Mechanism of excimer-laser-induced absorption in fused silica fibers

Transmission characteristics and luminescence spectra of fused silica fibers under delivery of KrF (wavelength 248 nm) and XeCl (308 nm) laser radiation was studied. Tested fibers were produced by various technologies based on PCVD and PMCVD methods. Multiple-shot transmission at both KrF and XeCl laser wavelengths was shown to be essentially dependent on the method of production and chemical composition of silica core. In particular, in fibers with high OH content the decrease of transmission during irradiation by XeCl laser is almost fully recoverable and restores after stop of irradiation in about 5 minutes. The maximum value of transmission fall depends on laser repetition rate. When irradiating by KrF laser either stable and unstable laser-induced absorption was observed. Additional losses in 'dry' silica fibers with low OH content are stable at both 248 nm and 308 nm wavelengths. The level of these losses can be significantly reduced by additional treatment of preform during production procedure. In order to determine the type of defects responsible for laser-induced absorption in fibers the study of luminescence spectra was carried out. It was shown that the only band varying during laser irradiation is that centered at 650 nm. Basing on the kinetics measurements which showed that the lifetime of this band is about 15 um luminescence was ascribed to non-bridging oxygen hole centers (NBOHC). The data of luminescence study as well as the results of measurements of induced absorption spectra enable to assume that NBHOC is the main type of defects which influences the multiple-shot transmission of fibers at 248 and 308 nm. Based on the information about the behavior of transmission and luminescence characteristics of fibers during KrF and XeCl lasers irradiation, few mechanisms of NBOHC generation from various precursors were suggested. It was shown that in spite of widespreading convinction hydroxyl groups are not the main precursors of non-bridging oxygen in OH-rich fibers as the value of transmission fall does not correlate with OH content. Moreover, the situation with unstable induced losses it quite opposite: the decrease of transmission as well as initial rate of this decrease diminish with the rise of OH content. The analysis of probable mechanisms of UV-induced losses enables a search for the way of improved fiber production technology to provide fibers characterized by high transmission resistance to multiple-shot excimer laser irradiation.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.