Investigation of structural change caused by UV irradiation of hydrogen-loaded Ge-doped core fiber

Structural changes caused by UV irradiation of hydrogen-loaded Ge-doped cores of commercial optical fibers were investigated using Fourier transform infrared (FTIR) spectroscopy. The IR reflection peak position of the silica structural band at ∼1120 cm−1 was used to determine fictive temperatures. The fictive temperatures of Ge-doped silica core glass, which are related to the density as well as the structure of the glass, were determined as a function of the UV irradiation. The fictive temperature increased with the UV irradiation from the original 1520 °C to 1640 °C under the highest irradiation. The hydroxyl (OH) concentrations in the core fiber were also determined using IR transmission at ∼3640 cm−1 as a function of the UV irradiation. The OH concentration increased from the initial zero ppm to the highest ∼1200 ppm. The hydroxyl increase in Ge-doped silica glass is accompanied by corresponding increases of the GeE′ center. The expected change of the refractive index due to changes in fictive temperature, OH concentration, and the GeE′ center was estimated. It was found that the increases in the refractive index by UV irradiation are predominantly due to the increase of the GeE′ center concentration, followed by the increases of hydroxyl concentration and fictive temperature (or densification).