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. To date, there has been no way to precisely measure laser power during the manufacturing process in real time; for example, while lasers are cutting or melting objects. Without this information, some manufacturers may have to spend more time and money assessing whether their parts meet manufacturing specifications after production.
A new laser power sensor — a chip-sized “smart mirror” — was developed for lasers of hundreds of watts — the range typically used for manufacturing processes. The radiation-pressure power meter could 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.
Conventional techniques for gauging laser power require an apparatus that absorbs all the energy from the beam as heat. Measuring temperature change allows researchers to calculate the laser's power. But 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. 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 (kW) 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 new device works essentially as a capacitor, measuring 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 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, or 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. The new sensor is 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.
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. 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.