## 5 Conclusions  
In order to apply the fiber optic TD-BOCDA sensor system for impact damage detection of a composite cylinder, a composite cylinder with an embedded optical fiber was fabricated through the filament winding process so that the optical fiber was positioned between the 90\({}^{\circ}\) hoop carbon layers to have an interval of 12 mm in the longitudinal direction of the cylinder. The optical fiber was wound 160 turns around the circumference of the cylinder, and the total straight length was about 125 m. The residual strain in the cylinder was measured using the proposed TD-BOCDA sensor system. Both the optical pump and probe in the system were phase-modulated in this measurement system using the same pseudo random bit sequence (PRBS) code pattern of 215-1, but with a bit delay relationship between the two. Modulation rates of up to 16 Gbps were applied, which was an eight-fold increase over our past optical-fiber-embedded composite experiments. This made it possible to measure a spatial resolution of 6.25 mm.  
A total of twelve impact events of 5, 10, 15, and 20 J were applied to the cylinder using a drop weight machine with a hemispherical impactor. Before impact, the Brillouin frequencies of the embedded optical fiber were measured, and the measurements showed well the stress environment of the optical fiber at each location, including different winding pressure and fiber adaptor pressure. After impact, the frequencies were again measured, and the residual strain was obtained using the difference of both the frequencies, and the proportionality constant. The damage images were derived by mapping this residual strain onto the structure of the cylinder. These damage maps were measured and compared using three spatial resolutions of 20, 10, and 6.25 mm. There were no hardware changes in the measurement system, such as needing to replace the optical delay line when changing the modulation rate to adjust the resolution. This is a clear advantage of the proposed  
Figure 9: The integrated residual strain according to the applied impact energy.  
system compared to the conventional BOCDA sensor system. The expansion deformation in the measured damage map due to the impact was measured at all impact points, and the impact locations on the map were observed to be exactly the same as their actual locations on the cylinder. As the measurement resolution increased, the damage showed a point-symmetrical shape on a two-dimensional plane more clearly. Furthermore, the total amount of measured residual strain at each impact point was linearly proportional to the applied impact energy.  
Conceptualization, I.-B.K.; methodology, B.-H.C.; investigation, D.-C.S.; resources, B.-H.C. and I.-B.K.; data curation, B.-H.C.; writing--original draft preparation, B.-H.C.; writing--review and editing, I.-B.K.; experiment, Y.-S.K. and D.-C.S.; supervision, I.-B.K.; project administration, I.-B.K.; funding acquisition, I.-B.K. All authors have read and agreed to the published version of the manuscript.  
This research was supported by \"Development of photonic sensing technology for anchor and composites integrity evaluation\", funded by the Korea Research Institute of Standards and Science (KRISS-2021-21011092). It was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2018R1A2B6003752).  
Not applicable.  
The authors declare no conflict of interest.