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We demonstrated a torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of two identical cascaded long-period fiber gratings (CLPFGs) inscribed in two-mode fiber (TMF) by high-frequency CO 2 laser. The transmission and mode coupling performances, as well as the spectral properties have been proved theoretically and experimentally. The proposed TMF-CLPFGs could not only be used as an efficient multi-wavelength LP 01 -LP 11 mode converter, but also achieve the vector torsion measurements. As the twist induced, the proposed TMF-CLPFGs present vector torsion ability through the intensity of interference dips varying. The torsion sensitivity of TMF-CLPFGs with spacing length 1 cm can reach −1.07 dB/(rad/m) in counterclockwise direction and 0.9 dB/(rad/m) in clockwise direction. Moreover, the temperature sensitivity of TMF-CLPFGs was measured to be 0.055 dB/°C, which could be negligible to torsion measurement. Therefore, the proposed TMF-CLPFGs could be potential in few-mode fiber-based communication systems and sensing fields. [ABSTRACT FROM AUTHOR]

A high-sensitivity bending sensor based on dual-dip long period fiber gratings (LPFGs) in a single-mode fiber is proposed. The length of the gratings is very short only 7.2 mm. The asymmetric perturbation in fiber is generated during the fabrication, which makes the dual-dip LPFGs show different bending sensing characteristics at opposite directions. Meanwhile, the highest bending sensitivity of −35.91 nm/m−1 and −20.74 dB/m−1 can be obtained at bending directions of 0° and 180°, respectively. Furthermore, the torsion and temperature sensing characteristics are investigated experimentally. The torsion sensitivities follow sine function showing potential for directional sensing. The temperature sensitivities are 41.9 p.m./°C and 45.1 p.m./°C are close, which can eliminate the cross-sensitivity to temperature. • A high-sensitivity bending sensor based on dual-dip long period fiber gratings. • The directional bending is realized by asymmetric electric arc discharge during the fabrication. • The torsion sensitivities follow sine function showing potential for directional sensing. • Cross-sensitivity of temperature can be eliminated for the same sensitivity. [ABSTRACT FROM AUTHOR]