-thermo-refractive (PTR) glass. PTR glass is a fluorosilicate glass doped with cerium and silver which demonstrates refractive index change after UV exposure and thermal development due to precipitation of NaF nanocrystals in the irradiated areas. This glass is widely used for volume holographic optical elements recording. Photosensitivity in PTR optical fibers has been shown after exposure to radiation at 325 nm for about 1 J/cm 2 followed by thermal development at 520°C. Refractive index difference between exposed and unexposed areas was about 1000 ppm. A Bragg mirror at 1088 nm was recorded in such fiber which showed narrow band reflection within 1 nm
