A first-ever comparison between National Metrology Institutes in the kW-power regime

NIST has continuously positioned itself at the forefront of high-power laser calibrations for over three decades [1]. To better meet needs of the manufacturing sector, the Sources and Detectors group acquired capability to provide laser instrument calibrations up to 10 kW in the infrared. This placed NIST at the top of the few National Metrology Institutes (NMIs) with capability to calibrate in the kW range. With this best-in-class capability, the Sources and Detectors group expanded the range of support routinely provided to laser power meter manufacturers, industrial welding/cutting applications and metal additive manufacturing markets.

Comparison against other NMIs is key to ensuring the quality of these calibration services. The typical method for such comparisons is use of a traveling transfer standard, which often yields higher uncertainty than primary standards which are delicate instruments, difficult to transport and re-deploy.

To enable such comparisons, the Sources and Detectors group at NIST developed the Radiation Pressure Power Meter (RPPM), a portable primary standard, which has allowed for significantly more efficient measurements with uncertainty metrics competitive with those legacy primary standards. The RPPM is a photon-momentum based device [2] that allows for measurement of high-power laser light (without absorption) by means of a commercial scale and high-reflectivity mirror. The RPPM is portable and thus can be taken to any laser site to measure different high-power laser sources. For the first time, in 2022 NIST scientists traveled with the RPPM to the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany to measure against another high-power standard, the Electronically Calibrated Power Monitor (EC-PM). The laser light passed through the RPPM and was incident on the EC-PM for a direct and simultaneous comparison. This marked the first comparison between NMIs in the kilowatt power range and the instruments demonstrated agreement over the measured power range [3]

In late 2024, PTB researchers traveled to NIST Boulder with an EC-PM to compare against the RPPM at the multi-kilowatt-level NIST facilities. Additionally, the EC-PM was directly compared with the High Amplification Laser-pressure Optic (HALO), which comes with a much smaller expanded uncertainty [4]. The HALO is also a non-absorptive device which utilizes multiple reflections from high-reflectivity mirrors to amplify the radiation pressure signal. The force detection method for the HALO is an Electrostatic Force Balance (EFB) developed by the Mass and Force Group in NIST Gaithersburg [5]. Analysis of this second comparison is ongoing.

The completion of these comparisons demonstrates both the portability and accuracy that are possible with state-of-the-art kilowatt level laser radiometry.

[1] J. A. Hadler, C. L. Cromer, and J. H. Lehman, “NIST Measurement Services: cw Laser Power and Energy Calibrations at NIST,” NIST Special Publication, vol. 250-75, (2007).

[2] P. Williams, J. Hadler, F. Maring, R. Lee, K. Rogers, B. Simonds, M. Spidell, M. Stephens, A. Feldman, and J. Lehman, “Portable, high-accuracy, non-absorbing laser power measurement at kilowatt levels by means of radiation pressure,” Optics Express, vol. 25, no. 4, pp. 4382-4392, (2017).

[3] K. Rogers, P. Williams, M. Pastuschek, H. Lecher, S. Kuck, M. Lopez, and J. Lehman, “Multi-kilowatt cw laser power measurement comparison between national standards,” Metrologia, vol. 61, 025006, (2024).

[4] A. B. Artusio-Glimpse, K. A. Rogers, P. A. Williams, and J. H. Lehman, “High Amplification Laser-Pressure Optic Enables Ultra-Low Uncertainty Measurements of Optical Laser Power at Kilowatt Levels,” Metrologia, vol. 58, 055010 (2021).

[5] G. A. Shaw, and J. Stirling, “Measurement of Submilligram Masses Using Electrostatic Force,” IEEE Transactions on Instrumentation and Measurement, vol. 68, pp. 2015-2020, (2019).