Novel real-time acousto-ultrasonic sensors using two phase-shifted fiber Bragg gratings

A novel real-time acousto-ultrasonic sensor system using a phase-shifted fiber Bragg grating rather than a normal fiber Bragg grating was investigated. The spectrum of the phase-shifted fiber Bragg grating was simulated and analyzed, which indicates that a phase-shifted fiber Bragg grating has superior properties when compared to a normal fiber Bragg grating. Based on theoretical considerations, a novel real-time acousto-ultrasonic sensor system was proposed. Two identical 5-mm phase-shifted fiber Bragg gratings, with slightly different Bragg wavelengths, were used as the filter and the sensor. An amplified spontaneous emission light source and an apodized fiber Bragg grating were connected to illuminate the phase-shifted fiber Bragg gratings. The design allows the strain resulting from the ultrasonic wave to be precisely received and converted to the fluctuation of the output voltage. An ultrasonic wave generated in a carbon fiber–reinforced plastic plate using a macrofiber composite actuator was detected by both the phase-shifted fiber Bragg grating sensor system and a sensor system based on a normal fiber Bragg grating and arrayed waveguide grating. Comparison of the temporal and spectral responses of these two systems indicates that the phase-shifted fiber Bragg grating sensor system has a higher sensitivity than the fiber Bragg grating system. The results show that the phase-shifted fiber Bragg grating system does not require data averaging for noise reduction to measure the Lamb wave modes in a carbon fiber–reinforced plastic plate.