Measuring Laser Power in Real Time

Scientists from the National Institute of Standards and Technology (NIST) have developed a laser power sensor capable of being integrated into manufacturing devices for real-time measurements. This could significantly help manufacturers to ensure that the lasers they use are firing at the correct power during a variety of manufacturing processes.

The device created by the team uses radiation pressure to measure the strength of the lasers. Radiation pressure is the force light exerts on any particular object. The ‘smart mirror‘ is the size of a chip, unlike older sensors that were far bulkier. And while its predecessors could only sense ultrahigh-power lasers, the new device is specially designed for lasers of hundreds of watts. The smart mirror is roughly 40 times more sensitive than previous models, allowing for greater accuracy in the readings as well.

Traditional laser measuring techniques use a device that absorbs the energy from the laser and stores it as heat. Scientists then measure the temperature change and use it to calculate the power of the beam. Unfortunately, this technique does not allow for measurements while the laser is being used. This means manufacturers must wait until a process is totally completed before getting the results.

Radiation pressure offers a more elegant solution. Light may not have mass, but it does have momentum. That momentum produces a force when it hits a surface. That force can be measured — thanks to the new sensors. Scientists can shine a laser onto a reflective surface. From there, they can measure the force of the laser. The force allows them to determine the power level of the laser, and make any needed adjustments. Additionally, users can now take the light that bounces off that surface directly and use it in manufacturing work.

To create the smart mirror, scientists took two charged plates. The top plate was coated with a distributed Bragg reflector. This makes the surface into a highly reflective material. When the laser hits the top plate, the force moves it closer to the bottom plate. This changes the capacitance — the plate’s ability to store electric charge. The more powerful the laser, the more force ends up on the top plate. To prevent external activity from impacting the plates, scientists made them insensitive to vibrations by using springs. Any external activity will impact both springs; the remaining difference can be directly attributed to the laser.

Scientists will continue to work on refining and streamlining this new device.

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