Lasers provide many different industries with greater efficiency, less waste, and higher precision, but they are not always as easy to fix as the machines they replace. A blunted drill bit is easier to observe and replace than an underperforming laser beam is to detect and correct. Fortunately, as laser technology has evolved, so have a variety of technologies that measure, monitor, and help to optimize the power, shape, and energy of a laser beam. What cannot be measured cannot be improved. Laser beam profiling describes how power is distributed spatially within the beam. A 2D profile showing the beam’s power distribution in false color (left), and a 3D representation of the scale of power discrepancy (right). The data that profiling provides not only helps to optimize laser performance, it also contributes to preventative maintenance and avoids costly downtime. Courtesy of MKS Ophir. Most of the technologies involved in beam measurement trace their origins to the ’70s, ’80s, and ’90s. Such technologies have all received updates of varying magnitudes along the way, with the advent of new sensor and materials technologies, but the fundamental mechanisms of beam measurement have remained the same. Today, the maturity of beam measurement technology has helped to drive the value of and demand for lasers by enabling real-time monitoring of laser parameters, leading to less downtime and greater production efficiency. In the past, when a laser that was used to cut metal began to diminish in performance after 100 h, the user might just turn up the power. And if it happened again after a month, he would do the same again, said Roei Yiftah, senior product line manager at MKS Ophir. But after a certain point, performance can no longer be remedied simply by increasing the power of the beam. The solution might then rely on the user being able to characterize the beam, he said. Mapping how energy is distributed across the axis of a laser beam relies on two basic approaches: camera-based profilers or a mechanical scanning-based system, such as a pinhole, knife-edge, or slit scanner. Each technology has its strengths and weaknesses. “Historically, scanning-slit profilers have been the default,” said Rocco Dragone, vice president of engineering at DataRay. “But as technology advances, people are using cameras more and more because they can provide the full 2D image.” Sean Bergman, product line manager at Coherent, said scanner-based profilers allow users to directly measure very small beams, but such profilers tend to be two or three times more expensive than a camera-based system. Some types of laser beams do not lend themselves to mechanical scanning. Scanner-based laser profiling remains the favored option when measuring narrow beams. In a representative example of the technology, a disk with narrow slits (left) is rotated to pass the slits through a laser’s beam path. A photodetector located behind the disk (right) records the summation of the energy entering the slit to profile the beam. Courtesy of DataRay. “If you have a really nice well-behaved Gaussian, circularly symmetric beam — that’s perfect. You get all the information you need,” Dragone said. “But as soon as you have anything that’s asymmetrical or multimode, the scanning-slit profilers start to lose some of their usefulness.” Camera-based profilers provide information about the shape, size, and intensity distribution of a beam. End users must be mindful of their laser’s specifications when choosing this type of profiling approach, however. There is a big difference in the distribution of energy when measuring a 100-W beam that is 1 cm in diameter compared to a 100-W beam that is 6 μm in diameter, Dragone said. Smaller beams not only can exceed a camera’s damage threshold but they also require a certain level of pixel coverage to provide an accurate measurement. DataRay typically recommends that a beam cover at least 10 pixels in a row to gain an effective profile. The fast-growing market for laser-based additive manufacturing requires measuring specific beam metrics, including power, beam shape, and beam shift — ideally in a compact device that requires no bulky active cooling. Solutions such as MKS Ophir’s BeamPeek solve the cooling issue with a system that integrates an interchangeable beam dump cartridge that can be removed and replaced between measurements to prevent overheating. Courtesy of MKS Ophir. PRIMES’ ScanFieldMonitor, an air-cooled laser profiler for additive manufacturing applications, is compact enough to be integrated within the printing chamber, where it connects wirelessly to a computer. Courtesy of PRIMES. When it comes to measuring the power of continuous-wave lasers, the dominant technologies employ thermopiles and photodiodes. Low-powered lasers in the picowatt and nanowatt ranges are usually measured by using photodiodes, which are typically composed of silicon or germanium. Higher-powered lasers are measured with thermopiles comprising thermocouples arranged in an annular array around a ring. Thermopiles cover a wide range of powers, from a few hundred microwatts up into the kilowatt range. Pyroelectric sensors are used for measuring the power of pulsed lasers, particularly those with low duty cycles in the nanosecond to microsecond range. What is driving innovation? Despite the relative maturity of beam measurement technologies, new applications are demanding further innovations. Sources interviewed unanimously identified additive manufacturing as one of the fastest growing markets for laser beam measurement, and the application requirements are very specific. Perspectiva’s software for its HUARIS system displays a laser’s performance over seven days. In-line beam profiling systems such as HUARIS’ are integrated into production processes to capture measurements in real time. They not only enable feedback loops to automatically adjust laser parameters, they can also enable a historical record of laser performance to support preventative maintenance. Courtesy of Perspectiva Solutions. A technician adjusts a camera-based profiler. Such profilers provide information about the shape, size, and intensity distribution of a beam. They are particularly effective at profiling beams with larger diameters that cover more pixels. Courtesy of Coherent. “[Users] want to measure the power. They want to measure the beam. They want to see the beam shift. They want to see all of that for a 1-kW laser,” Yiftah said. “And they need to measure it without any kind of cooling device.” To meet this stack of challenges, MKS Ophir introduced BeamPeek, an integrated beam analysis and measurement system capable of providing simultaneous beam profiling, focal spot analysis, and power measurement in less than 5 s. It also allows up to 2 min of continuous measurement at 1 kW, along with passive cooling. Successive measurements of different systems can be performed quickly with the help of an interchangeable beam dump cartridge that can be quickly replaced to prevent overheating. One solution to the rigorous demands of additive manufacturing processes is PRIMES’ ScanFieldMonitor. The system is air-cooled, which allows it to be integrated into the printing chamber, where it can connect wirelessly to a computer. The device measures a laser’s caustic, scanning speed, focus shift, beam diameter, vector length, and other details. According to PRIMES, the system is currently the only device on the market capable, on its own, of satisfying ISO/ASTM’s 52941 standard for laser measurement in metal powder bed fusion additive manufacturing for aerospace applications. Industrial materials processing, especially metal processing, is another rapidly growing market for beam profiling, said Nicholas Prefontaine, product manager at Gentec-EO. The reason is not just the growth of this industry but also a widespread drive to replace traditional metal processing tools with lasers. “Even the classic machine toolmakers like Mitsubishi and Haas are all coming out with optical processing tools,” Prefontaine said. “So, this whole industry is moving towards optical processing from traditional methods.” The nature of metal processing applications — such as cutting, welding, and ablation — necessitates the use of high-powered lasers. Measuring the power of these sources is not necessarily challenging for a standard thermopile, but measurements can take longer than many end users would like. In certain metal processing applications, the laser runs almost constantly, finishing parts every minute or every 30 seconds, according to Bergman. “One of the disadvantages of a thermopile is its speed. When you take your machine offline to run over and measure the laser, you might lose an opportunity to braze a car body,” he said. To address this issue, Coherent developed a transverse thermoelectric material that enables it to combine thermopile, pyroelectric, and photodiode sensors into its PowerMax-Pro measurement tool. According to the system’s datasheet, it delivers a response time under 10 ms, allowing users to take quick measurements between jobs. Profiling the beam of a high-powered laser can be more complicated than measuring laser output. Best practice is to profile a laser beam as close as possible to the source, Dragone said. But compromises often need to be made when a laser’s operational power exceeds the damage threshold of the profiler. Some users choose to run lasers at a lower power to profile the beams. “And that’s good enough for a lot of applications,” Dragone said. “But, on the other hand, we have another end user who sees differences in their laser when they run it at 10% power versus 100% power. So, for them it was necessary to add quite a bit of attenuation.” Attenuating a laser beam involves the use of optical filters and other elements to reduce the power of a beam while still retaining its original shape and appearance. The challenge is to avoid thermal lensing in the optical elements, which can happen if an attenuator overheats to the point that its optical properties change and thereby alter the profile of the beam. “We always use a beam sampler whenever the power is above a watt,” Dragone said. A beam sampler uses reflection to pick off a fraction of the beam to be measured. Preventative maintenance A faulty laser is typically out of commission for four to eight weeks, which gives users a powerful motivator to invest in a beam profiling system for preventative maintenance. In-line measurement systems can further reduce downtime because they can profile the beam during natural pauses in production. With an in-line process, an operator can monitor for changes in the laser’s focus position during production and stop the process to adjust the beam as necessary, according to Christof Blumenstein, head of sales and marketing at PRIMES. He said he expects to see greater adoption of in-line measurements in the future, as well as increased autonomy in the measurement process. Newer sensor devices are already inching toward greater autonomy by integrating onboard data analysis and sending data to the machine as feedback. Gentec-EO’s IS50A-1KW-RSI integrating sphere detector. Measuring the output power of high-energy lasers often requires using an attenuation method to get an accurate measurement of the beam while retaining its original shape and appearance. Integrating sphere detectors offer one solution. They permit light to enter the hollow sphere through a small aperture and diffuse the light to create multiple reflections on the sphere’s inner coating. This allows a small, uniformly lit aperture at another position in the sphere to sample a portion of the diffused light and send it to a sensor. Courtesy of Gentec-EO. Perspectiva Solutions developed a sensor platform with an architecture familiar to systems designed for the Internet of Things. The platform is backed by artificial intelligence. “We believe that the [laser] measurement process has to be automated to some extent,” said Krzysztof Jakubczak, Perspectiva Solutions’ CEO. “So, for this reason, we have developed artificial intelligence algorithms which analyze the measurement data, and they inform you if they detect some artifact in your beam that is unwanted.” The measurements and the calculations are all stored in a cloud system that sends notifications to users over SMS (short message service), email, or a web application when something is off with a monitored laser. Part of the appeal of this system is the ability to take measurements remotely. A laser manufacturer who sells metal-cutting systems worldwide could then offer customers real-time support or maintenance advice, rather than waiting for them to call when something breaks, Jakubczak said. “Instead, you might want to offer them a remote check of their laser.” Another attractive aspect of this approach is the wealth of data it offers and supports. Profiling laser beams creates a lot of data. “For example, a camera that has 25 million pixels at every frame that it sends to the computer [has] 25 million points [of data], and you’re refreshing that many times a second, so it can accumulate very fast,” said Gabrielle Thériault, Gentec-EO’s marketing director. A cloud-based system enables data to be collected over the long term to provide both the user and the manufacturer with valuable information that can inform improvements in the production process, as well as in the laser itself. What is next? Thériault said he has recently seen greater demand for integration to allow embedded power meters that continuously monitor beam conditions, for example. Such systems would require a solid basis of communication with computers or other data acquisition systems. Achieving this continuous communication sounds easier than it actually is. Coherent’s Bergman said that, in some cases, RS-232 cables — a standard introduced in 1960 — are still the norm for connecting components to external computers. There are no obvious alternatives to this approach. USB cables are significantly faster than RS-232 connectors, but the cables’ latency becomes problematic at lengths over 5 m, which stands in contrast to RS-232’s 15-m maximum. This means that advancements in wireless communications will be crucial for enabling integration. Camera-based beam profiling is another potential area of improvement. “We’re always looking for sensors that have smaller pixels,” DataRay’s Dragone said. With small beams, camera-based profilers require magnification to achieve the pixel coverage necessary for an accurate measurement. Fewer pixels means less magnification. “It’s much easier to find optics that can focus to a very small spot size than optics that can magnify a small spot to a larger size,” Dragone said. Suppliers of beam profilers are also monitoring opportunities among laser applications that are only beginning to emerge or are still in development, such as free-space optical communications and Li-Fi. How these systems will be measured remains an open question. But they will undoubtedly pose new challenges to beam measurement. “There’s a lot of different ways to go because these things are still being figured out,” Gentec-EO’s Prefontaine said. “It’s going to be fun to see how it pans out.”
