High Power Beam Characterization with Laser Diagnostics

High power solid state fiber and disk lasers with output power levels in excess of 10 kW are introducing new capabilities and becoming increasingly popular for welding. The higher powers afforded by these lasers combine faster processing speeds, deeper weld penetrations, and lower levels of work piece distortion, which provide greatly improved process efficiencies. Both transmissive optics, which are fabricated using glass or ceramic materials, and reflective optics systems, which are composed of water-cooled metallic mirrors, can be used to direct and focus the laser light. When using transmissive optics at high laser powers and prolonged periods of operation, though, changes in the beam propagation parameters, such as the focal length and beam diameter, have been observed. Without continuous control of these beam parameters, the quality of the resulting product will be negatively impacted. 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 systems studied here, 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. The impact of the anti-reflective coating damage in the collimator on the beam properties was also investigated.  On the other hand, the performance of the reflective optics was not impacted by either increases in power or time, and a stable beam was produced at powers up to 12 kW during prolonged laser operation.