Low levels of backscattering confirmed in hollow-core fibres

Researchers from the University of Southampton in the UK and Université Laval in Canada have successfully measured back-reflection in cutting-edge hollow-core fibres that is around 10,000 times lower than conventional optical fibres — one of many optical properties in which hollow-core fibres are capable of outperforming standard optical fibres. Their discovery has been published in the journal Optica.

A small portion of the light that is launched into an optical fibre 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 fibre and limits the performance of many fibre-based devices, such as fibre-optic gyroscopes that navigate airliners, submarines and spacecrafts. However, the ability to accurately measure backscattering can be beneficial in other instances, such as the characterisation of installed fibre cables where the backscatter is used to monitor the condition of a cable and identify the location of any breaks along its length.

The latest generation of hollow-core nested antiresonant nodeless fibres (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. As noted by Professor Radan Slavik, Head of the Coherent Optical Signals Group at Southampton’s Optoelectronics Research Centre, “The long-term, world-leading research of my design and fabrication colleagues has led to the lowest-loss and longest-length hollow-core fibres ever made. My work has focused on measuring the unique properties of these fibres, which is often challenging.”

To resolve this challenge, ORC researchers teamed up with colleagues from the Centre for Optics, Photonics and Lasers (COPL) at Université Laval, who specialise in research into highly sensitive optical instrumentation. Together, they developed an instrument that enabled the team to measure the extremely weak signals back-scattered in the latest ORC-fabricated hollow-core fibres — confirming that scattering is over four orders of magnitude lower than in standard fibres, in line with theoretical expectations.

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

“Moreover, the ability to measure such low backscattered signal levels is also critical in the development of hollow-core fibre technology itself, in providing a critical route to distributed fault-finding in fabricated hollow-core fibres 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 fibres and this work demonstrates a solution to this problem.”

Image caption: Light propagates through a central hole of the hollow-core fibre, pioneered in Southampton.

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