Sensitivity of an extrinsic fabry-perot interferometric sensor with respect to the alignment direction of the sensor for detecting lamb waves

Lamb wave is a good method to detect some imperfections in a thin plate. In order to use this method, a sensor as well as an actuator is needed. Usually, a piezoceramic transducer is a good sensor and also a good actuator. Nowadays, fiber optic sensors are good alternative transducers for detecting Lamb wave as well as other ultrasonic waves. However, in the case of the fiber optic sensor, its sensitivity has directivity; that is, the sensitivity is variable according to the alignment direction of the sensor because the sensor dominantly measures the displacement induced by the change of gage length along the parallel direction to the sensor. Thus, considering the change of the sensitivity with respect to the alignment direction of the sensor to an ultrasonic source is essential in order to detect the ultrasonic wave using a fiber optic sensor and to determine the absolute amount of the measured value correctly. In this paper, the directivity of the fiber optic sensor was investigated through both a theoretical analysis and an experimental one. The theoretical analysis showed that the sensitivity was related to the alignment angle of the sensor and to the ratio (L/λ) of the gage length (L) of the sensor and the wavelength (λ) of the Lamb wave. In the experimental analysis, an extrinsic Fabry-Perot interferometric sensor was used for detecting the Lamb waves which were excited by a lot of piezoceramic transducers. One fiber optic sensor was attached on the center of the aluminum plate; otherwise these piezoceramic transducers were attached around the fiber optic sensor according to the alignment direction of the fiber optic sensor. Finally, the theoretical results were verified in the experimental analysis.