Real-Time Monitoring of High-Power Lasers

The device in this picture is a prototype of a capacitive pressure sensor designed to measure 1,000 W lasers in the infrared with better than 1% uncertainty. (Source: A. Artusio-Glimpse, NIST)

High-power lasers are now widely used in additive manu­facturing and laser welding systems to precisely cut and weld metal, making all kinds of metal parts for medical devices, aerospace appli­cations, automotive industries, and more. With the rise in industrial use of high-power laser processing, manu­facturers increa­singly seek high-accuracy, point-of-use laser power meters that can quickly report laser powers at any time in the manu­facturing process – a vital aspect to control­ling product quality. Traditional laser power meters, however, are often bulky in size and slow in response time. Power measure­ments can also only be taken separately, inter­rupting the manu­facturing process.

Now, a group of researchers from National Insti­tute of Standards and Tech­nology NIST in Boulder, Colorado, have developed a smaller, faster and more sensitive laser power meter in the form of a folding mirror – a “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 package. The four-centimeter-on-a-side cubes can be conve­niently embedded into laser optical systems or laser-welding systems for point-of-use, real-time laser power measure­ment and cali­bration.

“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,” said Alexan­dra B. Artusio-Glimpse, a scientist of NIST. “Unlike any other optical power measure­ment techniques, our method allows us to continue using the laser for work while a measure­ment is being taken.” 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 has to tempo­rarily stop the laser beam from work for around tens of minutes.

“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 laser power 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. 200 watts of laser power, for example, exerts a force equivalent to 100 micro­grams, which is 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, making real-time moni­toring of laser power or laser cali­bration possible.

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. In the new version of the Smart Mirror, the researchers improved the measurement sensi­tivity by 100 times and decreased the response time by 50 times. They 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 – 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 100 watts 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. (Source: OSA)

Reference: A. B. Artusio-Glimpse et al.: Non-Absorbing, Point-of-Use, High-Power Laser Power Meter, Session ATh2A.2, OSA Imaging and Applied Optics Conference 2018, Orlando, USA

Link: Precision Measurement Laboratory, National Institute of Standards and Technology NIST, Boulder, USA

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