A story about using photon momentum to measure laser power, and the Rocky Mountains

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I had the opportunity to work at National Institute of Standards and Technology (NIST), USA, studying photon momentum based optical power sensing for high-power lasers. A measurement of radiation pressure caused by the transfer of momentum from photons to matter offers the possibility for a power measurement traceable to the kilogram and to the Planck's constant through the redefined SI.

Optical power is typically measured by either observing the heating effect of absorbed photons, or by converting photons into electrons using quantum detectors, such as conventional photodiodes. It is often forgotten that photons also have momentum, as it is never observed in our daily lives. In fact, the miniscule force exerted by a flux of photons is difficult to measure with any means – unless you have an enormously powerful light source.

Very high-power lasers, ranging from hundreds of watts to kilowatts, are used in applications like laser welding, cutting, and metal additive manufacturing. At these power levels, absolute power measurements become difficult, as one must either absorb or attenuate the high power. However, the high power enables the use of photon momentum for optical power sensing. National Institute of Standards and Technology (NIST) pioneered this approach about a decade ago. NIST is a physical sciences laboratory and a part of the U.S. Department of Commerce, with a mission to promote U.S. innovation and industrial competitiveness.

I had the privileged to work at NIST, in Boulder, CO, as guest researcher under the Fulbright Foundation’s post-doctoral research grant. I got to work on the High Amplification Laser pressure Optic (HALO), which is the state-of-the-art primary standard for kilo-watt range lasers. The HALO has been demonstrated to measure a 10-kW laser power with a total expanded uncertainty of 0.26 %, while the ultimate target is 0.01 %. It should be noted that the HALO - and absolute radiation pressure measurements in general - are traceable to the SI kilogram.

One of my main tasks during my visits were to characterize the properties of the very power laser source, such as stability and spectral distribution. To accomplish this, various methods for attenuating the high laser power by many orders of magnitude where examined. These high-power attenuators are also needed when systems like HALO are compared to conventional low-power detectors. This requires the measurement of absolute attenuation, which was another of my major tasks.

Finally, I would like to point out that my visit was not only about learning and understanding new science, but also to experience the great work atmosphere and awesome colleagues. In addition to work, I was fortunate to make many new friends during my stay and enjoy the unique atmosphere of Boulder. All in all, it was a truly unique experience. If you ever have such an opportunity to work abroad, I encourage you to seize it.

High Amplification Laser pressure Optic (HALO)
The High Amplification Laser pressure Optic (HALO). Thirteen small mirrors redirect the beam towards the sensing mirror at the center, multiplying the force and reducing the measurement uncertainty. Credit: Aly Artusio-Glimpse / NIST.
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Timo Dönsberg
Timo Dönsberg