By Josh Perry, Editor
[email protected]

Researchers at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. have unveiled a prototype of a new chip-level, laser power sensor that can be built into manufacturing devices for real-time measurement.

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. (Jennifer Lauren Lee/NIST)

This new sensor could help manufacturers, especially those using 3-D printing or other additive manufacturing techniques, by ensuring that the laser is firing at the desired power.

According to a report from NIST, this sensor is based on similar principles as a previous sensor from 2017, which  used radiation pressure, but this smart mirror is designed for smaller lasers that put out hundreds of watts and are commonly used in manufacturing. The new sensor is about 250 times faster than the larger version and 40 times more sensitive.

“Conventional techniques for gauging laser power require an apparatus that absorbs all the energy from the beam as heat,” the article said. “Measuring the temperature change allows researchers to calculate the laser’s power. The trouble with this traditional method is that 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 for something.”

Radiation pressure measures the force that light exerts on an object. Shining a light at a reflective surface and measuring how much the surface moves when the laser hits, scientists can measure the laser’s force and still use that light for manufacturing processes.

The chip-sized mirror works as a capacitor. The sensor measures the change in capacitance between two half-dollar-sized charges plates.

“The top plate is coated with a highly reflective mirror called a distributed Bragg reflector, which uses alternating layers of silicon and silicon dioxide,” the article explained. “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.”

The sensor is also insensitive to vibration using springs attached to both plates to ensure that it is not affected by the lab environment. In this way, the mirror only measures forces that impact only the top plate.

“Right now, the prototype sensor has been tested at a laser power of 250 watts,” the article added. “With further work, that range will likely extend to about 1 kW on the high end and below 1 watt on the low end.”

The research was recently published in IEEE Sensors. The abstract stated:

“We introduce a micromachined force scale for laser power measurement by means of radiation pressure sensing. With this technique, the measured laser light is not absorbed and can be utilized while being measured. We employ silicon micromachining technology to construct a miniature force scale, opening the potential to its use for fast in-line laser process monitoring. Here, we describe the mechanical sensing principle and conversion to an electrical signal.

“We further outline an electrostatic force substitution process for nulling of the radiation pressure force on the sensor mirror. Finally, we look at the performance of a proof-of-concept device in open-loop operation (without the nulling electrostatic force) subjected to a modulated laser at 250 W and find its response time is less than 20 ms with noise floor dominated by electronics at 2.5 W/ √ Hz.”

See how the smart mirror works in the video below: