Real-time laser power sensor embeds into mfg equipment
The new device works in a similar way to a previous sensor made by the team, which uses radiation pressure. This shoebox-sized Radiation Pressure Power Meter (RPPM) measures ultrahigh-power lasers of thousands of watts, while the new chip-sized “smart mirror” is designed for lasers of hundreds of watts, the range typically used for manufacturing processes.
“It’s still a radiation-pressure power meter, but it’s much smaller and much faster,” with 250 times the measurement speed of their larger sensor, said NIST’s John Lehman. The smart mirror is also about 40 times more sensitive than the RPPM.
The smart mirror sensor can be used for manufacturing in aerospace and automotive as well as cellphones and medical devices. The smart mirror could also be integrated into machines employed in laser-based additive manufacturing 3D printing. The aim is that the sensors could be in every additive manufacturing machine and in every laser weld head.
“This would put the high accuracy of NIST power measurements directly in the hands of operators, providing standardized quality assurance across laser-based systems and helping to accelerate the process of part qualification,” which ensures that manufactured objects meet engineering specifications, said NIST’s Alexandra B. Artusio-Glimpse.
A 1kW laser beam has a small but noticeable force–about the weight of a grain of sand. By shining a laser beam on a reflective surface, and then measuring how much the surface moves in response to light’s pressure, researchers can both measure the laser’s force (and therefore its power) and also use the light that bounces off the surface directly for manufacturing work.
The RPPM shines the laser onto a laboratory weighing scale, which depresses as the light hits it, but this is too big to be integrated into welding heads or 3D printers. The “smart mirror” works essentially as a capacitor, measuring the changes in capacitance between two charged plates..
The top plate is coated with a highly reflective mirror called a distributed Bragg reflector, which uses alternating layers of silicon and silicon dioxide. Laser light hitting the top plate imparts a force that causes that plate to move closer to the bottom plate, which changes the capacitance. However the kW laser light used for manufacturing is not powerful enough to move the plate very far. That means that any physical vibrations in the room could cause that top plate to move in a way that wipes out the tiny signal it’s designed to measure.
So NIST researchers made their sensor insensitive to vibration by attaching the top and bottom plates to the device by springs. Ambient influences cause both plates to move in tandem, but a force that affects only the top plate causes it to move independently. “If the device gets physically moved or vibrated, both plates move together,” Lehman said. “So the net force is strictly the radiation pressure, rather than any ambient influences.”
This allows the sensor to measure real-time power measurements for lasers of hundreds of watts, with a background noise level of just 2.5 watts.
“I’m just surprised how well it works. I’m really excited about it,” Lehman said. “If you told me two years ago that we’d do this, I’d say ‘no way!'”
Right now, the prototype sensor has been tested at a laser power of 250W. With further work, that range will likely extend to about 1 kW on the high end and below 1W on the low end. Lehman and colleagues are also working to improve the sensitivity and stability of the device.
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