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For a number of years we have been investigating laser-induced damage mechanisms that can occur during the transmission of Q-switched, Nd/YAG laser pulses through fused silica fibers. We have found that fiber end-face characteristics, laser characteristics, and aspects of the laser-to-fiber injection typically determine dominant damage mechanisms. However, an additional damage process has been observed occasionally at internal sites where fibers were experiencing significant local stresses due to fixturing or to bends in the fiber path. A transmission reduction prior to damage was typically not measureable at these sites. Damage would not always occur during initial testing, but sometimes occurred later in time at laser levels that previously had been transmitted without damage. In these cases the time at stress appeared to be more important than the number of transmitted shots prior to damage. A possible relation between internal damage thresholds at stressed sites and the total time under stress is suggested by the fact that silica fibers experience static fatigue processes. These processes involve the slow growth of local defects under tensile stress at rates that depend upon environmental conditions. Defects reaching sufficient size and having appropriate location could be sites for reduced laser-induced damage thresholds. This possibility could have important implications for high-power fiber transmission systems that must satisfy extended lifetime requirements. The needs of the telecommunications industry have motivated extensive studies into initial fiber defect characteristics and their likely growth mechanisms. The present work used the understanding developed in these studies to guide a preliminary experimental investigation into the possibility that static fatigue processes can affect damage thresholds. The experiments used a laser injection and fiber routing configuration that produced significantly elevated fluences within fiber core regions under tensile stress. In one set of experiments, internal damage thresholds were determined in available fiber samples that had been assembled in stress-imposing fixtures for periods up to 24 months. A decline in mean thresholds with time was observed, although measured values showed significant scatter. In order to establish initial strength and fatigue properties for these fibers, a number of additional samples were used to generate time-to-failure data at various stress levels. Based on these results, other fiber samples were subjected to conditions that greatly accelerated fatigue processes. Internal damage thresholds were then measured in these fibers and compared to thresholds measured in fresh fibers. Conclusive comparisons were frustrated by sample-to-sample and lot-to-lot variations in fiber defects.
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