Directional torsion sensor based on aluminum-coated tapered capillary hollow core fiber

Abstract

Based on capillary hollow core fiber (CHCF) that has a tapered section, a Mach-Zehnder interferometer is constructed to measure torsion, and it is capable of discriminating torsion direction using a signal processing method. A novel technique of splicing and tapering CHCF is developed to construct this sensor, using a standard splicer, where a section of SMF is spliced to 1 mm of MMF and then to 1.5 mm of CHCF; the process is repeated to have two identical fiber structures. This original program splices these fiber structures (both sections of CHCF) and then tapers to a waist of 30 μm. During this process, the central hollow does not exhibit deformation or collapse. For clockwise (CW), the torsion sensitivity is 0.674 ± 0.009 dB/(rad/m) in the range from 0.727 to 5.091 rad/m; for counterclockwise (CCW), the torsion sensitivity is −0.918 ± 0.012 dB/(rad/m) in the range from −4.363 to −0.727 rad/m. The performance of the sensor is upgraded by depositing two diametrically opposed aluminum films (30 nm). For CW, the torsion sensitivity is 4.270 ± 0.093 dB/(rad/m) in the range from 0 to 2.909 rad/m; for CCW, the torsion sensitivity is −4.762 ± 0.107 dB/(rad/m) in the range from −2.909 to 0 rad/m. Applying the Fast Fourier transform (FFT) to the transmission spectra of the coated sensor is feasible to distinguish the torsion direction. The proposed fiber structure used standard equipment for its construction, low-cost fiber, and simple fabrication, and it is efficient in identifying the twist direction. All these qualities make the sensor adequate for structural monitoring.