Instead of calorimetry for measurement of high-power laser output levels, a contrary approach uses the force of radiation pressure on a highly-reflective mirror.
Engineers often have to measure power, and this power can take many forms: DC or low-frequency, wired or wireless RF, or optical. In addition, the dynamic range of power to be measured spans many decades spanning femtowatts to kilowatts are common, and it’s at the very low or very high extremes that challenges become most daunting for both making the measurement and assessing its accuracy.
In some ways, miniscule optical power levels are the most difficult. The reason is that photons just do not like to be captured or measured, and the process of doing so, by assessing their wavelengths and quantity, and thus power, is difficult as doing almost anything will upset the scenario. (That’s why single-photon detectors are a fascinating subject in the test and measurement world.) As optical-power levels increase from the femtowatt regime into nano, micro, milli, and just-plain watts, the measurement situation eases, of course.
But what happens when that optical power reaches into the kilowatt, and even tens of kilowatt region? Lasers are now used for industrial welding at these power levels (and there are those long-distance laser-based “ray guns” under development, as well). The welding systems need to know the laser power with reasonably good accuracy so they can carefully tailor the thermal profile.
The standard way to measure higher laser power levels is via calorimetry with a calibrated thermal sensor, which transforms the impinging optical power into heat and measures the resultant temperature rise. That rise, in turn, can be modeled and correlated with the power level. It’s accurate but impractical in an ongoing manufacturing process. Also, the thermopile sensor has to be cooled so the intense laser power does not burn up the sensor itself.
That’s why a system developed by NIST in conjunction with Scientech Inc. is an interesting alternative. Instead of converting the laser power to heat and then dissipating it, this system measures the force of the massless photons of the laser – formally known as radiation pressure – as the laser impinges on a highly reflective mirror assembly (Figure 1). This mirror is coated to reflect 99.9% of the incident light, so the self-heating is manageable.
Figure 1 Why dissipate when you can reflect? Rather than measure thermal rise, this approach measures the radiation pressure produced by an incident beam on a highly reflective mirror. (Source: NIST)
The new system offers some interesting attributes:
- It is non-destructive to the laser beam.
- It works in real time and can actually be used as part of a laser-welding system to monitor the power levels during the welding cycle.
- Although it doesn’t work well at very low power levels, it gets better (more accurate) as the laser power and thus the radiation pressure increase.
- It’s simple, at least in principle (these things can never be simple in reality, when you are dealing with kW of power), and has far fewer cooling issues due to dissipation; for a 10-kW beam, dissipation at the mirror is a modest 10 W.
- The parameter of interest is force, which can be measured with high accuracy and precision even at fairly low levels; a 100-kilowatt laser beam produces an equivalent weight of about 330 milligrams.
Standard calorimetry-based laser-power units can measure up to 500 kW (Wow, that’s serious power, especially for a laser beam) and the new radiation-pressure power meter (RPPM) can go to the same levels. It was recently tested and calibrated against the existing calorimetry standard to 20 kW, still a substantial power level, and results were accurate to about 3% (Figure 2).
Figure 2 NIST tests versus the established calorimetry standard showed accuracy of about ±3% over the entire 1 to 20-kW power range. (Source: Laser Focus World)
The present unit is about the size of a large toaster oven, and the next step will be to develop an RPPM based on a silicon MEMS capacitive-force scale, similar to the technology of MEMS-based pressure sensors and accelerometers. This would make the RPPM approach smaller and even more compatible with permanent installation and use at a manufacturing site.
What’s your experience with difficult optical-power measurements? Did you have to develop new approaches and instrumentation? How did you verify the performance?
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.Reference
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