Sapphire fibre sensor ushers in significantly cleaner air travel

The work, carried out by Oxford University, uses a sapphire optical fibre – a thread of industrially grown sapphire less than half a millimetre thick – which can withstand temperatures over 2000°C. 

When light is injected onto one end of the sapphire fibre, some is reflected from a point along the fibre which has been modified to be sensitive to temperature (known as a Bragg grating). The wavelength (colour) of this reflected light is a measure of the temperature at that point.

The research resolves a 20-year-old problem with existing sensors that, whilst the sapphire fibre seems very thin, in comparison to the wavelength of light, it is huge. This means that the light can take many different paths along the sapphire fibre, which results in many different wavelengths being reflected at once. 

The researchers overcame this problem by writing a channel along the length of the fibre, such that the light is contained within a tiny cross-section, one-hundredth of a millimetre in diameter. With this approach, they were able to make a sensor reflecting predominantly a single wavelength of light.

The initial demonstration was on a short length of 1cm-long sapphire fibre, but the researchers predict that lengths of up to several metres will be possible, with several separate sensors along this length. This would enable temperature measurements to be made throughout a jet engine, for example. Using this data to adapt engine conditions in-flight has the potential to significantly reduce nitrogen oxide emissions and improve overall efficiency, reducing the environmental impact. The sapphire's resistance to radiation also gives applications in the space and fusion power industries.

Rob Skilton, Head of Research at RACE, UK Atomic Energy Authority said:
“These sapphire optical fibres will have many different potential applications within the extreme environments of a fusion energy powerplant. This technology has the potential to significantly increase the capabilities of future sensor and robotic maintenance systems in this sector, helping UKAEA in its mission to deliver safe, sustainable, low carbon fusion power to the grid.”

Dr Fells, who is leading the research, concluded:
“We are very grateful to the UK Engineering and Physical Sciences Research Council (EPSRC) for supporting this work and to the reviewers who saw the potential for the challenging work we proposed. We are now working with our partners to further develop the technology to the point where it can be integrated into suitable infrastructure.”