Axial Force Radiometer For Primary Standard Laser Power Measurements Using Photon Momentum

We have demonstrated operation of a new photon momentum radiometer for measuring laser power levels above 1 kW by use of radiation pressure. The 'axial' design allows the input and output laser beams to remain collinear and the force sensing can be carried out with any analytical balance having a vertical sensing direction and sufficient readability (sensitivity). The design also provides for a parasitic fast photodiode power measurement that is calibrated in situ by the slower radiation pressure-based power measurement. This permits a short-time-constant measurement limited primarily by the photodiode bandwidth, rather than the response time of the force balance. Uncertainty contributions yield a relative expanded uncertainty of 2.1% for (1-2) kW and 1.2% above 2 kW (for laboratory environmental conditions). We perform a direct comparison between this new power meter and the existing primary standard radiation pressure power meter. We find the two techniques agree with an average inequivalence of 0.12% for continuous wave (CW) optical powers from (1-10.8) kW. This disagreement is well-explained by the quadrature-summed uncertainties of the two techniques.