Quasi-Distributed Directional Bending Sensor Based on Fiber Bragg Gratings Array in Triangle-Four Core Fiber

A quasi-distributed directional bending sensor based on fiber Bragg gratings (FBGs) array in triangle-four core fiber (TFCF) is proposed and experimentally demonstrated. The proposed sensor was composed of a single mode fiber (SMF) and a TFCF. A piece of TFCF is heated by a thermal diffusion technique (TDT) to fabricate an integrated coupler, which couples the optical power from central core into the side cores. Considering the size of the Gaussian beam and the effective power distribution on the cross section of excimer laser, we fabricated FBGs samples by a defocusing phase mask method. We fabricated some FBGs array samples in TFCF with peak wavelengths in the range from 1528 nm to 1566 nm with a step of 4 nm. The error and variance of peak wavelengths and reflectivity are also measured. Obviously, the consistency of wavelength and strength are satisfactory. The curvature, temperature, and strain responses of FBG array samples with the central core and side cores are studied. Based on the unique characteristics of the proposed sensor, cross-sensitivity of temperature, and strain can be solved by subtracting the wavelength changes of side cores from that of central core. In addition, by adding temperature compensating sensor, we analyzed the combined standard uncertainty for simultaneous measurement of curvature, temperature, and strain. The proposed sensor have high potential to monitor the deformation of the smart wing on the spot and provide information for the flight control system to evaluate the aerodynamic performance of flight conditions.