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Point-of-Use Power Meter Can Monitor High-Power Lasers in Real Time

Industrial Photonics
Jul 2018
A high-accuracy point-of-use laser power meter, in the form of a folding mirror called the Smart Mirror, could make in situ, real-time measurement of laser power possible. Unlike most traditional meters, the Smart Mirror is compact, fast, and can report laser powers at any time during the manufacturing process without interrupting work.

The design, from a team at the National Institute of Standards and Technology (NIST), uses a capacitor-based force transducer and merges a high-reflectivity mirror and sensing elements into a compact cube package. The 4-centimeter-on-a-side cubes can be embedded into laser optical systems or laser-welding systems.

Smart Mirror high-power laser power meter, NIST.
The Smart Mirror is a force scale that relates the force of a laser shining on it to the power (i.e., the brightness) of that laser. Scientists at NIST are developing these devices to embed into laser-based manufacturing tools for improved performance and reliability. Shown is a prototype of their capacitive pressure sensor designed to measure 1000-W lasers in the IR with less than 1 percent uncertainty. The next phase of devices like this one will be standard reference instruments that report absolute power of any color laser from 1 mW up to 100 kW of power. Courtesy of A. Artusio-Glimpse/NIST.

The Smart Mirror laser power meter operates by measuring the radiation pressure of the laser. When a laser beam strikes an object, such as a mirror, the momentum of the beam will exert a minute force on the mirror known as the radiation pressure, which directly relates to laser power. For example, 200 W of laser power will exert a force equivalent to 100 µg.

The Smart Mirror’s capacitor-based compact force transducer is used to calculate radiation pressure. A silicon spring supports a circular plate with a high-reflectivity mirror on one side and an electrode on the other. An identical spring with an electrode is placed close to the first spring, so that the two electrodes face each other, forming a variable capacitor.

A laser beam reflecting off the mirror on the first spring will cause that spring to move toward the second, changing the capacitance between the two electrodes. To calculate the laser power, researchers use a fixed reference capacitor for comparison. After reflecting off the mirror, the laser beam can be used for work purposes, allowing real-time monitoring of laser power or laser calibration.

“Manufacturers can measure the laser power continuously during every weld and monitor the laser calibration in real time," said researcher Alexandra B. Artusio-Glimpse. "They would know right away whenever the laser has a problem and wouldn’t risk wasting metal parts with bad welds."

An earlier version of the system was about the size of a shoebox, with a measurement sensitivity of 50 µg and response time of 5 s.

In the new version of the Smart Mirror, the measurement sensitivity has been improved by 100× and the response time has been decreased by 50×. The team also mitigated static sagging errors of the device caused by gravity when the device is rotated. This allows the sensor to be embedded at the end of a robotic arm or in additive manufacturing and laser welding systems where the laser head will move and rotate.

Based on preliminary tests, the new meter is sensitive enough to measure 100 W of laser power with no more than 1 percent uncertainty, and with a response time faster than any other absolute high-power laser meter.

The researchers are now continuing to validate these results with more tests. Artusio-Glimpse said the NIST team expects to establish a primary standard version of the Smart Mirror laser power meter in the near future.

“Measuring laser power by measuring the pressure of a laser beam hitting a mirror is a very unique technique, [and] so far it is the only laser power measurement technique that is truly an in situ process,” Artusio-Glimpse said. “Unlike any other optical power measurement techniques, our method allows us to continue using the laser for work while a measurement is being taken.”

The research will be presented at 10:30 a.m. on June 28 at the OSA Imaging and Applied Optics Congress, which runs June 25-28, 2018, in Orlando, Fla.

Research & TechnologyAmericaslasersopticsTest & MeasurementindustrialmirrorsSmart Mirrorhigh-power laserlaser power meterhigh power meterlaser measurementbeam measurementLaser Beamadditive manufacturinglaser weldinglaser meterTechnology News

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