defocused laser beam is directed onto the surface of the thermal power meter, which is coated by a substance that is highly efficient at absorbing light at the wavelength of the laser beam. The meter then operates by absorbing all the laser light and measuring the resulting temperature increase. This means the laser power is being used in the measurement and is not available for the welding process. We are approaching the measurement of high laser powers in a different way. Instead of absorbing all the laser light, we prefer to reflect as much as possible from a mirror. And, rather than measuring heat, we measure the very small force of the light as it pushes on the mirror. It may be surprising that light itself can push on anything, but as will be explained, this idea of “radiation pressure” has been proven repeatedly, and is used in other scientific fields. The idea of measuring highpower lasers without appreciable heating then opens several inviting possibilities. With a radiation pressure power meter, the laser power could potentially be measured to within 1–2% accuracy during the welding operation and would allow for quality monitoring and could simplify the setup process for a weld. We have done preliminary testing of this idea by welding with an ytterbium fiber laser at up to 5 kW optical power while simultaneously and accurately measuring the laser’s output power using a prototype radiation pressure power meter. To our knowledge, this is the first such measurement of its kind. Exploiting the Force of Light to Measure Laser Welding Power Since the 1700s, scientists have predicted that light has a momentum associated with it. This means that like any other travelling object, when light hits something, it pushes on it, but it was not until the early 20th century that this tiny push was actually measured. Of course, the push (or force) is very small. As a simple analogy, picture a tennis ball thrown toward a nonmoving racket — Fig. 1. The ball will bounce away, while the racket is pushed in the opposite direction. The force experienced by the racket will be in proportion to the mass of the ten- Fig. 1 — Just as a tennis ball bouncing off a racket pushes the racket back, light also pushes with a measurable force when it reflects off a mirror. Fig. 2 — Diagram (overhead view) of radiation pressure power meter (RPM) in place for laser welding of stainless steel pipe. MARCH 2016 / WELDING JOURNAL 31
Welding Journal | March 2016
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