High-temperature gas flow measurement using optic fibre

A team of researchers at the University of Pittsburgh has created an all-optical high-temperature sensor for gas flow measurements that operates at record-setting temperatures above 800°C.

By fusing together the concepts of active fibre sensors and high-temperature fibre sensors, they have developed a technology that is expected to find industrial sensing applications in harsh environments ranging from deep geothermal drill cores to the interiors of nuclear reactors to the cold vacuum of space missions - and it may eventually be extended to many others.

The team describes their all-optical approach in a paper recently published in The Optical Society’s (OSA) journal Optics Letters. They successfully demonstrated simultaneous flow/temperature sensors at 850°C, which is a 200°C improvement on an earlier notable demonstration of MEMS-based sensors by researchers at Oak Ridge National Laboratory.

An artist's rendering of the fibre-optic flow sensor. The glowing red sections along the fibre are the sensors. (Image: Kevin Chen, University of Pittsburgh)

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 P. Chen, an associate professor and the Paul E. Lego Faculty Fellow in the University of Pittsburgh’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 said.

In microgravity situations, for example, it’s 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 - a problem 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 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. I couldn’t help thinking there must be a better way to do it.”

It turned out, there is. The team looked to optic 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.”

Next, the team plans to explore common engineering devices that are often taken for granted and search for ways to enhance them. “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.”

Reference:

Chen R et al 2014, Fiber-optic flow sensors for high-temperature-environment operation up to 800°C, Optics Letters, Vol. 39, Issue 13, pp. 3966-3969.