Long-Period Fiber Grating Based on Side-Polished Optical Fiber and Its Sensing Application

A novel side-polished long-period fiber grating (LPFG) sensor was proposed and experimentally validated. Side-polished can provide a stronger evanescent field than traditional grating and bring superior sensitivity. The greater the side-polished depth, the higher the refractive index (RI) sensitivity. When
$d = 44\,\,\mu \text{m}$
, the RI sensitivity reached 466.85 nm/RIU in the range of 1.3330–1.3580, which is fourfold higher than the LPFG prepared by the electric-arc discharge (EAD) method. A graphene oxide (GO) nano-film is coated on the LPFG to make it realize high sensitivity relative humidity (RH) sensing. Humidity sensitivity reached −0.193 nm/%RH in the range of 40%–80% RH. In addition, side-polished breaks the symmetry of the distribution of the cross-sectional light field, which determines the ability to achieve vector curvature measurement. It shows good sensing performance in the same/opposite bending direction as the side polished surface. When the input light polarization is 90°, the average sensitivity reaches 5.03 and −5.9 nm/
$\text{m}^{-1}$
in the range of 0–19.67
$\text{m}^{-1}$
, respectively. This strongly indicates that the fabricated sensors show high sensitivity, low-cost materials, and robust performance and break the limitations of the EDA method to prepare gratings, which have good application potential for biomedicine and the field of construction.