Sensor Image
Oxford University researchers have developed a sensor made of sapphire fiber that can tolerate extreme temperatures, with the potential to enable significant improvements in efficiency and emission reduction in aerospace and power generation. (Image courtesy of Julian Fells, University of Oxford)

Researchers at Oxford University have used a sapphire optical fiber – a thread of industrially grown sapphire less than half a millimeter thick – that can withstand temperatures over 2000°C. When light is injected into one end of the sapphire fiber, some is reflected back from a point along the fiber which has been modified to be sensitive to temperature (known as a Bragg grating). The wavelength (color) 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 fiber 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 fiber, which results in many different wavelengths being reflected at once. The researchers overcame this problem by writing a channel along the length of the fiber, such that the light is contained within a tiny cross-section, one-hundredth of a millimeter 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 sapphire fiber 1 cm long, but the researchers predict that lengths of up to several meters will be possible, with a number of 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.

Engineering Science research team member Dr Mohan Wang says, "The sensors are fabricated using a high-power laser with extremely short pulses and a significant hurdle was preventing the sapphire from cracking during this process."

The work is part of a £1.2M EPSRC Fellowship Grant held by the Department's EPSRC Research Fellow Dr Julian Fells and was carried out in partnership with Rolls-Royce, the UK Atomic Energy Authority (Remote Applications in Challenging Environments – RACE), Cranfield University, Halliburton and MDA Space and Robotics.

Rob Skilton, Head of Research at RACE, UK Atomic Energy Authority says, "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."

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