Back-reflection in novel hollow-core fiber is 10,000x lower than regular fiber

15 Feb 2021

Partners ORC and Laval measure high-performance Nested Antiresonant Nodeless Fibers developed by LightPipe program.

Researchers from the UK’s Optoelectronics Research Centre (ORC), Southampton, and Université Laval, Québec Canada, have successfully measured – for the first time – back-reflection in cutting-edge hollow-core fibers, which is around 10,000 times lower than conventional optical fibers.

This discovery, published this week in Optica, highlights another optical property in which hollow-core fibers can outperform standard optical fibers.

Research into improved optical fibers is key to enable progress in numerous photonic applications. Most notably, these would improve Internet performance that heavily relies on optical fibers for data transmission where current technology is starting to reach its limits.

A small portion of the light that is launched into an optical fiber is reflected backwards as it propagates, in a process known as backscattering. This backscattering is often highly undesirable as it causes attenuation of signals propagating down the optical fiber and limits the performance of many fiber-based devices, such as optical gyroscopes that navigate airliners, submarines and spacecrafts.

However, the ability to reliably and accurately measure backscattering can be beneficial in other instances, such as the characterisation of installed fiber cables where the backscatter is used to monitor the condition of a cable and identify the location of any breaks along its length.

Novel applications

The latest generation of hollow-core Nested Antiresonant Nodeless Fibers (NANFs), which have been pioneered in the Southampton-led LightPipe research program and applied to novel application fields within the Airguide Photonics program, exhibit backscattering that is so low that up until this point it remained unmeasurable.

To resolve this challenge, ORC researchers within the University of Southampton's Zepler Institute for Photonics and Nanoelectronics teamed up with colleagues from the Centre for Optics, Photonics and Lasers at the University of Laval, who specialise in research into highly-sensitive optical instrumentation.

They developed an instrument that enabled the team to reliably measure the extremely weak signals back-scattered in the latest ORC-fabricated hollow-core fibers - confirming that scattering is over four orders of magnitude lower than in standard fibers, in line with theoretical expectations.

Professor Radan Slavik, Head of the ORC's Coherent Optical Signals Group, said, “I am fortunate to work in the ORC, where the long-term, world-leading research of my design and fabrication colleagues has led to the lowest-loss and longest-length hollow-core fibers ever made. My work has focused on measuring the special properties of these fibers.”

Dr Eric Numkam Fokoua, who carried out the theoretical analysis at the ORC to support these findings, said, “The experimental confirmation of our theoretical prediction that backscattering is 10,000 times less in our latest hollow-core fibers than in standard all-glass fibers demonstrates their superiority for many applications.

“Moreover, the ability to measure such low backscattered signal levels is also critical in the development of hollow-core fiber technology itself, in providing a critical route to distributed fault-finding in fabricated hollow-core fibers and cables as needed to drive forward improvements in their manufacturing processes. Existing technology is simply not sensitive enough to work with these radical new fibers and this work demonstrates a solution to this problem.”