Fiber-Optic Interferometric Sensor Based on the Self-Interference Pulse Interrogation Approach for Acoustic Emission Sensing in the Graphite/Epoxy Composite

Fiber-optic acoustic emission sensors are highly attractive for non-destructive testing applications in composites due to their small size, galvanic isolation, and immunity to electromagnetic interference (EMI). The significant part of such sensors is based on the Fabry-Perot interferometers (FPIs) because of their small size, high sensitivity, and their ease of embedding into a composite structure. However, linear demodulation techniques of their signals suffer from the limited dynamic range and require an additional operating point control. In this paper, we propose the novel self-interference interrogation approach for the fiber-optic FPI-based acoustic emission sensor, which allows the FPI to be interrogated as a two-beamlike interferometer with current-induced frequency modulation of the light source. Then, with using of a PGC-ATAN demodulation algorithm or similar, phase signals can be measured independently of the interferometer operating point with the extended dynamic range. The sensor is based on the fiber-optic FPI that consists of the optical fiber segment bounded by two TiO2 mirrors: the semitransparent one (20%-25% reflecting and -1.6-dB loss) in the fiber and the external one (99.9% reflecting) at the fiber tip. The mirrors are spaced 7 mm apart. Two FPI sensors were fabricated and experimentally investigated. One sensor was mounted on the surface of the graphite/epoxy composite plate, and another one was embedded into its structure. The experimental investigation results of two considered interferometric acoustic emission sensors in comparison with the reference piezoelectric transducer GT301 are presented. Obtained results confirmed the efficiency of the proposed interrogation approach in terms of FPI usage as a two-beamlike interferometer for acoustic emission signals detection and might be used for the implementation in existing and new FPI-based acoustic emission sensing systems.