Photosensitive Polymer-Based Micro-Nano Long-Period Fiber Grating for Refractive Index Sensing

In this paper, we propose and demonstrate a photosensitive polymer-based micro-nano long-period fiber grating (PPMN-LPFG) for refractive index (RI) sensing. We coat liquid photosensitive polymer on the surface of micro-nano fiber (MNF) and obtain a series of periodically distributed droplets on the MNF due to the Plateau-Rayleigh instability (PRI) property. Then we cure the polymer droplets by using the 532 nm laser illumination. We configure the cladding modulated LPFG based on the periodic polymer points along the MNF. The PPMN-LPFG combines with a strong evanescent field of MNF and cladding modulated LPFG, and thus, it is much more sensitive to the external environment. The experimental results indicate that the spectral dip redshifts from 1404 nm to 1654 nm when the RI changes from 1.3391 to 1.4164. The maximum RI sensitivity is 10419 nm/RIU in the RI range of 1.4102–1.4164. The temperature sensitivity is 0.89 nm/°C in the temperature range of 17.1 °C–50.5 °C. Compared with the RI sensitivity, the temperature crosstalk is negligible. Our simulated and experimental results verify the feasibility of the proposed sensor and show the advantages such as high sensitivity, compact size, and good stability.