New fibre optic sensors increase range

Researchers in the US are on their way to solving a problem that is limiting the range and number of sensors used to safeguard civil and industrial infrastructure.

Researchers in Virginia Tech’s Center for Photonics Technology are on their way to solving a problem that is limiting the range and number of sensors used to safeguard civil and industrial infrastructure.

Real-time monitoring of oil and gas fields and pipelines, power networks, bridges, dams, and buildings is important to economic and national security. A limitation to using sensors with different capabilities to create large, economical sensor networks has been the distance a signal will travel and the limited multiplexing capability. Sensor multiplexing in a fibre sensor system means the sensors along a fibre cable can be interrogated by one optoelectronic signal-processing unit. The optical signal is first converted by light detection into an analogue signal, which is then digitised for further signal processing.

Researchers in the Center for Photonics Technology (CPT) have discovered new methods for fabricating and spacing sensors within optic fibres, called UV-induced intrinsic Fabry-Perot interferometers (IFPIs), and have demonstrated that the resulting fibre optic sensors have a greater range.

“We believe we could place many more sensor elements along a single fibre cable and these fibre cables, placed in different geographical areas, could be linked by a computer network,” said Anbo Wang, professor of electrical engineering at Virginia Tech and director of the Center for Photonics Technology. “In theory, many such computer ‘networks’ can be linked to form a nation wide system.”

Now Wang and his colleagues have received a $500,000 Sensor Initiative Research Grant from the US National Science Foundation (NSF) to develop ‘Highly Multiplexed Optical Fibre Sensing Networks for Infrastructure Monitoring’.

Infrastructure monitoring requires sensors that can cover a large area with minimum maintenance, are low in cost per unit, and have the ability to operate in harsh environments of different kinds. Semiconductor-based electronic sensors are low cost and can use wireless transmission to cover a large area. However, they are susceptible to electromagnetic interference (EMI), and restricted to relatively low temperatures of mostly below 125 C.

One solution is to combine electronic sensors with the more-expensive optical fibre sensors, which are insensitive to EMI, offer great resolution and accuracy, and have much higher temperature capability. A roadblock has been the rather limited multiplexing capability usually within several hundred sensors along a single fibre cable.

Wang’s group is confident that their sensor technology will increase the multiplexing capability by at least one order of magnitude. “In addition, this capability can be multiplied by many fold through sensor data fusion and computer networking so millions of sensors of different types in one system may become possible for real-time key infrastructure monitoring,” says Wang.

FP interferometers have been used for temperature, strain, pressure, electromagnetic field, and ultrasound sensing. The Virginia Tech experiments demonstrated that the new UV-induced IFPI fibre sensor can also measure different physical parameters, including temperature, strain, and pressure, and have the ability to operate at temperatures above 600 degrees C.

Sensor performance characteristics will be optimised through continued work on sensor fabrication and multiplexing using both mathematical modelling and experimental analysis.