## 1 Introduction  
The application of non-destructive structural evaluation techniques to the structural condition monitoring of composite materials is useful to investigate the material properties, and find defects, without inducing sample breakage. Ultrasonic inspection, acoustography, low-frequency methods, acousto-ultrasonic inspection, shearography, and radiographic inspection have previously been tried as such evaluation techniques [1]. However, because of the size and weight of these technique systems, it is hard or impossible to use such evaluation techniques in an operational structure. Nondestructive techniques with optical signals have been recently suggested to overcome these drawbacks using fiber Bragg gratings as point sensors [2, 3], or the optical fiber itself as a distributed sensor [4]. Sensing optical fibers are small and lightweight, and can either be attached to the surface of the tested structure, or embedded inside, acting in many ways as the equivalent of the human nervous system, while having little impact on the structure itself [1, 5]. Standardtelecommunication fibers, despite being very small, have a size of 125 \(\upmu\)m, which is much larger than that of the commonly used composite fibers of (5-10) \(\upmu\)m for carbon fiber. However, when standard telecommunication fibers are embedded in parallel to composite fibers, they exhibit minimal intrusive behavior, similar to that of composite fibers [4].  
Two of our previous papers applied optical nondestructive techniques using Brillouin scattering in optical fiber to composite materials, and the residual strain measurement after external impact was successfully reported for the first time [6; 7]. Other authors had earlier found that, because of the difficulty of detection and strength degradation of composite materials, the detection of barely visible impact damage (BVID) caused by low-velocity impacts was a major obstacle to detecting the structural damage to composite materials [8; 9; 10]. Our next paper then showed that it was possible to detect the BVID by measuring the residual strain in aluminum-coated optical fiber after impact events [11]. In our most recent paper, we embedded a standard single mode optical fiber in composite material, and measured the residual strain as a two-dimensional damage image according to the external impact energy [12].  
In recent years, fiber optic Brillouin scattering sensor technologies have been dramatically improved to measure the strains distributed along sensing optical fibers. These technologies used the stimulated Brillouin scattering (SBS) effect, which occurs due to the correlation between the probe and pump lights traveling in opposite directions. One of these sensing technologies is termed Brillouin optical correlation domain analysis (BOCDA) [13]. Our previous works applied this BOCDA sensing measurement system to damage detection of composite materials [11; 12]. However, because the system must use a fixed optical delay line selected by calculation, freely changing measurement spatial resolution is limited for this technology. We proposed the system without an optical delay line, which is called time differential (TD)-BOCDA, and demonstrated that the system can measure the sensing fiber while freely varying the spatial resolution without changing the system hardware [14].  
Although our group first reported the two-dimensional damage mapping technique of composite materials using the conventional BOCDA technique, the spatial resolution was low, because a 2 Gigabit per second (Gbps) modulated optical signal was used. In this study, we applied a 16 Gbps light signal of eight-fold higher modulation speed to improve the spatial resolution. Furthermore, we also used the novel measurement system without an optical delay line, called time differential (TD)-BOCDA, for damage detection in an optical-fiber-embedded composite cylinder. Thus, following the impacts on the cylinder, the damage maps were drawn and compared using different spatial resolutions.