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Researchers develop chip-sized sensor for real-time beam power 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. (Image: Jennifer Lauren Lee/NIST)

Researchers from the National Institute of Standards and Technology (NIST), in the US have developed a chip-sized sensor that could be built into manufacturing systems in order to measure laser beam power in real time.

The device, presented in October’s issue of IEEE Sensors, operates in a similar way to a previous sensor developed by the team which uses radiation pressure – the force that light exerts on an object – to determine the power of a laser beam. This shoebox-sized ‘Radiation Pressure Power Meter (RPPM)’ was designed to measure ultrahigh-power lasers in the multi-kilowatt range, while the new chip-sized sensor, dubbed ‘smart mirror’, is instead designed for lasers that have power in the range of hundreds of watts.

Conventionally, gauging laser power has required apparatus that can absorb all the energy from a laser beam as heat, with the resulting temperature change then being used to calculate the laser’s power. The issue with this however is that while all the energy from the beam is being absorbed to make a measurement, no energy remains available to process material with, meaning manufacturers could never use the technique to perform power measurements in real time during production.

Radiation pressure can be used to solve this problem. While light has no mass, it does have momentum, meaning it produces a force when striking an object. A 1kW laser beam for example has a small but noticeable force, approximately equal to the weight of a grain of sand. Therefore, by shining a laser beam on a reflective surface, and then measuring how much the surface moves in response to light’s pressure, not only can the laser’s force – and therefore power – be measured, but the light that reflects off the surface can then be used for materials processing .

While the RPPM uses radiation pressure in this way, it is too big to be integrated into manufacturing equipment such as processing heads or 3D printers to perform real-time measurements. The researchers therefore designed the new smart mirror to not be only smaller than its predecessor, but also 250 times faster at performing measurements and 40 times more sensitive and than the RPPM, enabling it to detect the significantly smaller forces (and therefore beam powers) used in everyday manufacturing processes. The smaller size of the smart mirror makes it easier to integrate into manufacturing equipment such as that in the aerospace, automotive, medical and consumer electronics industries.

‘This would put the high accuracy of NIST power measurements directly in the hands of operators, providing standardised quality assurance across laser-based systems and helping to accelerate the process of part qualification,’ commented NIST researcher Alexandra Artusio-Glimpse.

The smart mirror operates by measuring the changes in capacitance – the ability to store electric charge – between two small charged plates. When laser light strikes the top plate, a force is imparted that causes that plate to move closer to the bottom plate, which changes the capacitance. By measuring this slight change in capacitance, the sensor can make precise, real-time power measurements for lasers of hundreds of watts.

So far the prototype smart mirror has been tested at 250W of beam power, however according to the researchers with further work it could measure powers as high as 1kW and as low as 1W. They are also working to improve the sensitivity and stability of the device. 

Paper: I. Ryger, A.B. Artusio-Glimpse, P. Williams, N. Tomlin, M. Stephens, K. Rogers, M. Spidell and J. Lehman. Micromachined force scale for optical power measurement by radiation pressure sensing. IEEE Sensors. Published Oct. 1, 2018. DOI: 10.1109/JSEN.2018.2863607

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