Scientists have developed an all-optical temperature sensor for gas flow measurements that can, for the first time, operate at temperatures above 800°C. The technology, described in The Optical Society’s (OSA) journal Optics Letters, could prove an invaluable tool for applications in harsh environments such as deep geothermal drill cores or the interiors of nuclear reactors.
The team of scientists from the University of Pittsburgh in the USA successfully demonstrated simultaneous flow/temperature sensors at 850°C, which is a 200°C improvement on an earlier demonstration of MEMS-based sensors by researchers at the United States’ Oak Ridge National Laboratory.
The basic concept of the new approach involves integrating optical heating elements, optical sensors, an energy delivery cable and a signal cable within a single optical fibre. Optical power delivered by the fibre is used to supply energy to the heating element, while the optical sensor within the same fibre measures the heat transfer from the heating element and transmits it back.
‘We call it a smart optical fibre sensor powered by in-fibre light,’ said Kevin Chen, an associate professor and the Paul E Lego Faculty Fellow in the University of Pittsburg’s Department of Electrical and Computer Engineering.
The team’s work expands the use of fibre-optic sensors well beyond traditional applications of temperature and strain measurements. ‘Tapping into the energy carried by the optical fibre enables fibre sensors capable of performing much more sophisticated and multifunctional types of measurements that previously were only achievable using electronic sensors,’ Chen added.
For example, in microgravity situations, it is difficult to measure the level of liquid hydrogen fuel in tanks because it doesn’t settle at the bottom of the tank. It’s a challenge that requires the use of many electronic sensors − this was a problem that Chen initially noticed years ago while visiting NASA, which was the original inspiration to develop a more streamlined and efficient approach.
‘For this type of microgravity situation, each sensor requires wires, aka "leads", to deliver a sensing signal, along with a shared ground wire,’ explained Chen. ‘So it means that many leads − often more than 40 − are necessary to get measurements from the numerous sensors.’
The team managed to overcome this challenge by looking to optical-fibre sensors, which are one of the best sensor technologies for use in harsh environments thanks to their extraordinary multiplexing capabilities and immunity to electromagnetic interference. And, they were able to pack many of these sensors into a single fibre to reduce or eliminate the wiring problems associated with having numerous leads involved.
‘Another big challenge we addressed was how to achieve active measurements in fibre,’ Chen said. ‘If you study optical fibre, it’s a cable for signal transmission but one that can also be used for energy delivery − the same optical fibre can deliver both signal and optical power for active measurements. It drastically improves the sensitivity, functionality, and agility of fibre sensors without compromising the intrinsic advantages of fibre-optic sensors. That’s the essence of our work.’
Based on the same technology, highly sensitive chemical sensors can also be developed for cryogenic environments. ‘The optical energy in-fibre can be tapped to locally heated in-fibre chemical sensors to enhance its sensitivity,’ Chen said. ‘In-fibre optical power can also be converted into ultrasonic energy, microwave or other interesting applications because tens or hundreds of smart sensors can be multiplexed within a single fibre. It just requires placing one fibre in the gas flow stream—even in locations with strong magnetic interference.’
The next stage for the team is to explore common engineering devices that are often taken for granted and search for ways to enhance them. ‘For fibre sensors, we typically view the fibre as a signal-carrying cable. But if you look at it from a fibre sensor perspective, does it really need to be round or a specific size? Is it possible that another size or shape might better suit particular applications? As a superior optical cable, is it also possible to carry other types of energy along the fibres for long-distance and remote sensing?’ Chen noted. ‘These are questions we’ll address.’