NIST develops novel high-power laser meter

30 May 2018

Compact design with high-performance opens door to in-situ, real-time monitoring of industrial apps and more.

Researchers at the US National Institute of Standards & Technology, NIST have developed a compact, high-accuracy, point-of-use high-power laser power meter in the form of a folding mirror. Called “Smart Mirror”, the technology will enable in-situ, real-time measurement of higher laser powers.

With the rise in industrial use of high-power laser processing, manufacturers are increasingly seeking high-accuracy, point-of-use laser power meters that can quickly measure laser powers at any time in the manufacturing process – a vital parameter for controlling product quality. NIST comments that “traditional laser power meters are often bulky and slow in response time,” adding, “Power measurements can also only be taken separately, interrupting the manufacturing process.”

The NIST researchers have developed what they call “a smaller, faster and more sensitive laser power meter in the form of Smart Mirror”. The novel design uses a capacitor-based force transducer and merges optical elements, namely a high reflectivity mirror, and sensing elements into a compact cube-style package.

The 40mm cubes can be conveniently embedded into laser optical systems or laser-welding systems for point-of-use, real-time laser power measurement and calibration. The researchers will be presenting their innovation at the OSA Imaging and Applied Optics Congress, being held June 25-28 in Orlando, Florida, United States.

"Measuring laser power by measuring the pressure of a laser beam hitting a mirror is a very unique technique. So far, it is the only laser power measurement technique that is truly an in-situ process,” said Alexandra B. Artusio-Glimpse, a scientist at NIST in Boulder, Colorado, and the Smart Mirror group’s primary author. “Unlike any other optical power measurement techniques, our method allows us to continue using the laser for work while a measurement is being taken.”

Dr Artusio-Glimpse explained that traditional high-power meters measure laser power by absorbing all the energy of a laser beam as heat and measuring the temperature change. The calorimetric measurement method requires temporarily stopping the laser beam from its work, typically for tens of minutes.

Uninterrupted operation

“Using our Smart Mirror laser power meter, that stop-measure-continue process is no longer needed. Manufacturers can measure the laser power continuously during every weld and monitor the laser calibration in real time, they would know right away whenever the laser has a problem and wouldn’t risk wasting metal parts with bad welds,” Artusio-Glimpse said.

The Smart Mirror meter is also referred to as a radiation pressure power meter (RPPM), as the operating principle of this meter is based on measuring the pressure of the laser, the radiation pressure. Light has no mass, but it has momentum and when a laser beam strikes an object such as a mirror, it will exert a tiny force known as the radiation pressure on the mirror, which directly relates to the laser power. For example, 200W of laser power exerts a force equivalent to 100 micrograms, roughly the weight of a single human eyelash.

The key part of the Smart Mirror design is a capacitor-based compact force transducer. It consists of a spiral planar silicon spring supporting a circular plate with a high reflectivity mirror on one side and an electrode on the other. An identical silicon spring with an electrode is placed close to the first spring such that the two electrodes face each other, forming a variable capacitor.

A laser beam reflecting off the mirror on the first spring will push the first spring to move toward the second and change the capacitance between the two electrodes. By comparing to a fixed reference capacitor, the researchers can calculate the radiation pressure and laser power. After reflecting off the mirror, the laser beam can be used directly for work, enabling real-time monitoring of laser power or laser calibration.

According to Artusio-Glimpse, the team has been developing the novel radiation pressure power meter for years and an earlier version of RPPM employed a commercially available scale with a mirrored surface as a force transducer. The final system was about the size of a shoebox, with a measurement sensitivity of 50 micrograms and response time of five seconds.

Sensitivity boosted

In the new version of the Smart Mirror, the researchers improved the measurement sensitivity by 100 times and decreased the response time by a factor of 50. They also mitigated static sagging errors of the device caused by gravity when the device is rotated.

This approach 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 – a key feature that the early version bulk RPPM lacks. It also meets the measurement requirements of many commercially significant applications.

Based on preliminary tests, the new meter is sensitive enough to measure 100W of laser power with no more than one 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.