Mode Dependence of Refractive Index Sensitivity of Long-Period Fiber Gratings Induced by Periodically Embedded Multimode Fiber

In this article, we present and verify the mode dependence of refractive index (RI) sensitivity of long-period fiber grating induced by periodically embedded multimode fiber (PEM-LPFG). The PEM-LPFG is first fabricated by periodically embedding multimode fiber (MMF) into the single mode fiber (SMF). The resonant mode is translated by reducing the cladding. Numerical simulations based on beam propagation method (BPM) are applied to calculate the transmission spectrum, mode distribution, and RI sensitivities of the PEM-LPFG. The cladding diameter after etching is determined to be 89 and 68 mu m, which corresponds to the mode translated from LP012 to LP07 and LP05, respectively. The experimental results indicate that the maximum RI sensitivity of 1322.26 nm/RIU in the range of 1.333-1.437 RIU is achieved when the resonant mode is LP07, which is four times as large as the original one (325.58 nm/RIU @ LP012 mode). PEM-LPFG working at LP05 mode exhibits a lower sensitivity of 844.72 nm/RIU. All of the experimental results are in good agreement with the simulation. The RI sensitivity optimization of PEM-LPFG is able to realize by mode converting. Furthermore, the temperature responses are measured to be 13 pm/degrees C and 4 pm/degrees C. This work provides a theoretical reference and experimental evidence for the dependence of the PEM-LPFG RI sensitivity on the resonant mode for the first time.