A Quantum Tool Set Could Reshape Laser Beam Profiling

JOE KUCZYNSKI, SENIOR EDITOR JOE.KUCZYNSKI@PHOTONICS.COM

With applications for lasers constantly expanding, the need to precisely monitor the lasing process to ensure optimal and repeatable performance places stringent requirements on beam-profiling mechanisms. Industry widely agrees on many of the standards that govern effective beam profiling as well as the parameters — such as power, shape, and energy — for which data is needed to determine the utility of the laser.

Courtesy of iStock.com/yuriz

Establishing a tool set to profile multiple degrees of freedom simultaneously is especially valuable now, in an age in which optics, such as metasurfaces, are increasingly prevalent, even in industry.

Similarly, in the context of profiling and measuring a beam, industry has also reached an understanding of which qualities are changeable and which are not. With these factors in mind, operators select the precise time to measure a beam in-process, gather data, and sort it in a way that yields information on the most meaningful parameters. This information enables users to calculate more than the physical parameter of the beam: It can also be used to deliver insights into how to best use the laser.

Still, current beam-profiling and measurement practices often rely on equipment that is big and bulky, which may present a bottleneck for many high-power and on-chip lasers. To acquire the full picture, a tool set based on quantum mechanics is needed to provide a more complete understanding of lasers on the extreme ends of the spectrum.

A whetstone to an axe

Beam profiling, by its simplest definition, describes the process of finding and analyzing the spatial intensity distribution of a beam. Laser beam profiling is not a new practice, and the fact that evaluating different parameters provides insights of varying significance to a given application is hardly novel.

A quantum tool set is poised to enable users to completely profile a beam, building on and enhancing the functionality of existing beam measurement methods. The tool set-based method involves several measurements — six different spatial patterns and six different polarization states — resulting in 36 individual measurements. OAM: orbital angular momentum. Courtesy of Andrew Forbes.

In many ways, the laws of physics, and even logic, can be applied to determine how and why a beam behaves. Consider for example that a polarization parallel to a laser cut leaves a smoother surface than an angled approach.

For this reason, beam profiling has historically been the means by which users gather spatial intensity distribution, polarization, and other data and tune their lasers accordingly, much as a logger may use a whetstone on an axe. According to Asger Jensen, senior market development manager at NKT Photonics, the quality of the beam profile has a direct tie to the overall efficiency of a given process. Sophisticated applications, such as those in the quantum realm, increasingly necessitate the use of specialty laser systems, including high-power fiber lasers.

“At a very high power, you do get distorted beam profiles even in fibers, and so of course beam profiling is an important part of what we do,” Jensen said.

Currently, high-power fiber lasers such as these are used in quantum optical computing, among other applications. These lasers are frequency-converted to achieve wavelengths that are appropriate for the application. According to Jensen, the quality of the beam profile dictates the efficiency of this process.

Photonic integrated circuits (PICs) create additional challenges. Just as very high-powered lasers typically generate too much heat to be successfully profiled, the extremely small size of on-chip lasers often makes these sources too small to measure.

“When you bring a laser on-chip, that’s really challenging as is,” said Klea Dhimitri, quantum technology lead for Hamamatsu Photonics. While fabrication and integration may eventually give way to considerations about optimizing the performance of the laser, many companies and institutions remain focused on laser on-chip integration.

Still, Dhimitri said, learning and optimizing the properties of an on-chip laser will undoubtedly help users in the future. And this is especially true regarding applications for which on-chip lasers are not yet known. “For quantum computers, whether it be modulators or bringing detectors on-chip, it’s the holy grail,” Dhimitri said.

With very high-power models on one side of the scale and on-chip lasers on the other, most users operate in the space between the two. And in many cases, these users do not require expanding profiling techniques. “The middle ground is like going to the department store: If you’re looking for a medium, there are lots of things on the rack,” said Andrew Forbes, a professor at the University of the Witwatersrand in South Africa. “But, if you’re looking for extra-extra-small or extra-extra-large, you’re going to have a problem.

“I don’t know if there is an answer to this question, in terms of looking at far ends of the spectrum for beam profiling,” Forbes said.

The need for new measurement

According to Forbes, the evolution of new and sophisticated lasers is bringing with it beams with degrees of freedom that are mixed together, as opposed to scalar beams with a single polarization. In applications using these beams, statistical tools may not suffice to achieve optimal profiles or measurements.

Quantum mechanics, Forbes said, offers the right machinery that may be used to extract new information.

Forbes is turning to a reconceptualized approach to laser beam profiling that uses what he calls a quantum “tool set.” In situations in which beams are constantly changing polarizations, such as when a horizontally polarized beam is joined with a vertically polarized beam, more than conventional beam profiling is needed to evaluate the beam with accuracy. “This beam is very hard to analyze with standard profiling techniques because it doesn’t capture the kind of salient property of this beam,” Forbes said. “Intensity is certainly one part of the equation, but how the polarization looks is actually the critical part of the equation. And this is called vectorial light.”

Digital holograms show how a quantum tool set can be used to profile beams. This allows the user to reconstruct the complete picture of the beam. Courtesy of Button Optics/Optics Letters.

Vectoral beams are used in industrial applications to drill smaller holes, make cleaner cuts, and ablate surfaces with better performance than with classic systems. The principal challenge in measuring and profiling these beams involves the pattern of light, or the intensity. According to Forbes, the pattern of two different lights used in a spectral beam, each with different polarizations, as well as their degrees of freedom, will look one way on camera. However, those images will not at all reflect the laser beam itself, which will appear to be in more of a quantum state with countless polarizations.

Ultimately, images of vectorial light produced by one camera may not paint an accurate picture.

Though the beam in this situation is not in a true quantum state, quantum tools could potentially be applied to the beam to offer the same data set provided by current iterations of beam profiling — plus additional information that could include the multitude of variations on polarization that would be critically important to modern applications, Forbes said.

“It’s kind of a modern manifestation of the old laser beam shaping,” Forbes said. “Previously, when we shaped light, we only shaped light in its amplitude, like what it looks like on the camera. We ignored polarization.”

Another example involves adding a doughnut beam — a beam carrying orbital angular momentum — with a Gaussian beam. Such a combination provides every possible polarization embedded inside it, Forbes said.

Establishing a tool set to profile multiple degrees of freedom simultaneously is especially valuable now, in an age in which optics, such as metasurfaces, are increasingly prevalent, even in industry.

Fiber lasers operating at a very high power can cause distorted beam profiles. Since beam quality affects the efficiency of laser operation, users of new and emerging types of lasers must find a way to fully profile beams. Courtesy of iStock.com/yuriz.

“Together with the ability to create, we need the ability to detect. And that’s where the modern profiling tools must come in,” Forbes said. He said that a full field analysis, complete with new layers of data, provided at the same precise time, will be key to unlocking the full potential of today’s lasers.

Rethinking benchmarks

A solution without a problem is a common occurrence, both with new and established technologies. Companies such as Button Optics, a spinout of the Structured Light Lab at the University of the Witwatersrand, use many standard beam-profiling practices. At the same time, the company is working to create novel structured light vector beams, thereby creating demand for an approach to beam profiling that extends beyond these established techniques.

Bertus Jordaan, an R&D scientist with Button Optics, said that his group is focusing on the generation and detection of new laser types and that new detection practices will rely on quantum inspired practices.

Further, Jordaan said, such new detection processes, with a quantum inspired tool set, are poised to support applications beyond standard profiling.

“Wavefront sensing and getting phase information that is more than the amplitude, along with the amplitude, can tell you a lot,” Jordaan said. “And this could support applications even in ophthalmology and astronomy, if you want to really understand what’s going on in your telescopes.”

The critical technology to consider is wavefront sensing, Jordaan said.

“If you have the full field information, phase and amplitude together, then you can predict everything from the beam. And wavefront sensing is doing that. Now, we can combine some of the aspects of that together with the vector analysis to really give a full perspective.”

According to Jordaan, many companies and researchers are now tasked with simultaneously analyzing multiple degrees of freedom. This analysis, along with the intensity profile and the polarization profile, can reconstruct the state of new, complex beams by running a modal analysis and a polarization analysis at the same time.

Though developing the process needed to support the quantum tool set for beam profiling is still in the works, it must meet certain benchmarks to become a new standard, even for sophisticated beam measurements; it must be widely recognized by the international photonics community, and it must be affordable enough to permeate all the fields using these lasers and beams. Perhaps most critically, it must deliver on its promise to accurately reconstruct new types of laser beams.