EVacuAted OptiCal Fibres for Ultimate UV-to-Infrared Light TransMission (VACUUM)

Over the last four decades, optical fibres have revolutionised telecommunications and enabled Internet as we know it today. Sensing is another area where optical fibres are used, for example for monitoring engineering structures (e.g., strain and vibration along bridges, tunnels, etc.), or to deliver light for advanced instruments such as next-generation microscopes that can see material/tissue properties that are invisible with traditional instruments. Optical fibres are also leading a revolution in manufacturing by generating and delivering intense laser light capable of welding and cutting.

However, conventional fibres, where light propagates through glass, cannot cope with such high powers due to the onset of nonlinear effects and material damage caused by the high light intensities. Glass absorption also limits the exploitation of fibre technologies in the visible and near/mid-infrared. These shortcomings are being addressed by the next generation of optical fibres, so-called hollow-core fibres that guide light through a central hole, thus avoiding significant light-glass interaction. Light in these fibres is guided thanks to a specially engineered glass microstructure built around a central hole. Recently, the design and manufacturing of this microstructure has been improved significantly and hollow-core fibres are now emerging with properties that surpass those of traditional fibres in almost every regard.

In these novel fibres, light propagates through the core; in most cases the core contains air which enters the fibre during fabrication or onward handling. Although light interacts with air significantly less than with glass, this interaction nevertheless still imposes appreciable limitations. One example is absorption at wavelengths such as 1300 nm (due to water vapour) or in the mid-infrared (absorption of atmospheric gases). Another example relates to the transmission of high-power pulses (e.g., as needed for laser based welding) where nonlinear optical interactions with the air result in significant beam distortions. The ultimate solution would be to evacuate the fibre core, thereby eliminating the air-light interaction.

Preliminary calculations show that evacuating a long length of hollow-core fibre (kilometres) would take impractically long (years) due to the small core diameter (typically ~0.03 mm). Techniques to characterize the gas pressure or content along the fibre length have also not been developed yet. Without such measurements, it is difficult to monitor the evacuation process, or to validate models that describe the evacuation process.

This project is dedicated to investigating, theoretically and experimentally, techniques to accurately characterize the (residual) air pressure along a length of hollow-core fibre. Subsequently, we will research several solutions to reliably evacuate them over long lengths and to seal them while enabling low loss coupling of light in and out. Finally, we will demonstrate how these improved hollow-core fibres will enable next-generation applications, targeting three selected areas:

1) telecommunications, where evacuation will enable communication over a large wavelength range, increasing several times how much data can be transmitted over a given time.

2) high-power laser pulses for welding/drilling/mining, but also bio-medical imaging, where we expect up to 100-1000 times larger powers to be deliverable through the evacuated hollow-core fibres as compared to air-filled ones and up to one million times more than with today's glass-core fibres.

3) transmission of mid-infrared light ("molecular fingerprint region") and demonstration of applications in remote hydrocarbon analysis, of interest, e.g., in oil wells.

Evacuated hollow-core fibres will offer superior performance to any other fibre technology, ranging from guiding in the UV all the way to mid-infrared, opening new opportunities in science, technology, and applications.