Introducing the Smart Mirror

New chip-sized device could help manufacturers measure laser power in real time.


Lasers assume jobs in many assembling forms, from welding auto parts to making engine segments with 3D printers. To control these tasks, makers must guarantee that their lasers fire at the right power.

Until now, there is no way to precisely measure laser power during the manufacturing process in real time. Some industries spend more time and money assessing whether their parts meet manufacturing specifications after production.

To address this need, analysts from the National Institute of Standards and Technology (NIST) have been building up a laser power sensor that could be built with assembling gadgets for real-time estimations.

The new device 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.

NIST’s John Lehman said, “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. The smart mirror is also about 40 times more sensitive than the RPPM.”

Scientists noted that the smart mirror could likewise be incorporated into machines utilized in additive manufacturing, a kind of 3D printing that constructs a protest layer by layer, frequently utilizing a laser to soften the materials that shape the object.

The NIST team’s past RPPM, for multi-kW beams, works by sparkling the laser onto basically a research center weighing scale, which discourages as the light hits it. Yet, that gadget is too huge to be coordinated into welding heads or 3D printers. Scientists likewise needed a framework that would be more delicate to the fundamentally smaller forces utilized for everyday manufacturing processes.

Rather than utilizing a laboratory balance, the new “smart mirror” works basically as a capacitor, a gadget that stores electric charge. The sensor estimates changes in capacitance between two charged plates, each about the span of a half dollar.

The top plate is covered with a highly reflective mirror called a distributed Bragg reflector, which utilizes exchanging layers of silicon and silicon dioxide.** Laser light hitting the top plate gives a force that makes that plate move nearer to the bottom plate, which changes the capacitance, its capacity to store electric charge. The higher the laser power, the more prominent the force on the top plate.

Laser light in the range utilized for assembling—in the many watts go—isn’t ground-breaking enough to move the plate exceptionally far. That implies that any physical vibrations in the room could make that best plate move in a way that wipes out the minor flag it’s intended to gauge.

Scientists 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.

Lehman said, “If the device gets physically moved or vibrated, both plates move together. So the net force is strictly the radiation pressure, rather than any ambient influences.”

“I’m just surprised how well it works. I’m really excited about it. If you told me two years ago that we’d do this, I’d say ‘no way!’”

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.

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.

Scientists have presented their paper in an October 2018 issue of IEEE Sensors.


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