Non-Uniform Distribution of Defects and Effects of Excimer Laser Radiation on Bulk Silica and Optical Fibers

The presence of defects in bulk silica and optical fibers, their distribution and the subsequent effect of ultraviolet excimer laser on their properties are the subjects of this research. Bulk silica rods manufactured by different methods which varied with respect to water content and oxygen stoichiometry were selected for the study. Absorbance and luminescence measurements were made in the UV and visible regions of the spectrum, which revealed the presence of an absorption band centered at about 5 eV (248 nm) in certain types of low water content silica. The presence of absorption bands and the effect of 248 nm excimer laser irradiation are presented and discussed for these latter samples as well as silicas of moderate to high water content. Investigations were carried out in the fiber form, which yielded results in conformity with the conclusions derived from studies on bulk silica, with respect to issues relating to the oxygen stoichiometry, water content and laser radiation effects. In addition, the draw induced 630 nm absorption band was also observed in certain types of fibers and results pertaining to this aspect are also presented. For the first time, spatially variant emissions generated by UV laser radiation in fluorine doped silica clad fibers with pure silica cores were reported through this work. Spatial and spectral correlations are made and the defects contributing to the various emissions are identified for each fiber type. Longitudinal effects observed in fibers with oxygen deficient cores are also presented and discussed. High sensitivity X-band electron spin resonance (ESR) experiments were carried out on different types of fibers, to probe into the origins of the spatially variant luminescence recorded in the laser irradiation studies. The fibers were successively etched down to lower diameters and ESR measurements were carried out at each step at low temperatures (typically at 110 K). By this cross-sectional profiling of the ESR defect signals it was established that, there is a nonuniform distribution of defects in the optical fibers. These results confirm the inferences from the spatially variant luminescence observed in the laser irradiation experiments. The correlations between the ESR experimental results and the spatially variant luminescent patterns obtained in laser irradiation studies are discussed.