To address this need, researchers from the National Institute of Standards and Technology (NIST) have been developing a laser power sensor that could be built into manufacturing devices for real-time measurements.
The device works in a similar way to a previous sensor made by the team, which uses radiation pressure, or the force that light exerts on an object. But unlike their older device - a shoebox-sized "Radiation Pressure Power Meter (RPPM) for ultrahigh-power lasers of thousands of watts – the chip-sized "smart mirror" is designed for lasers of hundreds of watts.
"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 kinds of manufacturing processes that could potentially use this new technology include everything from airplanes and automobiles, to cellphones and medical devices. The smart mirror could also be integrated into machines employed in additive manufacturing.
"This would put the high accuracy of NIST power measurements directly in the hands of operators, providing standardised 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.
The NIST team's previous RPPM, for multi-kW beams, works by shining the laser onto essentially a laboratory weighing scale, which depresses as the light hits it. But that device is too big to be integrated into welding heads or 3D printers. Researchers also wanted a system that would be more sensitive to the significantly smaller forces used for everyday manufacturing processes.
Instead of employing a laboratory balance, the smart mirror works essentially as a capacitor. The sensor measures changes in capacitance between two charged plates, each about the size of a half dollar. 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, its ability to store electric charge. The higher the laser power, the greater the force on the top plate.
Laser light in the range 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. As such, the NIST researchers made their sensor insensitive to vibration.
"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."
With this technique in place, the sensor can make precise, real-time power measurements for lasers of hundreds of watts, with a background noise level of just 2.5 watts.
The prototype sensor has been tested at a laser power of 250 watts. With further work, that range will likely extend to about 1kW on the high end and below 1watt on the low end. Lehman and colleagues are also working to improve the sensitivity and stability of the device.
