We describe a non-traditional optical power meter which measures radiation pressure to accurately determine a laser's optical power output. This approach traces its calibration of the optical watt to the kilogram. Our power meter is designed for high-accuracy and portability with the capability of multi-kilowatt measurements whose upper power limit is constrained only by the mirror quality. We provide detailed uncertainty evaluation and validate experimentally an average expanded relative uncertainty of 0.016 from 1 kW to 10 kW. Radiation pressure as a power measurement tool is unique to the extent that it does not rely on absorption of the light to produce a high-accuracy result. This permits fast measurements, simplifies power scalability, and allows high-accuracy measurements to be made during use of the laser for other applications.

This work describes a metrological comparison between nanonewton force references derived from an electrostatic force balance and photon pressure from laser optical power in the 1-Watt range. An Electrostatic Force Balance is used to measure photon pressure force in the 10 nanonewton range from the reflection of a laser from a low (approximately 10 -5 ) loss III-V semiconductor distributed Bragg reflector mirror while the power of the reflected beam was simultaneously monitored with a traceable thermopile detector. This work demonstrates a method to link mass, force and laser power within the International System of Units with explicit treatment of absorption, diffuse reflection, and a detailed uncertainty analysis. Additionally, it demonstrates a viable method to scale this force continuously using a pulsed laser technique while maintaining the constant thermal load necessary for precision measurement of nanonewton forces with a mechanical balance.

We introduce a micromachined force scale for laser power measurement by means of radiation pressure sensing. With this technique, the measured laser light is not absorbed and can be utilized while being measured. We employ silicon micromachining technology to construct a miniature force scale, opening the potential to its use for fast in-line laser process monitoring. Here we describe the mechanical sensing principle and conversion to an electrical signal. We further outline an electrostatic force substitution process for nulling of the radiation pressure force on the sensor mirror. Finally, we look at the performance of a proof-of-concept device in open-loop operation (without the nulling electrostatic force) subjected to a modulated laser at 250 W and find its response time is less than 20 ms with noise floor dominated by electronics at 2.5 W/√Hz.

Laser power metrology at the National Institute of Standards and Technology (NIST) ranges 20 orders of magnitude from photon-counting (10 3 photons/s) to 100 kW (10 23 photons/s at a wavelength of 1070 nm). As a part of routine practices, we perform internal (unpublished) comparisons between our various power meters to verify correct operation. Here we use the results of these intercomparisons to demonstrate an unbroken chain tracing each power meter's calibration factor to the NIST cryogenic radiometer (our lowest uncertainty standard, whose SI traceability is established through the volt and ohm units). This yields the expected result that all the NIST primary standard measurement techniques agree with each other to within their measurement uncertainty. Then, these intercomparison results are re-mapped to describe the agreement of the various techniques with our radiation-pressure-based power measurement approach, whose SI traceability is established through the kilogram. Again, agreement is demonstrated to within the measurement uncertainty. This agreement is reassuring because the measurements are compared with two entirely different traceability paths and show expected agreement in each case. The ramifications of this agreement as well as potential means to improve on it are discussed.We demonstrate SI measurement traceability of our single-photon power measurement through the kilogram with less than 3 % relative expanded uncertainty (obtained for a coverage factor k=2 defining an interval having a level of confidence of approximately 95 %). I. (cesium hyperfine splitting frequency), and c (the speed of light in vacuum). *At the time of publication, the NextGenC is not yet fully validated as a primary standard but is included for completeness.

Calibrated optical choppers are used in high-power laser calibration services at the National Institute of Standards and Technology for beam power reduction due to their advantages in performance and safety over wedges and semi-transparent materials. While the design, operation, and calibration of such choppers is generally straightforward, fabrication tolerances and edge geometry must be taken into account for low transmission (<5%) choppers now required to accommodate the increased powers (upwards of 10 kW) available for the calibration service. The slit edge is presented to the beam at non-zero angles of incidence and for edge thicknesses on order of 10% of the slit width, the relative error from design transmission can exceed 2% (depending on beam diameter and angle of incidence). Both slit edge presentation at non-zero angles of incidence and displacement of the slit edges due to minor variation in fabrication or coating yield significant changes in transmission with radial displacement of the beam over the chopper face. While diffraction effects play a smaller role under typical setup conditions, they become an appreciable factor for detectors having small entrance apertures or located far from the chopper wheel. Here we demonstrate a close agreement between the calculated and measured transmission ratio for two optical choppers when considering these three effects.