Understanding confinement loss in hollow-core fibres

Progress has been made in recent years to increase the efficiency of optical fibres through the design of cables that allow data to be transmitted faster and at broader bandwidths. The greatest improvements have been made in the area of hollow-core fibres — a type of fibre that is ‘leaky’ yet also essential for many applications.

Now, scientists from the University of Bath have determined why some air-filled fibre designs work more efficiently than others. The research was conducted by Dr Leah Murphy and Emeritus Professor David Bird. Their theoretical and computational analysis provides an explanation for a phenomenon that other physicists have observed in practice; that a hollow-centred optical fibre incorporating glass filaments into its design causes ultra-low loss of light as it travels from source to destination.

“The work is exciting because it adds a new perspective to a 20-year-long conversation about how antiresonant, hollow-core fibres guide light. I’m really optimistic that this will encourage researchers to try out interesting new hollow-core fibre designs where light loss is kept ultra-low,” Murphy said.

Optical fibres play a vital role in enabling the growth of fast data transmission. Specially designed fibres are also key in the fields of imaging, lasers and sensing. The best fibres feature a range of properties — for example, a pulse of light can travel over 50 km along a standard silica glass fibre and still retain more than 10% of its original intensity (an equivalent would be the ability to see through 50 km of water). However, the fact that light is guided through a solid material means current fibres have drawbacks. Silica glass becomes opaque when the light is attempting to transmit falls within the mid-infrared and ultraviolet ends of the electromagnetic spectrum. This means applications that need light at these wavelengths (such as spectrometry and instruments used by astrophysicists) cannot used standard fibres. High-intensity light pulses are also distorted in standard fibres and can even destroy the fibre itself.

Researchers have been working to develop optical fibres that guide light through air rather than glass. However, a fundamental property of light is that it doesn’t like to be confined in a low-density region like air. Optical fibres that use air rather than glass are intrinsically leaky (the way a hosepipe would be if water could seep through the sides). The confinement loss (or leakage loss) is a measure of how much light intensity is lost as it moves through the fibres, and researchers aim to improve the design of the fibre’s structure to reduce this loss.

Researchers therefore designed a central hollow core surrounded and confined by a specially designed cladding. Slotted within the cladding are hollow, thin-walled glass capillaries attached to an outer glass jacket. Using this set-up, the loss performance of the hollow-core fibre is close to that of a conventional fibre. Murphy and Bird described their model in a paper published in the journal Optica.

The theoretical and computational analysis focuses on the role played by sections of the glass capillary walls that face neither the inner core nor the outer wall of the fibre structure. As well as supporting the core-facing elements of the cladding, the Bath researchers showed that these elements play a crucial role in guiding light, by imposing a structure on the wave fields of the propagating light. The authors named the effect of these structures ‘azimuthal confinement’.

While the basic idea of how azimuthal confinement works is simple, the concept is powerful in explaining the relationship between the geometry of the cladding structure and the confinement loss of the fibre. “We expect the concept of azimuthal confinement to be important to other researchers who are studying the effect of light leakage from hollow-core fibres, as well as those who are involved in developing and fabricating new designs,” Murphy said.

Bird, who led the project, said the research provides a new way for researchers to think about leakage of light in hollow-core fibres and will lead to new designs being tried out. “This was a really rewarding project that needed the time and space to think about things in a different way and then work through all the details,” Bird said.

Image credit: iStock.com/Thomas-Soellner