Optical fiber sensing technology for smart structures
Aging degradation and seismic damage of civil infrastructure pose a serious problem for society. One promising technology for monitoring the condition of structures is optical fiber sensing. By embedding long optical fibers into structures as “nerves”, strain and temperature distributions along the fibers can be detected. If this artificial nerves or fiber-optic nerve systems function, the structures themselves react as human beings do, saying, “I have a pain here” and “It is too hot here.” Structures equipped with such artificial nerves, which enhance their maintenance efficiency, are called “smart structures.”
Our group is working on optical fiber sensing technology for implementing smart structures. We are especially focusing on the two topics: 1) distributed optical fiber sensing and 2) plastic (polymer) optical fiber sensing. In this page, we briefly describe these two topics along with 3) plastic fiber fuse effect, which was accidentally discovered in 2014.
Distributed optical fiber sensors
Optical fiber sensors that can measure strain, temperature, etc. at arbitrary positions along optical fibers are called “distributed optical fiber sensors.” Our distributed optical fiber sensors are based on optical frequency shift caused by Brillouin scattering. We are focusing on an original distributed sensing technique referred to as “Brillouin optical correlation-domain reflectometry (BOCDR).” This technique was invented by Assoc. Prof. Mizuno in 2008 (under the supervision of Prof. Hotate, who was a professor at the University of Tokyo and is currently the President at Toyota Technological Institute). BOCDR operates by light injection only to the single end of a sensing fiber and has high spatial resolution, random accessibility, and cost efficiency. Recently, real-time operation has also been achieved (see this movie), but we still need to tackle some difficulties to put BOCDR into practical use. Our first mission is to keep on developing distributed fiber-optic sensing technology with world-top performances (including BOCDR).。
References: Opt. Express 16, 12148 (2008); J. Lightwave Technol. 28, 3300 (2010); Light: Sci. Appl. 5, e16184 (2016).
Plastic (polymer) optical fiber sensors
Optical fibers used for sensing have mainly been silica glass optical fibers, which are widely used for telecommunication as well. However, as silica glass fibers break at relatively small strains of several percent, it was difficult to measure larger strains. To tackle this issue, we have been working on fiber-optic sensing using plastic (or polymer) optical fibers (POFs), which are sufficiently flexible to withstand over 100% strain. To date, we have succeeded in observing Brillouin scattering in POFs for the first time and clarified its numerous unique properties such as memory function and Brillouin frequency shift hopping. We have also demonstrated distributed Brillouin sensing along a POF for the first time in the world. In addition, our POF-related work includes fiber Bragg gratings (FBGs), long-period gratings (LPGs), and intermodal interference. Thus, our second mission is to develop novel POF-based sensing technology. We are in collaboration with research groups at Cyprus University of Technology and BAM, Germany.
※ For details, refer to the proceeding of IEICE Invited Talk (Japanese)
References: Appl. Phys. Lett. 97, 021103 (2010); J. Lightwave Technol. 32, 3999 (2014); Opt. Lett. 44, 2097 (2019).
Plastic optical fiber fuse and its applications
Optical fiber fuse is the continuous self-destruction of a fiber by propagating light. High-power light propagating through the fiber results in local heating and the creation of an optical discharge that is then captured in the fiber core and travels back along the fiber toward the light source, consuming the light energy. Its properties have been well investigated only in glass fibers (see Dr. Todoroki’s website). In 2014, we succeeded in observing the fiber fuse in plastic optical fibers (POFs) for the first time (see this movie). Since then, we have clarified the unique properties of the POF fuse, including its slow propagation velocity (1–2 orders of magnitude slower than that in silica fibers) and low threshold power density (1/180 of the value for silica fibers), etc. We have also shown that an oscillatory carbonized continuous curve is formed after the fuse passage, which exhibits electrical conductivity. These days, fused POFs have attracted considerable attention as novel optical materials, and their engineering applications have been extensively studied. We have recently developed a fused-POF-based high-sensitivity magnetic field sensor in collaboration with Brazilian and Portuguese groups.
References: Appl. Phys. Lett. 104, 043302 (2014); Sci. Rep. 4, 4800 (2014); Opt. Express 26, 12939 (2018); Adv. Photon. Res. 2, 2000078 (2021).