Sensitivity Enhancement of FBG Sensors for Acoustic Emission Using Waveguides

Sensors created from optical fibers are often used to measure strain due to their high accuracy. However, they are not used as often for higher frequency applications as a result of low sampling rates in their data acquisition systems. Recently, acquisition systems have been developed that satisfy the required specifications for use in acoustic emission (AE), but these types of sensors are not used due to their low signal-to-noise (S/N) ratios. However, there are many studies on the sensitivity enhancement conferred by the fiber Bragg grating sensor (FBG sensor). This sensor is easy to use and does not require specific reference fibers when sensing. In this study, acoustic waveguides, which are usually used in ultrasonic microscopes, were fabricated and used in FBG AE sensors to enhance the S/N ratio. Two types of waveguides were designed; the refraction type and the reflection type. FBG fibers were attached to the waveguides, which were then secured using adhesive to a thin aluminum plate. Impulses were inputted into the sensor by breaking pencil lead on the plate. The results showed that the amplitude of the first wave was increased using these waveguides. Using finite-element method (FEM) analysis, the enhancement of the wave amplitude was also evaluated such that the acoustic waveguide could be used for enhancement of the S/N ratio of FBG sensors.