Physics colloquium: “Metrology for High Power Laser Manufacturing”

Brian Simonds

NIST

Abstract: Continuous-wave high-power lasers have evolved from bulky, inefficient tools with only niche applications to reliable photon appliances that have rapidly been adopted by industry. Accordingly, the metrology of these laser systems has also advanced. The Sources and Detectors group at the National Institute of Standards and Technology (NIST) develops and maintains traceable, primary standards for high-power lasers from 1 watt to over 100 kilowatts. In this talk, I will give an overview of high-power laser metrology at NIST as well as how this metrology is being specifically applied to laser manufacturing. Historically, high-power laser detectors have been thermal. As an alternative, we have recently developed radiation pressure detection schemes. A radiation pressure-based sensor measures laser power by using a precision balance to measure the force imparted by laser light incident to a mirror. As light no longer needs to be absorbed, very large laser powers can be measured with a much smaller footprint detector and in significantly less time. This approach has led to the lowest uncertainty measurement of a 10-kW laser (0.26 %) by using a multiple reflection approach known as High Amplification Laser-pressure Optic, or HALO. In addition to improving absolute laser power metrology, the influx of lasers in manufacturing for cutting, welding, and additive manufacturing has led to the development of application specific metrology techniques. Precision laser manufacturing like metal laser powder bed fusion additive manufacturing (LPBF-AM) necessitates tight processing windows and accurate knowledge of all process parameters, including laser power. For this reason, we are surveying a sample of commercial LPBF-AM systems across the United States to determine the accuracy with which they deliver laser power. This work is ongoing but has already shown that the discrepancy in laser power delivery is limited by the uncertainty of the laser power meter used for calibration, typically 4 % to 5 %. In addition to knowing the laser power delivered, process developers also want to know how much of this light energy is being absorbed by the material. We have developed an in situ, time-dependent approach for measuring laser power absorption. This technique has been applied to metal solids and powders and has been combined with other in operando techniques such as high-speed synchrotron X-ray imaging and inline coherent imaging to reveal underlying mechanisms affecting laser power absorption. Data from these experiments have recently been used for the NIST Additive Manufacturing Benchmark Challenge where simulation experts competed to blindly model our experimental results. Although some participants performed very well, the challenge results illustrated several areas for improvement from both modeler and experimentalist.

Biographical Sketch – Brian is a physicist in the Applied Physics Division of the National Institute of Standards and Technology in Boulder, CO. His expertise is in high-power laser metrology for absolute radiometry and industrial laser processes. He holds a bachelor’s degree in physics from Illinois Wesleyan University and a PhD in applied physics from the Colorado School of Mines. Outside of work, Brian enjoys rock climbing, ski mountaineering, and just being outside.

All lectures in CoorsTek 140 unless otherwise noted