2. Wuhan National Lab for Optoelectronics (WNLO) & National Engineering Laboratory for Next Generation Internet Access System, School of Optics and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
Overview: The paper introduced four primary approaches for fiber core-rod preform fabrication: the MCVD (modified chemical vapor deposition) process, the PCVD (plasma chemical vapor deposition) process, the OVD (outside vapor deposition) process and the VAD (vapor axial deposition) process. Due to the flexible control of sophisticated refractive index profile and the capability of rare-earth element doping, the PCVD and MCVD processes are widely utilized for specialty fiber preform fabrication.
The progresses of specialty fibers used for sensing were reviewed, organized in four sections, namely, specialty fibers based on polarization, functional specialty fibers as a gain medium for fiber-optic sensing, specialty fibers for harsh environment, and fibers for distributed sensing. The reduced-diameter polarization-maintaining fibers (PMF), such as PMFs with 80 μm (cladding)/135 μm (coating) and 60 μm/100 μm, have been developed to meet the demand of smaller-size gyroscope applications with comparable mechanical and optical performance. YOFC has successfully fabricated high-birefringence spun fiber for fiber-optic current transformer (FOCT) applications. The results of tested FOCT system with the spun fiber coil as the sensing element showed great agreement with temperature change; Rare-earth doped fibers, which are an essential part of fiber amplifiers and fiber lasers, have achieved high absorption coefficient and high light/light conversion efficiency for lasers. The 20/125 double-cladding ytterbium-doped fiber fabricated by YOFC has ensured a laser output of M2 < 1.5; Specialty fibers for harsh environment use also play a crucial role in fiber-optic sensing in various extreme environment applications. High strength bend-insensitive fibers have been developed in order to meet the requirement of optical hydrophone applications with 200 kpsi proof test and bending radius as small as less than 5 mm. The radiation hard fibers with pure silica core produced by YOFC have passed the test in the environment of up to 10000 Gy. For high temperature resistant fibers, test results showed the polyimide-coated fiber was able to withstand as high as 300 ℃ and also ensure the optical and mechanical properties; Additionally, the paper reviewed a number of novel specialty fibers that have been used in distributed sensing applications, such as the optimized multimode fiber (MMF) for distributed temperature sensing (DTS), few-mode fiber (FMF) for DTS. The results showed the DTS system employing the optimized MMF and FMF could effectively extend the measurement range without deterioration of the spatial resolution and temperature resolution. Also, the Brillouin optical time domain analysis systems based on multicore fibers and FMF have been reported for distributed shape sensing.
In conclusion, the paper reviewed the progress of specialty fibers used for fiber sensing and envisioned the trend of specialty fibers to be more compact, more robust, and suitable for extreme environment.