Brillouin Optical Correlation-Domain Technologies Based on Synthesis of Optical Coherence Function as Fiber Optic Nerve Systems for Structural Health Monitoring

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.

Distributed cladding mode fiber-optic sensor

“…The generator was programmed to repeatedly apply a 43-bit-long binary sequence, with a symbol duration of 800 ps. Phase coding restricted the Brillouin interaction between the two pumps to correlation peaks of 8 cm width [61,62]. The optical frequency of the pumps source could be scanned in a few megahertz increments using a suppressed-carrier single-sideband (SC-SSB) electro-optic modulator, driven by the output of a microwave generator.…”

“…Several mitigation schemes have been developed as part of B-OCDA setups and may be applicable to distributed cladding mode sensing as well. These include time-gating of pump pulses [61,62,64], and dual-layer hierarchal coding of both amplitude and phase [65]. In addition, off-peak Brillouin interactions may be suppressed by changing B (z) along the fiber.…”

“…Distributed sensing, however, requires that the Brillouin interactions be confined to discrete and narrow fiber sections of arbitrary locations. Several schemes for the localization of steady-state SBS have been proposed towards distributed sensing of temperature and strain [58][59][60][61][62]. In those protocols, which came to be known as Brillouin optical correlation domain analysis (B-OCDA), the two input optical waves are jointly modulated in frequency or phase [58][59][60][61][62].…”

“…Therefore, BDGs induced by phase-coded pumps would provide dynamic, random-access coupling between core and cladding modes in arbitrarily located, short fiber segments. Phase coding in SBS is addressed with more detail in Supplement 1 and is discussed at length in numerous references [46,52,[60][61][62]. The technique serves in distributed sensing of media outside the fiber cladding with a few-centimeter spatial resolution, as described next.…”

“…Several schemes for the localization of steady-state SBS have been proposed towards distributed sensing of temperature and strain [58][59][60][61][62]. In those protocols, which came to be known as Brillouin optical correlation domain analysis (B-OCDA), the two input optical waves are jointly modulated in frequency or phase [58][59][60][61][62]. The concept was successfully carried over to BDGs in polarization-maintaining fibers [44][45][46]52].…”