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NIST refines laser power meter for industrial apps

19 Sep 2018

Chip-sized "smart mirror" technology could help manufacturers measure laser power from 1W to several 100W in real time.

To effectively control increasing number and diversity of industrial tasks now performed by lasers, manufacturers must ensure that their lasers fire at the correct power.

But to date, there has been no way to precisely measure laser output during the often high-powered manufacturing process in real time – while lasers are cutting or melting objects, for example. Without this precise information, some manufacturers are forced to spend more time and money assessing whether their parts meet manufacturing specifications after production.

Researchers from the US National Institute of Standards and Technology (NIST) have been developing a compact laser power sensor that could be integrated into manufacturing devices for real-time measurements. The results from the latest NIST prototype are reported in IEEE Sensors.

The new device works in a similar way to a previous sensor (reported in May 2018) developed by the same team, which uses radiation pressure, or the force that light exerts on an object.

Now chip-sized

But unlike their older device – a shoebox-sized Radiation Pressure Power Meter for ultrahigh-power lasers outputting thousands of watts – the new, chip-sized smart mirror is designed for lasers delivering hundreds of watts; the range typically used for manufacturing. “It’s still a radiation-pressure power meter, but it’s much smaller and much faster, with 250 times the measurement speed of the larger sensor,” said NIST’s John Lehman. “Also, the smart mirror is also about 40 times more sensitive than the earlier RPPM.”

The kinds of manufacturing processes that could potentially use this new technology include those for airplanes and automobiles through to cellphones and medical devices. The smart mirror could also be integrated into machines employed in additive manufacturing, which often uses a laser to melt the materials that form the desired objects. The NIST researchers say the new, tiny meters could one day be fitted “in every additive manufacturing machine and in every laser weld head”.

Dr Alexandra Artusio-Glimpse, a scientist at NIST in Boulder, Colorado, and the Smart Mirror group’s primary author, commented, “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. This would ensure that manufactured objects meet engineering specifications.”

Radiation pressure

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 problem with this traditional method is that if the measurement requires absorbing all the energy from the laser beam, then manufacturers cannot 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. For example, 1kW laser beam has a small but noticeable force — equivalent to 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 use the light that bounces off the surface directly for manufacturing work.

Device development

The NIST team’s previous RPPM, for multi-kW beams, works by shining the laser onto essentially a laboratory weighing scale, which is depressed as the light hits it. But that device was too big to be integrated into welding heads or 3D printers. Researchers also said that they wanted a system that would be more sensitive to the significantly smaller forces used for everyday manufacturing processes. So 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 30mm in diameter.

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.

Therefore the NIST researchers made their new 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 is physically moved or vibrated, both plates move together,” said Lehman. “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.

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 1W on the low end. Lehman and colleagues are also working to improve the sensitivity and stability of the device.

The video below shows a demonstration of the basic working principles of the smart mirror – a chip-sized device designed to measure the power of a laser in real time:

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