Thermally stable vector torsion sensing based on cascaded long-period fiber gratings in two-mode fiber

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 CO2 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 LP01-LP11 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.