###### Abstract  
Brillouin optical correlation-domain technologies are reviewed as \"fiber optic nerve systems\" for the health monitoring of large structures such as buildings, bridges, and aircraft bodies. The Brillouin scattering property is used as a sensing mechanism for strain and/or temperature. Continuous lightwaves are used in the technologies, and their optical coherence properties are synthesized to realize position-selective measurement. This coherence manipulation technology is called the \"synthesis of optical coherence function (SOCF)\". By utilizing SOCF technologies, stimulated Brillouin scattering is generated position-selectively along the fiber, which is named \"Brillouin optical correlation domain analysis (BOCDA)\". Spontaneous Brillouin scattering, which takes place at any portion along the fiber, can also be measured position-selectively by the SOCF technology. This is called \"Brillouin optical correlation domain reflectometry (BOCDR)\". When we use pulsed lightwaves that have the position information, sensing performances, such as the spatial resolution, are inherently restricted due to the Brillouin scattering nature. However, in the correlation-domain technologies, such difficulties can be reduced. Superior performances have been demonstrated as distribution-sensing mechanisms, such as a 1.6-mm high spatial resolution, a fast measurement speed of 5000 points/s, and a 7000-\(\upmu\)c strain dynamic range, individually. The total performance of the technologies is also discussed in this paper. A significant feature of the technologies is their random accessibility to discrete multiple points that are selected arbitrarily along the fiber, which is not realized by the time domain pulsed-lightwave technologies. Discriminative and distributed strain/temperature measurements have also been realized using both the BOCDA technology and Brillouin dynamic grating (BDG) phenomenon, which are associated with the stimulated Brillouin scattering process. In this paper, the principles, functions, and applications of the SOCF, BOCDA, BOCDR, and BDG-BOCDA systems are reviewed, and their historical aspects are also discussed.  
distributed optical fiber sensors; optical correlation-domain technologies; Brillouin scattering +
Footnote †: journal: Applied Soft  
the time-domain techniques have inherent difficulties. To have a high spatial resolution, a high-speed data acquisition system is necessary. Especially, when we use fiber Brillouin scattering as a sensing mechanism for strain and/or temperature, the time-domain technologies show trade-off between the spatial resolution and the measurement accuracy.  
On the contrary, our group proposed and developed other ways of distributed measurement that can reduce the above-mentioned difficulties. These are called \"Brillouin optical correlation-domain technologies\" [8; 9]. In the technologies, continuous lightwaves are utilized, whose interference or optical-coherence properties are synthesized to realize position selective measurement along the sensing fiber. This coherence manipulation technology was also proposed and developed in our group, which is called the \"synthesis of optical coherence function (SOCF)\" [13; 14]  
In this paper, Brillouin optical correlation-domain technologies based on the synthesis of optical coherence function are reviewed, as are the fiber optic nerve systems, and the historical aspects are also described.