A fiber-optic ultrasonic sensing system was constructed with fiber Bragg gratings based on the intensity modulation of light reflected from fiber Bragg grating sensors. This fiber-optic system consists of fiber Bragg gratings for sensing and filtering, a broadband light source as well as a photo detector. The feasibility of damage monitoring using the fiber-optic system was investigated. Transient ultrasonic Lamb waves generated with a piezoceramic pulser were propagated in a cross-ply CFRP with visible impact damage. The response of the fiber Bragg grating sensor to the Lamb wave propagated through the damaged area was compared with a reference response in an intact area. Frequency characteristics of the response signal were analyzed in order to evaluate Lamb wave interaction with damage. Moreover, Lamb wave detection using a piezoceramic sensor was performed to compare with the response of the fiber Bragg grating sensor. The experimental results demonstrated that fiber Bragg grating sensors are comparable to piezoceramic sensors in ultrasonic detection and the fiber-optic system proved to be effective in impact damage detection of CFRP.

The influence of grating length and bend radius of long-period gratings (LPGs) on refractive index sensing was examined. Sensitivity to refractive indexes smaller than that of silica could be enhanced by bending LPGs. Bent LPGs lost sensitivity to refractive indexes higher than that of silica, whereas a 20-mm-long LPG arranged in a straight line had considerable sensitivity. These experimental results demonstrated that the sensitivity characteristics of LPGs to refractive index could be controlled by grating length and bend radius.

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