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Damage studies in high-power fiber transmission systems

For several years we have been investigating laser-induced damage mechanisms encountered when transmitting Q-switched Nd/YAG laser pulses through step-index, multimode, fused silica fibers. Previous studies primarily addressed end-face breakdown and damage processes and how corresponding thresholds could be affected by different preparation techniques. However, we frequently encountered two internal mechanisms for damage that influence test procedures and results. An `entry' mechanism is related to the laser mode structure and to the geometry of laser injection into the fiber entrance face. Internal damage is also observed at sites where a fiber is experiencing significant local stresses, either due to fixturing or to severe bends in the fiber path. The present study continued to address these fundamental issues, and began to address additional concerns that may arise in the design of practical high-power fiber transmission systems. End-face preparation issues were examined through a comparison between purely cleaved faces and mechanically polished faces.