Modeling an Omnidirectional Fiber-optic Strain Transducer J. Agbodjan P., F. Kohl, H. Steiner, T. Sauter Center for Integrated Sensor Systems, Danube University Krems Wiener Neustadt, Austria just.agbodjan@donau-uni.ac.at F. Keplinger Institute for Sensor and Actuator Systems, Vienna University of Technology Vienna, Austria franz.keplinger@tuwien.ac.at M. Stojkovic, Z. Djinovic Austrian Center for Medical Innovation and Technology Wiener Neustadt, Austria zoran.djinovic@acmit.at Abstract—We present a modeling study devoted to an omni- directional fiber-optic transducer for elastic ultrasound waves. A coil-wound single mode fiber is attached to the surface of the device under test. The interferometric detection of strains is accomplished with the help of a reference coil, a 3x3 coupler, a laser or superluminescent LED, and two avalanche photodiodes. Since practical coil diameters of 25 mm are comparable to typical wavelengths of the mechanical excitation, we deal with distinct spatially distributed transduction. Finite element analyzes (FEA) demonstrate the achievable transducer sensitivity to Lamb waves in thin rigid plates. Moreover, FEA are capable to resolve minor distortions of wave propagation by the attached fiber coil. I. I NTRODUCTION The monitoring of the structural integrity of large areas of the shell of an aircraft using a minimum of sensors, draw many research group’s attention. It is highly relevant to investigate and to characterize damages in advanced materials like laminated carbon fiber reinforced polymer composites. Delamination, interlaminar debonding, micro-cracks, micro- buckling and inclusions are frequent damages in composite material. Delamination is a kind of invisible damage and, therefore, calls for the development of reliable damage detec- tion techniques. The actually available methods for material damage detection methods are based on radiation inspection, eddy current measurement, visual inspection technique, and ultrasound technique. The present contribution deals with the detection problem in connection with ultrasound waves that are typically excited with piezoelectric actuators. Lamb waves [1] are a form of elastic perturbation that can propagate in a solid plate with free surfaces. Lamb wave modes are sensitive to mechanical discontinuities, which may cause wave reflection, diffraction, or even mode conversion. Conventional Lamb wave inspection uses either pulse-echo or pitch-catch techniques with piezoelectric transducers for generation and detection [2], [3], [4], [5]. Contrary to piezo- electric sensing, fiber optic based detection techniques are insensitive to electromagnetic interferences induced, e.g., by the piezoelectric actuators. Fiber-optic transduction offers a high bandwidth, excellent sensitivity and a lack of strong mechanical resonances. The most common fiber-optic sensor proposed for Lamb waves detection is the fiber Bragg grating (FBG). FBGs are wavelength-shift coding transducers that can be applied to both strain and temperature monitoring [6] if directly bonded to the host material. They are very sensitive to strain in the grating region, whose length is several centimeters. However, this high sensitivity is achieved only for strains oriented parallel to the fiber axis. In contrast, the transducer described below offers a high and uniform sensitivity in arbitrary directions. It comprises a conventional single mode fiber with a mirrored end face that is coiled to form a flat spiral which is fixed to the plate under investigation. II. TRANSDUCTION SETUP A system overview of the transduction method is illustrated in Fig. 1. The arrangement is built around a 3x3 fiber cou- Fig. 1. Essential components of the fiberoptic transduction setup. PD1, PD2 - photodiodes, LC - light source, IMG - index matching gel, the sensing coil is exposed to an external perturbation as indicated. pler. Light from a laser or a superluminescent light emitting diode is guided to the fiber-optic coils. The third output is terminated by an index matching absorber. The light hitting the photodiodes comprises the reflected wave of the same fiber and, additionally, a corresponding component received via fiber-to-fiber transmittance between neighboring fibers in the coupler. Since the optical length of the sensing path matches that of the reference path, reflected light portions from both fiber coils interfere at both photodetectors PD1, PD2. A deformation of the plate leads to a change of the time needed for forward and backward propagation of guided light in the measurement or sensing coil. Both photodetector signals vary then according to the changing phase retardation which in turn is coupled to the plate deformation. The 3x3 fiber 978-1-4799-5326-4/14/$31.00 ©2014 IEEE