Smart Mirror for a Radiation-Pressure Power Meter

A prototype of the smart mirror. Laser light bounces off the highly reflective surface of a silicon plate, visible in the middle of a thick black ring of plastic. (Source: J. L. Lee, NIST)

Lasers play roles 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. But 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 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 new 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, 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 researcher 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, a type of 3D printing that builds an object layer by layer, often using a laser to melt the materials that form the object. Someday, the researchers say, these tiny meters 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 Alexandra B. Artusio-Glimpse.

Conven­tional techniques for gauging laser power require an apparatus that absorbs all the energy from the beam as heat. Measuring the tempera­ture change allows researchers to calculate the laser’s power. The trouble with this traditional method is that if the measure­ment requires absorbing all the energy from the laser beam, then manufacturers can’t measure the beam while it’s actually being used for something. Radiation pressure solves this problem. Light has no mass, but it does have momentum, which allows it to produce a force when it strikes an object. A 1-kilowatt 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 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 new smart mirror works essen­tially as a capacitor, a device that stores electric charge. 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, 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. So the 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 if someone closes a door in the room or walks past the table, 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.” 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. “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 250 watts. With further work, that range will likely extend to about 1 kW on the high end and below 1 watt on the low end. Lehman and colleagues are also working to improve the sensi­tivity and stabi­lity of the device. (Source: NIST)

Reference: I. Ryger et al.: Micromachined Force Scale for Optical Power Measurement by Radiation Pressure Sensing, IEEE Sensors J. 18, 7941 (2018); DOI: 10.1109/JSEN.2018.2863607

Link: Sources and Detectors Group, National Institute of Standards and Technology NIST, Boulder, USA

Speak Your Mind