Simultaneous detection of SRI and temperature with a SLPFG sensor operating near PMTP

In our paper, a superimposed long-period fiber grating (SLPFG) sensor based on the dual-peak resonance near the phase-matching turning point (PMTP) for simultaneously detecting surrounding refractive index (SRI) and temperature is proposed. For the SLPFG, two fiber gratings with different periods are overlapped and written in the same fiber core. There are two resonant peaks in the transmission spectrum corresponding to the above gratings and different cladding modes. By adjusting the grating periods, one resonant peak corresponds to a little off-resonant state near the PMTP, while the other one corresponds to the single peak state. When the SRI and temperature change, the transmissivity of the resonant peak in the little off-resonant state changes accordingly at fixed wavelength, and the resonant peak in the single peak state shifts. Results of numerical simulations demonstrate that SRI and temperature simultaneous measurement can be achieved by monitoring the wavelength shift of the left peak and the transmissivity of right peak. For the left peak, the SRI and temperature sensitivities are − 628.57 n m / R I U and − 0.20 n m / ∘ C , respectively. For the right peak, the SRI sensitivity is from − 217.73 d B / R I U to − 682.47 d B / R I U when the SRI varies from 1.330 to 1.360. The temperature sensitivity is from − 0.044 d B / ∘ C to − 0.101 d B / ∘ C when the temperature varies from 20°C to 120°C. The SRI and temperature sensitivities of such sensors are 7 times and 20 times higher than those of other sensors with dual-parameter modulation of intensity and wavelength. The proposed sensor with high sensitivity and simplified structure can be extended to multi-parameter measurement.