Helical Intermediate-period Fiber Grating for Refractive Index Measurements with Low-Sensitive Temperature and Torsion Response

AbstractWe inscribe and characterize a series of helical intermediate-period fiber gratings (HIPFGs) on single-mode fiber with pitches from 24 m to 48 m. The fabricated HIPFGs have low temperature sensitivity, low torsion sensitivity, and high refractive index sensitivity. A series of separated peaks with low insertion losses and high coupling strengths are observed in the transmission spectra of the HIPFGs. For the HIPFG with grating pitch of 36 m (HIPFG-3), the peaks located at around 1596 nm and 1603 nm are chosen to evaluate the sensor performance of the fiber gratings. The temperature sensitivities are 6.86 pm/C and 7.53 pm/C for peaks around 1596 nm and 1603 nm, respectively. The corresponding values of the torsion sensitivities are 8.67 nm/(rad/mm) and 9.49 nm/(rad/mm) for those two peaks, respectively. In addition, the average refractive index sensitivity of the grating reaches 393.9 nm/RIU in the range of 1.310-1.408. The sensitivity of the HIPFG-3 at the RI of 1.408 is calculated to be 1291 nm/RIU. Therefore, we conclude that the HIPFGs proposed in this paper can be used as a high-sensitivity refractive index sensor which is not sensitive to temperature and torsion.