Each thread of industrially grown sapphire is less than half a millimetre thick but can withstand temperatures over 2000°C. When light is injected onto one end of the fibre, some of it is reflected back at a point along its length which has been modified to be sensitive to temperature. The wavelength (colour) of this reflected light is a measure of the temperature at that point.
Whilst the sapphire fibre seems very thin, in comparison to the wavelength of light it carries, 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.
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