Sapphire fiber could enable cleaner energy and air travel

Sapphire optical fiber.

Julian Fells/University of Oxford

Researchers at the University of Oxford have developed a sapphire fiber sensor that can tolerate extreme temperatures, with the potential to dramatically improve efficiency and reduce emissions in aerospace and power generation.

The work, published in the journal Express Optics, uses sapphire optical fiber – an industrially grown sapphire thread less than half a millimeter thick – that can withstand temperatures in excess of 2000°C. When light is injected onto one end of the sapphire fiber, some of it is reflected from a point along the fiber that has been modified to be temperature sensitive (known as a Bragg grating). The wavelength (color) of this reflected light is a measure of the temperature at that point.

The research solves a 20-year-old problem with existing sensors that although sapphire fiber looks very thin, compared to the wavelength of light, it is huge. This means that light can take many different paths along the sapphire fiber, causing several different wavelengths to be reflected at the same time. The researchers overcame this problem by writing a channel along the length of the fiber, so that the light was contained in a tiny cross-section a hundredth of a millimeter in diameter. With this approach, they were able to fabricate a sensor that primarily reflects a single wavelength of light.

The initial demonstration was on a short length of sapphire fiber 1cm long, but the researchers predict that lengths of up to several meters will be possible, with a number of separate sensors over that length. This would make it possible, for example, to take temperature measurements throughout a turbojet engine. Using this data to tailor in-flight engine conditions has the potential to significantly reduce nitrogen oxide emissions and improve overall efficiency, thereby reducing environmental impact. Sapphire’s resistance to radiation also gives applications in the space energy and fusion industries.

Research team member Dr Mohan Wang, Department of Engineering Science, University of Oxford, said:

“The sensors are made using a high-powered laser with extremely short pulses and a significant obstacle prevented the sapphire from cracking during this process.”

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

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