Recently realized UCF results published in the journal Optica could lead to a new generation of fiber-based laser beam delivery systems with applications in directed energy, precision machining, medical sciences and power beaming over fiber.

Matthew Cooper, who completed his optics and photonics doctorate from CREOL, tested the power, efficiency and quality of laser transmission via a hollow-core fiber just prior to his graduation in late 2023.

Hollow-core fibers are special types of optical fibers that look a bit like a straw, with a tiny hole running through the middle. They can transmit light faster and with less distortion than commonly used solid-core fibers, making them important for artificial intelligence, data centers, laser technology, and medical devices.

However, the widespread application has been limited by needs for more research and development.

That’s where Amezcua and Cooper’s research comes in. Their study demonstrated the successful transmission of high-power laser beams through gas-filled, reduced distortion (antiresonant) hollow-core fibers. The researchers achieved a record 2.2 kW of laser power through a single-mode (one path) antiresonant hollow-core fiber with remarkable efficiency and beam quality. Until now, the transmission of precise wavelength (narrow-linewidth) single-mode lasers at multi-kW power levels through a hollow-core fiber was not yet achieved.

The findings provide critical data in the emerging field of laser transmission with implications for directed energy and enhanced optical communications, Cooper says.

“The results of this particular experiment proved out the ability for these types of fiber structure to handle such extreme powers,” he says.

Applying the Research

Cooper says he believes hollow-core fiber research is on the precipice of addressing critical needs in communications and high-power lasers, and he is intrigued by them potentially being adopted on a larger scale.

“We need long distance transmission lines,” Cooper says. “In solid core fiber, there are nonlinear effects, which accumulate over long distance and distort and disrupt optical characteristics.”

Concentrated high-power lasers for use in industrial, medical or defense applications also may benefit from hollow-core fiber delivery, he says.

Precision and Focus

Most optical fibers are solid core, meaning the core surrounding the light transmission is made entirely of a single material like glass. Generally, they are easy to manufacture and are widely used in communications or laser applications. However, they are susceptible to disruption or scattering at higher power levels.

Hollow-core fibers, on the other hand, are filled with gasses to accommodate high power output, which helps them deliver and maintain laser power levels.

“This was a prototype fiber we fabricated within the fiber draw tower at UCF CREOL, where the structure was never tested at these levels before,” Cooper says.

For the tests, the hollow core was filled with atmospheric air, and the results showed potential advantages over the solid core fibers.

The experiment consisted of testing two hollow-core fiber lengths at various linewidths while carefully increasing the power in increments of 220 watts at a time to monitor the stability and efficiency of the power delivery. The laser ultimately was stabilized and observed at 2.2 kW – which previously had not been achieved.

A high level of precision and vigilance was necessary when dealing with lasers operating at 500,000 times greater intensity than that of a common laser pointer, Cooper says.

“I was truly excited when we first reached these results because we just didn’t know what would happen at these power levels,” Cooper says. “When working in this regime, any slight misalignment or piece of dust in the optics could quite literally cause a fire and ruin your whole experiments or thousands of dollars of equipment if not extremely careful.”

Overall, the experiments were hailed as a success as they demonstrated the hollow-core fiber’s suitability for high-power transmission, provided valuable performance metrics, and explored nonlinearity effects for potential applications.

The Next Steps

Cooper is stationed in South Korea as part of his active service in the Space Force, but he plans to put his expanding optics knowledge to work as a professor at the Air Force Institute of Technology upon return.

He’s hopeful that he and his former colleagues at CREOL will keep cultivating hollow-core laser delivery methods and helping to realize their potential.

“The first step is to keep pushing the power limits of UCF’s hollow-core fiber designs,” Cooper says. “In theory, the fiber should handle power levels well beyond what the coupling optics could ever handle. Secondly, the previous result about rotational Raman shifting [working at different wavelengths] has opened a brand-new door for potential laser sources. By tailoring the source wavelength, the gases inside the fiber, and the fiber design, we should be able to generate a multi kW class laser at any wavelength between 1um and 5um, which has never been done for most frequencies of light within that range.”

UCF will continue studying the limitations and properties of hollow-core fibers, says Amezcua, who also served as research advisor to Cooper.

“We have been working on hollow-core fibers for almost 20 years,” Amezcua says. “We don’t yet understand the limits, and it is a very new regime for light.”

The focus on hollow-core fibers signals a shift in focus in optics and an opportunity to expand the field of communications and high-powered lasers, he says.

“Basically, we were doing things you cannot do with any other conventional fibers,” Amezcua says. “They’re quite remarkable. It opens up possibilities that weren’t available before. There is a lot of interest for many areas of applications.”

The results of Cooper’s experiments are a big step forward for optics research, he says. Cooper spent nearly three years pursuing his Ph. D. with CREOL, and he had achieved many unique results in that time, Amezcua says.

“We are able to expand these kinds of experiments now,” he says. “We have a few news students who have come on recently to continue working on this and we’re hoping to become one of the leading places for hollow core fibers here at UCF.”

UCF CREOL researcher and professor Axel Schülzgen also served in an advisory role for this research. UCF CREOL research scientist Jose Enrique Antonio-Lopez and post-doctoral researcher Stephanos Yerolatsitis fabricated the prototype fibers while UCF post-doctoral scholar Daniel Cruz-Delgado and research associate professor Ivan Divliansky provided support for equipment and analysis techniques. Cooper’s fellow UCF students Joseph Wallen and Dan Parra assisted in conducting and recording experiments.

Researchers from Coherent Corp. and the Florida Institute of Technology also contributed to the study.

Researchers’ Credentials

Amezcua is a professor at UCF’s College of Optics and Photonics where he leads the Optical Fiber and Fiber Devices Laboratory. He received his doctorate from Southampton University. After that, he joined the University of Bath and worked at Powerlase Photonics developing industrial lasers. His main interests are advanced fiber design and fabrication, hollow core fibers, space division multiplexing optical fiber communications, high-power fiber lasers, nonlinear fiber sources, optical sensors and laser components.

Cooper earned his doctoral degree from UCF in December of 2023. He currently is serving as an officer in Space Force after joining the Air Force at 17 years old. Cooper received two master’s degrees in electrical engineering and aeronautical engineering from the Air Force Institute of Technology. He also earned an MBA from the University of South Dakota and his bachelor’s in electrical engineering from the Pennsylvania State University.