Highly Sensitive Bending Sensor Based on C-Shaped-Core Long-Period Fiber Gratings

Optical fiber structures for bending sensing have attracted extensive attention in recent years with their unique advantages. In this article, innovative C-shaped-core long-period fiber gratings (C-LPFGs) with two resonant peaks are proposed and experimentally tested. This special fiber microstructure is achieved by methods of CO2-laser polishing and oxyhydrogen-flame heating. During the process of heating, the core at polished area micro-bends to the surface of the structure due to the surface tension effect, which forms a “C”-shaped core. Each microbending Section of core results in partial energy leaking into the cladding to excite the cladding modes. As the number of periods increases, stronger effective refractive index modulation forms multiple resonant peaks. According to the experimental results, sensors with 17 periods represent excellent contrast and repeatability of two resonant peaks. The bending sensitivity of the proposed structure achieves −24.01/−21.3 nm/m−1 at 0° and −4.88/−10.17 nm/m−1 at 90° of Dip
$_{1/2}$
. Furthermore, the temperature sensitivity of both dips is 64/33 pm/°C, and the total length of the sensor is only 0.85 cm. With considerable bending sensitivity, the compact sensor possesses latent potential in fields of dual-parameter measurement in the future.