Improving reliability and reducing damage in high-energy solid-state UV lasers
Abstract
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.