Burn bright like a sapphire

Fiber Bragg Grating (FBG) sensors are used to measure a variety of engineering parameters, including temperature. They work by being modified to be sensitive to a certain temperature point (known as a Bragg Grating).

When light is shone through the sensor, the wavelength of the reflected light indicates the temperature that was reached. While silica optical fibre is predominantly used, its operational temperature is constrained below 1000°C. This can be a limitation, as gas turbines in aeroplane engines, for example, can operate at over 1300°C.

A solution has been found through research from the Department of Engineering Science, at the University of Oxford, UK, using industrially grown sapphire optical fibre, which can withstand temperatures over 2000°C. The work has been published in the journal Optics Express.

There were some tricky challenges to overcome to succeed with this material.

Sapphire optical fibre. Credit: Julian Fells/University of Oxford

While the sapphire fibre is quite thin (less than half a millimetre), it’s enormous compared to wavelengths of light which are 380 to 700 nanometers. When light is shone through the sapphire fibre, it can take different paths and reflect in multiple wavelengths at once, making detection difficult.

The researchers overcame this problem by marking a channel along the length of the fibre, which contained the light within a tiny cross-section 100th of a millimetre in diameter. Using this technique, they were able to make a sensor that predominantly reflected a single wavelength of light.

An annealing test was performed to verify the high-temperature performance of the Bragg Grated sapphire fibre. Annealing is a heat treatment process where the material is heated up to a temperature just below its limit, then cooled down slowly. This process helps make the material less brittle and also helps to remove internal stresses. After being heat treated at 1000°C, the wavelength stabilised to a size of 1548.43 nanometres.

While the initial demonstration sample of the sapphire fibre was only one centimetre long, researchers are aiming to produce lengths of up to several metres, with a multiple separate sensors along the length.

This type of sensor array would enable such applications as the temperature throughout a jet engine to be measured – the technology would allow more accurate and detailed data to be collected, which could be used to significantly improve the engine efficiency, reducing environmental impacts through less nitrogen oxide emissions and fuel use.

Sapphire is also resistant to radiation, which would make is suitable for taking measurements inside nuclear reactors, making it a potentially invaluable material for use in the space and fusion power industries.

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