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Multimode fiber acts as a sensitive strain sensor

06 Aug 2007

Interference of higher order modes produces a strain sensitivity almost twice that of sensors based on fiber Bragg gratings.

The ability of optical fibers to act as strain sensors, using fiber Bragg gratings and microstructured fibers, has been covered previously on optics.org.

But Enbang Li of Tianjin University in China claims that he has developed a fiber-based strain sensor that's both more sensitive than a sensor based on a fiber Bragg grating (FBG), and simpler in design than a microstructured fiber sensor (IEEE Photonics Tech Lett, 19, 1266).

Li told optics.org that his experiment is based on so-called multimode interference (MMI) among the higher-order modes in a step-index multimode optical fiber (MMF). "The MMI-based sensor has an improved sensitivity over an FBG-based strain sensor by a factor of 1.92," Li said. "We also found that the strain polarity of the sensor is opposite to that of the FBG sensor."

In the experiment, a 50 mm segment of MMF was connected between two lengths of standard singlemode fiber using fusion splicing. A 1550 nm amplified spontaneous emission (ASE) source with a power of 5 mW and a bandwidth of about 30 nm provided the light source.

To make a direct comparison between the two types of sensor, an FBG-based sensor was also connected to the source and put under identical strain. Li found that as the tensile stress increased the FBG sensor produced red-shifts in wavelength, while the MMI sensor moved to shorter wavelengths.

"The most obvious benefits of the MMI sensor are its extremely simple structure, easy fabrication using normal splicing, compactness, and low cost thanks to the use of standard commercially available fiber," said Li. "As with FBG sensors, the MMI sensors are wavelength-encoded, in that the sensing information is detected and transmitted using the wavelength of light rather than the intensity."

The increased sensitivity and opposite strain polarity could help to eliminate the effects of temperature changes, which will also cause a shift in the output wavelengths irrespective of any applied strain.

"We are currently working on sensor packaging and wavelength demodulation, and we are also extending the MMI-based sensing from physical parameters like strain and temperature to biosensing and chemical sensing. Patents have been applied for, and we are looking at ways to commercialize the sensors," said Li.

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