Improving reliability and reducing damage in high-energy solid-state UV lasers
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Developing a commercial high-energy solid-state ultraviolet (UV) laser with good stability and long-term reliability presents a number of engineering challenges. Issues such as dielectric coating damage, aging of the nonlinear frequency-conversion crystals and the sensitivity of crystal phase-matching characteristics to temperature make this an extremely difficult task. The laser of interest in this work was a dual-cavity 355-nm Q-switched laser, based on frequency tripling using LiB3O5 (LBO) crystals and producing an average output power of 80 W. By employing various microscopy, interferometry and surface analysis inspection methodologies, the performance and damage morphology of the laser optics and crystals coated by proprietary techniques from different vendors were investigated and explained. Use of an uncoated LBO crystal was also examined, and the performance of such a crystal with and without accelerated neutral atom beam post polish treatment was investigated. By comparing crystals with different coating designs in different environmental conditions, it was found that crystals purged with nitrogen and having a simpler coating design with fewer coating layers exhibited the best resistance to laser damage. Multi-peaked temperature-dependent behaviour of the LBO crystal tuning curve was studied in depth theoretically and empirically. It was discovered that the shape of the temperature tuning curve resulted from the combined effect of crystal phase matching and birefringence modulated infrared power. New approaches to crystal mounting were exploited to improve the discovered hysteresis behaviour during crystal temperature tuning. After applying an improved mounting method, no hysteresis was found during a five-day test period. At the time of writing, 26 such laser systems are commissioned in the field serving the semiconductor and organic light-emitting diode (OLED) flexible display industry.