## 7 Conclusions  
Brillouin optical correlation-domain technologies have been reviewed as fiber optic nerve systems for structural health monitoring (SHM) applications. Continuous lightwaves are utilized in the technologies, whose optical coherence properties are synthesized to realize the position-selective measurement along the sensing fiber. The synthesis of optical coherence function (SOCF) technology has also been reviewed, as the basic principle for the Brillouin optical correlation-domain technologies. Distributed or multiplexed sensing schemes utilizing the SOCF technology, which could be used for SHM, have also been discussed. In the Brillouin optical correlation domain analysis (BOCDA) systems and the Brillouin optical correlation domain reflectometry (BOCDR) systems, superior performances have been demonstrated for the individual performance indicators, such as a spatial resolution of 1.6 mm, a measurement speed of 5000 points/s with the random accessibility, and a strain dynamic range of 7000 \(\mu\epsilon\). One other important indicator is the ratio between the total measurement length and the spatial resolution. As this indicator, ratios of 1,000,000:1 and 134,000:1 have been demonstrated in a BOCDA system and a BOCDR system, respectively. In these systems, the performance improvement schemes have been introduced, such as the intensity modulation scheme and the differential measurement scheme (the phase modulation scheme) to reduce the background spectrum, and the temporal gating scheme and the double modulation scheme to elongate the measurement range. However, studies on the limitations of the total performances of the BOCDA and BOCDR systems have not been sufficiently accumulated. Mutual relations between the various performance indicators, such as the spatial resolution, measurement length, measurement speed, random access speed, strain dynamic range, strain accuracy, and so on, should be studied in detail to make sure of the total performance limitations. Recently, we have been studying the total performance of a BOCDA system mainly from the viewpoint of random accessibility. Performances of a seven-cm spatial resolution,  
Figure 46: Experimental verification of the spatial resolution improvement by the intensity modulation (IM) scheme [109]. In the experiment, a fiber portion is put into the cooled water, and the portion length is changed. The correlation peak position is set at the center of the cooled portion. (**a**) BDG reflection spectrum obtained without using the IM, which has the two peaks corresponding to the room temperature and the cooled fiber portion. (**b**) Plots of the two peaks’ power shown in Figure a. The crossing point of the two curves shows the equivalent spatial resolution. (**c**) BDG reflection spectrum obtained with using the IM. (**d**) Plots of the two peaks’ power shown in Figure c. The crossing point shows the equivalent spatial resolution. 1000-m measurement range, 2500-times/s random access speed, and 80-\(\upmu\epsilon\) strain accuracy has been totally demonstrated. However, more studies on the total performances or the limitation of the system performance need to be done, at this moment, in order to enhance the varieties of applications of BOCDA and BOCDR technologies. Studies to simplify the system configuration should also be performed. We have the possibility to do so while also keeping the system performance. An example was shown briefly, in this paper, in the subsection of the phase modulation scheme for the BOCDR systems. As for the BDG-BOCDA system, basic research items still remain, such as ways to enhance the spatial resolution in the measurements of the BDG reflection spectrum. The Brillouin optical time-domain technologies have already demonstrated systems that have quite a long measurement range beyond 100 km, with a 10-m order spatial resolution. For applications that require such a long measurement range, such as for example the SHM of pipelines, BOTDA systems are suitable.