Mode dependence of refractive index sensitivity of long-period fiber gratings induced by periodically embedded multimode fiber

In this paper, 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 firstly 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 μm and 68 μm, which corresponds to the mode translated from LP 012 to LP 07 and LP 05 , respectively. The experimental results indicate that the maximum refractive index sensitivity of 1322.26 nm/RIU in the range of 1.333 -1.437 RIU is achieved when the resonant mode is LP 07 , which is 4 times as large as the original one (325.58 nm/RIU @ LP 012 mode). PEM-LPFG working at LP 05 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/°C and 4 pm/°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.