Simultaneous measurement of bending and temperature using phase-shifted long-period fiber gratings

We experimentally investigate the simultaneous measurement of bending and temperature using the phase-shifted long-period fiber gratings (LPFGs) based on the resonant wavelength shift and wavelength spacing change by the bending and temperature change, respectively. The phase-shifted LPFGs can be fabricated by UV post-exposure on a half of the grating region. The UV post-exposure has the effect of inducing a positive phase shift for the core mode while decreasing the coupling strength between the core and cladding modes. The spectral evolution of the transmission thus depends on the initial coupling strength of the grating. We applied the phase-shifted LPFG fabricated with the second saturated LPFG to the simultaneous measurement of bending and temperature. The left and right resonant peaks shifted into the longer wavelength due to the variation of the grating period as the bending curvature increased and the wavelength spacing between two peaks was not changed by the bending. However, since the phase-shifted LPFG has two different sections with different photo-induced average indices, the wavelength spacing between left and right peaks decreases as the temperature increases. Based Consequently, it is possible to discriminate two effects between bending and temperature based on the proposed schematics.