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Spliced fibres yield sensitive strain sensor

09 Apr 2008

The spherical microbubble formed when a single-mode fibre is spliced to a hollow-core photonic crystal fibre produces a high-precision strain sensor.

When a single-mode fibre (SMF) is joined to a microstructured photonic crystal fibre (PCF) using fusion splicing, an air-filled microsphere is created at the end of the SMF's core. Such air bubbles are normally to be avoided because they can introduce large insertion loss, but Engbang Li at China's Tianjin University has shown that they can be put to good use (App Phys Lett 92 101117).

"The micro-sphere has a perfect silica surface wall and aligns exactly with the core of the SMF," Li explained to optics.org. "Light transmitting in the core is partially reflected back by the front surface of the cavity, and also from the rear surface on the opposite side of the bubble. These reflections interfere with each other, forming a self-aligned Fizeau interferometer."

A Fizeau instrument uses two partially reflective surfaces to produce the reference and measurement beams. Changing the relative locations of the surfaces alters the interference pattern between the two beams.

In Li's device, applying an axial strain to the fibre segment containing the microsphere changes the length of the cavity and so varies the wavelength of the maximum or minimum interference. As a result, the wavelength shift can be used to determine the strain experienced by the fibre.

Li's team produced their interferometer by creating a microcavity 39 µm in diameter in a spliced fibre, and connected it to a broadband source having a central wavelength of 1550 nm and a bandwidth of 35 nm. A fibre Bragg grating (FBG) with a known strain sensitivity of 1.22 pm/µε was used to provide a reference for the new interferometer's performance.

"We demonstrated that our sensor has a sensitivity of 3.36 pm/µε, which is much higher than other fibre optic strain sensors based on FBGs," said Li. "The improvement is because the effective cross-sectional area of the fibre is reduced by the cavity. With the same 125 µm diameter fibre and the same tensile stress, our interferometer will produce larger elongation."

The presence of the microcavity inside the fibre appears not to adversely affect the maximum strain that the fibre can withstand. "In our experiments we found the microcavity to be quite robust, as it was formed under relatively strong arc fusion," commented Li. "We tested the sensor at strains larger than 3000 µε without breaking it."

One disadvantage of the sensor is the relatively weak interferogram it produces, due to the low reflectivity of the surfaces forming the interferometer. However, this will not affect the measurement accuracy as long as an interfering peak or trough can be identified.

The next steps will be to control the formation of the cavity, since the splicing conditions affect the size of the resulting bubble. "We believe that adding air pressure to the microstructured fibre could control the size, but no tests have been done on this," noted Li. "We will also work on packaging the sensors for practical applications, and seek possible routes to commercialization."

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