Helical Intermediate-Period Fiber Grating for Refractive Index Measurements With Low-Sensitive Temperature and Torsion Response

We inscribe and characterize a series of helical intermediate-period fiber gratings (HIPFGs) on single-mode fiber with pitches from 24 μm to 48 μm. 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 μm (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.