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The optical fibre — which comprises a thread of industrially grown sapphire less than half a millimetre thick — is capable of withstanding temperatures in excess of 3,632F [2,000C]. When light is shone down one end of the fibre, part of it is reflected back from a point along the fibre called a “Bragg grating” that the team modified to make sensitive to temperature. The wavelength of the light returned depends on the temperature of the fibre at the Bragg grating, allowing the system to be used as a heat sensor.
The fibre sensor was developed by engineer Dr Mohan Wang of the University of Oxford and his colleagues.
Dr Wang said: “The sensors are fabricated using a high-power laser with extremely short pulses. A significant hurdle was preventing the sapphire from cracking during this process.”
The fibre made by the researchers as a proof-of-principle was but 0.4 inches long, however, the team said that they predict it will ultimately be possible to fabricate ones with lengths of up to several metres — so, likely in excess of 10 feet.
These larger versions could be equipped with multiple sensor points along their length, and could find application, for example, in allowing thermal measurements to be taken throughout an aircraft’s jet engines.
A better understanding of heat distribution in this situation could allow engineers to develop engines that adapt their conditions mid-flight to both reduce emissions of nitrogen oxides, while also improving overall engine efficiency.
Nitrogen oxides play a role in the formation of ozone, a greenhouse gas, in the lower atmosphere.
Sapphires’ ability to withstand extremes of radiation also makes it well-suited for applications in both the space industry and the development of fusion power facilities.
Rob Skilton, roboticist and head of research at the UK Atomic Energy Authority’s Remote Applications in Challenging Environments (RACE) test facility said: “‘These sapphire optical fibres will have many different potential applications within the extreme environments of a fusion energy power plant.
“This technology has the potential to significantly increase the capabilities of the future sensor and robotic maintenance systems in this sector, helping the UK Atomic Energy Authority in its mission to deliver safe, sustainable, low carbon fusion power to the grid.”
RACE was one of the partners involved in the research, along with Cranfield University, Halliburton, MDA Space and Robotics, and Rolls Royce.
Rolls-Royce university research liaison chief Mark Jefferies said:
“This is exciting news and yet another important scientific achievement resulting from our long-standing partnership with Oxford University.
“This fundamental research could in time enable more efficient and accurate multi-point temperature measurement in harsh environments, improving control, efficiency, and safety.”
In developing the fibre design, the researchers had to overcome a long-standing optical challenge that has confounded experts for some two decades.
The issue they encountered stems from the fact that while the fibre may seem very thin from our point of view, it is in fact huge when compared to the wavelength of light.
This means that light bouncing along inside the fibre can take many different paths, resulting in reflections at many different wavelengths simultaneously.
To address this, the researchers wrote a channel along the length of the fibre which acts to contain the passage of the light within a tiny cross-section just one-hundredth of a millimetre in diameter — a tenth of the thickness of a human hair — thereby almost completely confining the sensor reflections to a single wavelength of light.
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The work was funded using part of a UK Engineering and Physical Sciences Research Council fellowship grant totalling £1.2million held by lead engineer Dr Julian Fells of the University of Oxford.
Dr Fells said: “‘We are very grateful to the UK Engineering and Physical Sciences Research Council 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.’
The full findings of the study were published in the journal Optics Express.
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