Cold-container processing promises improved fibre optics for medical imaging

Portable scanners that detect cancer and endoscopes that analyse the content of tissue could be made possible using optical fibres that emit long-wavelength mid-infrared light.

Now, in an EPSRC-funded project, researchers at Nottingham University are developing a new technique to produce these fibres far more quickly and cost-effectively than existing methods, opening up the possibility of using the technology in a range of applications.

Mid-infrared light waves oscillate at frequencies that match the typical range of vibrations of molecular bonds. This makes them ideal for analysing the molecular makeup of a range of samples, including greenhouse gases, explosives, food, and biological tissue.

However, existing mid-infrared spectrometers emit only a very weak light signal, meaning they are only suitable for use in laboratory settings where the sample can be placed close to the detector.

Laser-pumped optical fibres, in contrast, have the potential to emit much brighter mid-infrared light. And in a paper published in the journal Nature Photonics in 2014, a team led by Prof Angela Seddon at Nottingham University, alongside researchers at the Technical University of Denmark, produced a long wavelength optical fibre that emitted a record broad range of frequencies of mid-infrared light.

This rainbow of mid-infrared light, known as a mid-infrared supercontinuum, can be shone at a sample, where it interacts with the molecular bonds within it. The light that hits the detector after this interaction can then be analysed to determine the molecular makeup of the sample.

However, the existing method of producing these optical fibres is laborious and time-consuming. Different glasses are first purified, and then melted together, before finally being shaped, in a process that takes eight weeks. The glass melting process alone can take 32 hours.

So the researchers are now investigating the use of microwave-assisted heating to speed up this process.

The materials used to produce the glass, called metalloids, contain free electrons. These electrons interact with the microwave field, causing the temperature of the material to rise, in a process known as resistive heating.

But because the containers in which the glass is melted do not interact with the microwaves, and remain cold, the process allows higher temperatures to be reached, said Seddon.

“It means the microwaves can penetrate into our metalloids, cause resistive heating, and raise the materials to very high temperatures, where the fluidity is very great, so mixing can occur readily,” she said.

This significantly speeds up the melting process, reducing it to just 30 minutes, she said.