Strain-Insensitive Simultaneous Measurement of Bending and Temperature Based on Cascaded Long-Period Fiber Gratings Inscribed on Double-Clad Fiber

We demonstrated an optical fiber sensor capable of performing strain-insensitive simultaneous measurement of bending and temperature using cascaded long-period fiber gratings (LPFG’s) inscribed on double-clad fiber with a CO2 laser at $10.6~\mu \text{m}$ . In the interference fringe spectrum of the fabricated cascaded LPFG’s, we selected three transmission dips as sensor indicators for the simultaneous measurement of bending and temperature. The measured bending sensitivities of the three indicator dips were −427.63, −911.29, and 233.33 pm/ $\text{m}^{-1}$ , and their measured temperature sensitivities were 60.49, 56.44, and 74.61 pm/°C. Owing to their linear and uncorrelated responses to bending and temperature, we could simultaneously estimate applied bending and temperature with two of the three indicators by converting the measured wavelength shifts of the two selected dips with their pre-determined bending and temperature sensitivities. In particular, our sensor showed a very low strain sensitivity less than 0.05 pm/ $\mu \varepsilon $ , 25 times less than that of a conventional LPFG, which can significantly reduce strain-induced measurement uncertainty. With two optimal indicator dips chosen considering the measured perturbation responses of the three indicators, we carried out the simultaneous measurements of bending and temperature for arbitrary sixteen points of applied bending and temperature. The standard deviations of the measured bending and temperature values were $\sim 0.059\,\,\text{m}^{-1}$ and ~0.85 °C, respectively.