Hundreds of Sensors Packed into Single Optical Fiber for Harsh Environments

Researchers at the University of Pittsburgh have created an all-optical, high-temperature sensor for gas flow measurements that operates at temperatures above 800 degrees Celsius.

The sensor, described in paper published in The Optical Society’s (OSA) journal Optics Letters, was created by combining the concepts of active fiber sensors and high-temperature fiber sensors. With its ability to operate at 850 degrees Celsius, which is 200 degrees Celsius higher than MEMS-based sensors developed at Oak Ridge National Laboratory, the sensor is expected to be used in industrial sensing applications in harsh environments such as deep geothermal drill cores, outer space and the interiors of nuclear reactors.

To build the new technology, researchers integrated optical heating elements, optical sensors, an energy delivery cable and a signal cable into a single optical fiber. Optical power delivered by the fiber supplies energy to the heating element, while the optical sensor measures the heat transfer from the heating element and transmits it back.

We call it a ‘smart optical fiber sensor powered by in-fiber light’,” Kevin Chen, an associate professor and the Paul E. Lego Faculty Fellow in the University of Pittsburg’s Department of Electrical and Computer Engineering, said. “Tapping into the energy carried by the optical fiber enables fiber sensors capable of performing much more sophisticated and multifunctional types of measurements that previously were only achievable using electronic sensors.”

Chen initially noticed a need for a more versatile sensor during a visit to NASA, a trip which became the inspiration for the project. In microgravity situations, multiple electronic sensors are needed to measure the level of liquid hydrogen fuel in tanks because the fuel does not settle at the bottom of the tank as it would on Earth.

“For this type of microgravity situation, each sensor requires wires, a.k.a. ‘leads,’ to deliver a sensing signal, along with a shared ground wire,” Chen said. “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.”

Chen and his colleagues selected optical fiber sensors to work with for their extraordinary multiplexing capabilities and immunity to electromagnetic interference, which make them one of the best sensor technologies for use in harsh environments. These sensors were also amendable to being packed into a single fiber, reducing the wiring problems associated with having numerous leads.

“Another big challenge we addressed was how to achieve active measurements in fiber,” Chen said. “If you study optical fiber, it’s a cable for signal transmission but one that can also be used for energy delivery—the same optical fiber can deliver both signal and optical power for active measurements. It drastically improves the sensitivity, functionality, and agility of fiber sensors without compromising the intrinsic advantages of fiber-optic sensors. That’s the essence of our work.”

The process can also be applied to highly sensitive chemical sensors for cryogenic environments, Chen added.

The research team plans to continue their work by examining methods for enhancing other common engineering devices.

“For fiber sensors, we typically view the fiber as a signal-carrying cable. But if you look at it from a fiber 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 fibers for long-distance and remote sensing?” Chen said. “These are questions we’ll address.”


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