Fiber-optic ultrasonic sensing systems have been developed for structural health monitoring of composite structures by introduction of phase-shifted fiber Bragg gratings (PS-FBGs). The systems can achieve the compatibility of high sensitivity and broadband performance. First, PS-FBG balanced sensing system was developed and succeeded in detection of small acoustic emission signals of composite laminates. Next, erbium fiber ring laser sensing system with inbuilt PS-FBG was developed. This system has high robustness due to its self-adjustment function for environmental disturbances and achieved much higher sensitivity and ultra-broadband respondency than piezoelectric ceramic sensors. These systems have large potential to realize the ultrasonic SHM.
Researching high-sensitivity flexible ultrasonic sensor is important in the field of structural health monitoring (SHM). In
this research, a novel ultrasonic sensor based on fiber ring laser with an in-built phase shifted fiber Bragg grating (PSFBG)
is proposed and demonstrated. The first function of the PS-FBG is to determine the wavelength of the laser. Thus,
this sensing system is robust to temperature change and quasi-static strain change because the PS-FBG is always
illuminated. The other function of the PS-FBG is a sensor with ultra-steep slope and short effective grating length. It is
beneficial for achievement of high-sensitivity and broad-bandwidth ultrasonic detection. The experimental evaluated
sensitivity was 58.5±3 dB, which is 7.5 dB higher than traditional PZT sensor. This may be the highest sensitivity
obtained by optical fiber sensing system. Because of the advantages including robustness, simple structure and low cost
in addition to the high sensitivity and broad bandwidth, this sensing system has potential practical applications in
A novel acoustic emission (AE) sensor system based on fiber Bragg grating is proposed. Two identical phase-shifted
FBGs (PS-FBGs) are cascaded after an amplified spontaneous emission (ASE) source, used as filter and sensor
respectively. By measuring the change of the output power, the AE signal and low frequency vibration can be detected.
Because of the sharp peak in the middle of the PS-FBG spectrum due to the phase shift, the sensitivity of the system is
guaranteed while preventing the photo-detector from saturation. The proposed system has the advantages of simple
configuration, no-resonance, electromagnetic interference immunity and low cost.