## 4 Damage Detection Results by TD-BOCDA Sensor System
Segment (4) shows the frequencies for another different SSMF attached to an optical isolator, which was explained as the end of the sensing fiber in Figure 2 of the experimental setup. Its manufacturing company is also different, and so the frequencies differ from those of segments (2) and (3). At the border of  
Figure 3: Photo of the drop weight impact machine equipped with an optical-fiber-embedded composite cylinder. the segments (3) and (4), there is a sharp frequency shift. This shift to low frequency means that compression stress was applied to this location. At its real position, an adaptor for fiber connectors was used to connect these two different SSMFs, and the adaptor mechanically applies a compressive force to the optical fiber at this position. This result means that the TD-BOCDA system can detect the optical fiber adaptor points. In segment (5), the frequency is not specified. This is because, due to the isolator, the pump can no longer propagate to this segment, so there is no peak gain frequency, but only noise. The Brillouin frequencies after impact events were also measured, and the top graph of Figure 5 plots the frequencies before and after impact as solid and dashed lines, respectively. The graph is enlarged for the position of around 21 m out of a total of 125 m, and this is the position impacted by 20 J. The Brillouin frequency difference was measured using three different phase modulations of 5, 10, and 16 Gbps, which correspond to the spatial resolutions of 20, 10, and 6.25 mm, respectively. When the resolution is varied, nothing in the experimental setup needs to be changed. This is the very strong advantage of our TD-BOCDA system. For the conventional BOCDA systems, in order to change the spatial resolution, the optical delay line has to be replaced; moreover, the real fiber location has to be recalculated for the changed phase modulation rate. However, for the TD-BOCDA, no optical delay is needed, and the fiber location only needs to be transferred to their points according to other modulation rates. The middle and bottom graphs give the difference between the two Brillouin frequencies before and after impact for the 5 and 16 Gbps modulations. The peaks in the graphs show the effects after external impacts. These shifts to higher frequency mean that the expansion deformation remains at the impact point. The intervals between these peaks in the graphs are exactly 786 mm, which corresponds to one circumferential turn of the embedded optical fiber around the cylinder. The linewidth of the peaks in the graph with 16 Gbps modulation is much narrower than that in the graph  
Figure 4: The Brillouin frequency measured throughout the whole fiber locations of 125 m (bottom graph), and enlargement of the fiber end location (top graph), with the Brillouin gain spectrum (inset). with 5 Gbps, because the graph with 16 Gbps has more resolvable measurement points than that with 5 Gbps. These one-dimensional measurement points in Figure 5 were cut into 79 cm segments of the length of the cylindrical circumference, and Figure 6 rearranges them along the vertical axis of the graph, so this figure realizes the damage map for the cylinder surface of 80 cm \(\times\) 190 cm. In Figure 6a, the photo of the cylinder and the points of impact on its surface are marked with white circles, and the amount of impact applied to each point is also marked on the circles. The impact group of 5, 10, 15, and 20 J was repeated three times, as previously mentioned. Figure 6b shows the damage map with 16 Gbps measurement. Figure 5: The frequencies before and after impact as solid and dashed lines (**a**), and the difference between both frequencies using 5 Gbps measurement (**b**), and 16 Gbps measurement (**c**). The graph means that the frequency shift increases from purple to red in the order of the colors of the rainbow. All impact points are clearly visible, except for the 5 J impacts in groups 2 and 3.