17 Feb 2006
A simple strain sensor with a high dynamic range combines the advantages of polymer and silica optical fiber.
Scientists in Australia, China and Hong Kong have come up with a simple strain sensor that combines a polymer fiber Bragg grating (FBG) and a silica long period fiber grating (LPFG). The resulting device not only has a small number of components but also offers a high dynamic range thanks to the combination of fiber types.
Bragg gratings written in silica single-mode optical fiber have found uses in applications ranging from underwater acoustics and smart structure monitoring to chemical and biological sensing. Despite this widespread adoption, they have all suffered from low sensitivity.
The problems stem from the fact that glass is a stiff material [with a large Young's modulus] and small thermal expansion coefficient. This means the Bragg wavelength of silica gratings cannot be easily changed by either mechanical or thermal means.
"Polymer's modulus is typically 30 times less than that of silica, making its stress sensitivity more than 30 times larger than silica's," said Gang-Ding Peng of the Photonics and Optical Communications Group at the University of New South Wales. "Polymer's breakdown strain is typically 10 times larger making its dynamic range as strain sensor 10 times larger."
The sensor, which is being developed by the New South Wales group and researchers from Chongqing University, China; and the City University of Hong Kong uses a polymer optical FBG as the sensor head and a LPFG as the wavelength shift interrogator.
The team launches a broadband light source into the grating via an isolator and a fiber coupler. One part of the wavelength component that is reflected from the polymer grating is directed onto the LPFG and collected by a photodetector. The other part of the reflected signal is directed straight onto a second photodetector.
The strain signal is then determined by the filtering action of the LPFG, which converts the wavelength shift of the reflected signal into the output voltage.
The output signal is obtained in electrical form by dividing the voltage from the LPFG photodetector by the voltage from the second photodetector. The researchers say the division is important because it eliminates the effect of the broadband source intensity fluctuations present in the signals entering both photodetectors.
The team believes it is too early to market the sensor. The next step is to improve the manufacturing technique to achieve a relatively high dynamic range in the simplest configuration. The researchers are also working on large strain sensing and biomedical engineering applications, including temperature sensing.
Paul Grad is a freelance journalist based in Australia.