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In this paper, we propose and investigate a novel long period fiber grating (LPFG) refractive index (RI) sensor, which is inscribed in a two-mode fiber and coated with the zinc oxide (ZnO) thin film. According to the coupled mode theory, the resonant wavelength, which appears at approximately 1550 nm, originates from the mode coupling between the LP11 core mode and the sixth order cladding mode. As a comparison, single-mode LPFGs (LPFG-SMFs) with and without ZnO thin film are fabricated and they are formed by coupling light from the LP01 core mode into the sixth order cladding mode. The sensing performance is researched by observing the shift of resonant wavelength with the increasing of surrounding refractive index (SRI) in the range from 1.3300 to 1.4577. The experimental results demonstrate that LPFG inscribed in a two-mode fiber (LPFG-TMF) has a higher RI sensitivity than the LPFG-SMF. And the LPFGs with coated ZnO thin film can achieve a higher RI sensitivity than the bare LPFGs in the mode transition region. The highest sensitivity of LPFG-TMF coated with ZnO thin film reaches 7578.94nm/RIU in the RI region between 1.4558 and 1.4577, which is 23.90 and 38.69 times higher than the bare LPFG-TMF and LPFG-SMF coated with ZnO thin film, respectively. The proposed sensor offers a promising platform to achieve a higher sensitivity for SRI.
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