Gauging Multi-Kilowatt Laser Power with the Pressure of Light

A multi-kilowatt laser beam can cut through steel and melt bricks into glass. Many industries use high-power lasers like these to precisely cut and weld metals for a range of applications, from manufacturing cars and cell phones to ships, planes, and medical devices. But industry professionals need tools to tell them exactly how much power their laser is producing at a given time.

“When you’re welding with a laser, the big thing that you care about is how much light you are using,” says Paul Williams of NIST’s Physical Measurement Laboratory (PML) in Boulder, Colo. “In some applications, to get a quality weld, you need to know the laser power to within plus or minus one percent.”

For years, NIST PML has been developing a method to gauge the power of high-kilowatt lasers by measuring the pressure of their light.* Now Scientech, a Boulder-based company that co-developed the technique with NIST, has commercialized the device, making it available for the first time to manufacturing companies such as those in the automotive and aerospace industries, as well as researchers in the U.S. Department of Defense. The new laser power meter is capable of measuring laser powers as high as 500-kilowatt (kW, a thousand watts).

Gauging Laser Power with the Pressure of Light

“Before now, high-power laser measurements relied on sensors that are inherently big and slow,” says John Lehman, leader of NIST PML’s Sources and Detectors Group. “Furthermore, the calibration of such instrumentation has been expensive and rare.”

Traditional techniques for gauging a laser’s power require using an apparatus to absorb all the energy from the beam as heat; measuring the temperature change allows researchers to calculate the laser’s power. But with very high laser powers, this conventional method becomes tricky to implement. If the instrument is not properly cooled, it could be destroyed by the laser beam that it is supposed to measure.

Instead of acting as a heat sink, the new device works by measuring the pressure of the laser light itself.

Light has no mass, but it does have momentum, which allows it to produce a small amount of force, or “radiation pressure,” when it strikes an object. For example, a 100-kilowatt laser beam has a small but noticeable force – the equivalent weight of about 68 milligrams (mg, thousandths of a gram), approximately the weight of two staples.

To measure this tiny force, the NIST-Scientech team took a commercially available laboratory balance and gave it a mirrored surface that reflects 99.9% of the light that hits it. Together, the balance and its housing are about the size of a shoe box. When the beam of a multi-kilowatt laser reflects off the mirror, the pressure it imparts is recorded by the balance. The force measurement is then converted to a power measurement.

Unlike the traditional heat sink power meters, the new device makes its measurements in real time: After the light hits the balance, it can be used directly for applications such as cutting and welding.

The NIST team is continuing to work on the technology. Recently, it completed a full analysis of the measurement process, which allowed the researchers to lower the uncertainties for radiation pressure measurements of multi-kilowatt lasers between 1 kW and 10 kW. Lower uncertainties mean manufacturers can have higher confidence in the accuracy of the equipment.

Lehman says the NIST PML team also has plans to establish a standard version of the new instrument in 2017, to be available as a NIST standard reference instrument.

-- Reported and written by Jennifer Lauren Lee

* “Force to be Reckoned With: NIST Measures Laser Power with Portable Scale” (October 22, 2013).