Researchers from the National Institute of Standards and Technology (NIST) have developed a laser power sensor that may be built into manufacturing devices to provide fast, real-time measuring. The new chip-sized device is an improvement on a past NIST laser that relied on radiation pressure.
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: Jennifer Lauren Lee/NIST
"It's still a radiation-pressure power meter, but it's much smaller and much faster," said NIST's John Lehman. Lehman is referring to the fact that the new sensor is 250 times faster than the previous one.
The new device could be used in a variety of products and industries including airplanes, cars, cell phones and medical devices. The smart mirror could even be used in additive manufacturing machines to speed up production.
"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 NIST's Alexandra B. Artusio-Glimpse.
Past devices absorb all their energy from laser beams as heat. But if the beam's energy is absorbed, it cannot be measured while the laser is in use.
Radiation pressure is a solution to this problem: it has momentum but no mass, so it can produce a force when in contact with an object. Therefore, by measuring how much an object's surface moves after shining a laser beam on it, researchers can measure the laser's force as well as reuse the reflected light for manufacturing work.
The device's "smart mirror" works as a capacitor, storing charge and measuring between two charged plates. The top plate is coated with a reflective mirror called a distributed Bragg reflector. The laser light hits the top plate of the device and moves the top plate to the bottom plate, changing the capacitance. Higher laser power results in higher force.
Manufacturing light is not powerful enough to move the plate very far, and vibrations in the room could cause the plate to move, resulting in an inaccurate reading.
NIST researchers solved this problem by making their sensor insensitive to vibration. The capacitor's plates are attached to the device using springs, so ambient influences 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."
A paper on the new device was published in IEEE Sensors.
