## 2 Fiber Optic Nerve Systems for Structural Health Monitoring  
Figure 1 shows schematically smart structures and smart materials that can feel their pain, based on the fiber optic nerve systems. The fiber optic nerve systems can also be used, besides the social infrastructures, for airplane bodies, trains, ships, and natural structures, as shown in Figure 1.  
Figure 2 shows fiber sensing systems that can act as the fiber optic nerve systems. Figure 2a-c show, respectively, multiplexed, distributed, and quasi-distributed fiber optic sensing systems. In the multiplexed one, a number of point sensors are set along an optical fiber, which sense, for example, the strain and/or temperature applied to the fiber. As the sensing element, fiber Bragg gratings (FBGs) are mainly used [1]. However, in this technology, we cannot have any information regarding when events take place at the fiber portion without setting the sensing element. From such a point of view, the distributed sensing systems shown in Figure 2b are more suitable. In the technology, the scattering phenomena caused in the fiber are mainly utilized as the sensing mechanism, such as Brillouin, Raman, and Rayleigh scattering. In the case where we do not need to have fully distributed information along the fiber, but we cannot expect the points to be measured exactly, quasi-distributed sensing technologies make sense. In this technology, for example, long-length FBGs are used as distributed sensing elements, which are set along the fiber as shown in Figure 2c. Figure 1: Fiber optic nerve systems to realize safety and security in the society. In this paper, the distributed sensing technologies using Brillouin scattering are mainly discussed, where the correlation-domain technologies are used as the distributed sensing mechanism. The multiplexed and the quasi-distributed sensing technologies, which utilize the same distributed sensing mechanism, are also discussed. As distributed sensing ways for the backscattering and reflections caused in fibers, time-domain pulsed lightwave technologies have been proposed and mainly used historically, which are called optical time domain reflectometry, or OTDR for short [1]. The main application of the technology was the diagnosis of fibers in telecommunication systems. Other ways, such as a frequency modulation (FM)-radar like technology, had also been studied as methods of distributed sensing [1]. In 1989, we proposed the \"synthesis of optical coherence function (SOCF)\" technology, as another way for distributed measurement [15; 16; 17]. In this technology, the optical frequency of a continuous lightwave is modulated, for example, with an appropriate periodical waveform, so that a periodical delta function-like optical coherence function is synthesized. When the sensing fiber length is shorter than the period of the synthesized optical coherence peaks, position selective measurement along the fiber can be realized. By additionally applying intensity modulation and phase modulation onto the lightwave, a periodically but arbitrarily-shaped optical coherence function can be synthesized by the SOCF technology [14]. Distributed measurement of, for example, Rayleigh scattering and Bragg reflection frequency along long-length FBGs have also been demonstrated with the SOCF technology. This technology requires neither short pulses nor post-data calculations. Next, we proposed \"Brillouin optical correlation domain analysis,\" or BOCDA for short, in 1999, in which stimulated Brillouin scattering was excited position-selectively along the fiber, by applying the SOCF to the continuous pump and the proven lightwave, which travel in opposite directions in the fiber [8]. Basically, the BOCDA output shows the Brillouin gain spectrum (BGS) which was caused at a selected position along the fiber, and the position can easily be changed by the FM frequency. Therefore, \"random accessibility\" to multiple points that are selected arbitrarily along the fiber can also be realized in BOCDA.