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-με 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.
Highlights
To realize a sustainable society, one important technology is “structural health monitoring (SHM)”for social infrastructures such as buildings, bridges, highways, pipelines, and so on
In 2008, we proposed and demonstrated “Brillouin optical correlation domain reflectometry”, BOCDR for short, in which the spontaneous Brillouin scattering that is caused along the sensing fiber is measured position-selectively [9]
Fiber Bragg grating (FBG), which are located along a fiber portion of a 10 m length, with a total measurement speed of 10 discussion will be done in Section 4.4.3, which is related to the random accessibility of Brillouin optical correlation domain analysis (BOCDA)
Summary
To realize a sustainable society, one important technology is “structural health monitoring (SHM)”. 1989, we proposed the “synthesis optical coherence function (SOCF)”oftechnology, as lightwave is modulated, for example, with an appropriate periodical waveform, so that a periodical another way for distributed measurement [15,16,17] In this technology, the optical frequency of a delta function-like optical coherence function is synthesized. When theperiodical sensing fiber length isso shorter continuous lightwave is modulated, for example, with an appropriate waveform, that a than the period the synthesized optical coherence peaks,is position selective measurement periodical deltaoffunction-like optical coherence function synthesized. In 2008, we proposed and demonstrated “Brillouin optical correlation domain reflectometry”, BOCDR for short, in which the spontaneous Brillouin scattering that is caused along the sensing fiber is measured position-selectively [9]. Varieties of research studies and developments related to optical correlation-domain distributed fiber sensors have been accumulated in various institutions, and these examples are shown briefly
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