Identifying performance differences in transmissive and reflective laser optics using beam diagnostic tools

High-power, solid-state fiber and disk lasers combine faster processing speeds, deeper weld penetrations, and lower levels of workpiece distortion. Both transmissive optics and reflective optics systems are commonly used. When using transmissive optics at high laser powers and prolonged periods of operation, changes in the focal length and beam diameter have been observed, and these changes adversely affect consistency of the processed materials. In this study, the properties of beams delivered using both transmissive and reflective optics systems from the exit of the process fiber through the final focusing optics have been characterized using commercial diagnostic tools. In the transmissive optics system, changes of nearly 8 mm in focal length have been measured with a 500-mm focal optic at 12-kW output power over several minutes of continuous operation. At powers above 4 kW, damage to the antireflective coating on a transmissive collimator resulted in a doubling in the beam diameter at the original focal position when using a 200-mm focal optic and a quadrupling of the beam diameter with a 500-mm focal optic. On the other hand, the performance of the reflective optics was not impacted by either increases in power or time at powers up to 12 kW during prolonged laser operation.