Although demand for composite structures rapidly increase due to the advantages in
weight, there are few effective assessment techniques to enable the quality control and guarantee
the durability. In particular, an invisible microscopic damage detection technology is highly
required because damages such as transverse cracks, debondings, or delaminations can lead to the
critical failure of the structures. Among many non-destructive evaluation (NDE) methods for
composite structures, fiber optic sensors are especially attractive due to the high sensitivity, the
lightweight, and the small size. In the current trend of the structural health monitoring technology,
fiber Bragg gratings (FBG) sensors are frequently used as strain or temperature sensors, and
Brillouin scattering sensors are also often used for a long distance distributed measurement. The
Brillouin distributed sensors can measure strain over a distance of 10km while a spatial resolution
was limited to 1m. Some novel sensing method is proposed to improve the spatial resolution. The
pulse-prepump Brillouin optical time domain analysis (PPP-BOTDA) is one of the latest distributed
sensing applications with a cm-order high spatial resolution. The PPP-BOTDA commercial product
has the spatial resolution of 10cm, and can measure the strain with a precision of ±25og. This
precision, however, can be achieved by using conventional single-mode optical fibers. In our
research, small-diameter optical fibers with a cladding diameter of 40om were embedded in the
CFRP laminate to avoid the deterioration of the CFRP mechanical properties. Thus, in order to
verify the performance of PPP-BOTDA, the distributed strain measurement was conducted with the
small-diameter optical fibers embedded in the CFRP laminate.