Twist measurement based on dual-wavelength ratio for helical long-period grating

Twist measurement based on the transmission of fixed wavelength of a helical long-period fiber grating is studied. The single-mode fiber is spiraled by a welding machine. There are two resonance dips near 1473 nm and 1517 nm, with the pitch length around 782 μm. The experimental results show that the transmission near 1473 nm decreases along with contra-direction twist and increases along with co-direction twist. In contrast, the transmission near 1517 nm decreases along with co-direction twist and increases along with contra-direction twist. The relationships between the twist and the resonance wavelength, the twist and the transmission of resonance dips, the twist and the transmission of fixed wavelengths are studied, respectively. They all show linear relationship with the twist, and the slopes are about 100 nm·mm/rad, 36 dB·mm/rad, and 46 dB·mm/rad, respectively. The dual-wavelength ratio is studied, and the ratio also shows linear relationship with the twist. The twist sensitivity of I1463.3 nm/I1526.5 nm is about 0.79 each 0.01 rad/mm. This has great potential application in twist sensors.