Chip-sized ‘smart-mirror’ could help manufacturers measure laser power in real time

Written by: Tom Austin-Morgan | Published:

Lasers are used in many manufacturing processes, from welding car parts to crafting engine components with 3D printers. To control these tasks, manufacturers must ensure that their lasers fire at the correct power.

To date, there has been no way to precisely measure laser power during the manufacturing process in real time, while lasers are cutting or melting objects, for example. Without this information, some manufacturers may have to spend more time and money assessing whether their parts meet manufacturing specifications after production.

To address this need, researchers from the US National Institute of Standards and Technology (NIST) have developed a laser power sensor that could be built into manufacturing devices for real-time measurements.

The kinds of manufacturing processes that could potentially use this new technology include everything from airplanes and automobiles to smartphones and medical devices. Someday, the researchers say, these tiny meters could be in every additive manufacturing machine and in every laser weld head.

The device 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. It also needed to 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, a device that stores electric charge and measures 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, 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 – in the hundreds of watts range – 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, the NIST researchers made their sensor insensitive to vibration. Both the top and bottom plates are attached to the device by springs. Ambient influences, such as vibrations, 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,” John Lehman, a NIST researcher, 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 2.5W.

“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!’”

Currently, the prototype sensor has been tested at a laser power of 250W. With further work, that range is expected to extend to about 1kW on the high end and below 1W on the low end. The researchers are also working to improve the device’s sensitivity and stability.

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