Measuring the optical output of a laser beam is traditionally done via absorption of the beam by an optical sensor or, for higher powers, a calorimeter. For very high-power laser beams (tens of kilowatts or more), this type of measurement is difficult, as the high power can damage the laser power meter unless heat transfer from the meter is handled very competently. Even so, commercial calorimetric laser power meters exist that can handle laser powers up to 100 kW.
A different approach, in which a laser beam is bounced off a mirror and the radiation pressure measured, actually gets more, rather than less, practical as the beam power increases. This approach has been pursued at the National Institute of Standards and Technology (NIST; Boulder, CO) and Scientech (Boulder, CO), which introduced a product that it says can measure beam powers of up to 500 kW. (Note: although Scientech worked with NIST on the technology, NIST does not endorse the company’s product, or indeed any commercial product.)
One advantage of this technique, according to NIST: a radiation-pressure power meter is based on the measurement of force, which is directly traceable to the kilogram, which can be easily measured to a better relative accuracy than the equivalent laser power.
Now, NIST has provided data on its radiation-pressure power meter (RPPM) in a manufacturing test environment for beam powers in the range of 20 kW, comparing these results to those gathered at NIST’s own research facility (see figure).1 They found a difference of about 3% in the results, which NIST says is an indication of the effect that uncontrolled or unknown conditions in a manufacturing environment can have on the laser power measurement results.
On-site and lab measurements
The experiment consists of manufacturing-site measurements taken at the same time with the RPPM and a calorimeter termed the device under test (DUT); the DUT was separately compared to the K-series calorimeter that NIST has in its own laboratory. The manufacturing-site testing was done at the former Edison Welding Institute, now called EWI (Columbus, OH), using EWI’s 20 kW ytterbium-doped fiber laser, which emits at a 1.07-μm wavelength. (The RPPM can be used to measure other wavelengths as well, as long as all optics are designed with the proper wavelength in mind.)
Laser power was measured at increments from 1 to 20 kW, with the 60-s-long measurements processed to remove thermal drift and the average of the RPPM-measured laser power calculated over the last 20 s of the time span. Noise fluctuations were also characterized.
Any discrepancy between the RPPM and the K-series power meters was determined to be <1%, so the 3% discrepancy between the test-site and lab results is best explained by measurement instabilities, according to NIST. The prime suspect was differences in cooling conditions for the DUT between the test site and NIST’s lab. For example, during the onsite measurements, the DUT’s cooling water rose in temperature by up to 5°C/min. While a thermopile like the one in the DUT is not affected by the static temperature of its environment, dynamic temperature changes can cause errors.
Such results demonstrate the utility of the RPPM over the DUT in the course of measurements that a laser-system user might be taking in an unstable industrial environment. The NIST scientists are now working on an RPPM based on a silicon micromachined capacitive force scale suitable for permanent installation and use at a manufacturing site.2
REFERENCES
1. P. A. Williams et al., Appl. Opt. (2017); https://doi.org/10.1364/ao.56.009596.
2. I. Ryger et al., "Silicon micromachined capacitive force scale: the way to improved radiation pressure sensing," in 13th International Conference on New Developments and Applications in Optical Radiometry (2017).
