The fabrication of single mode fiber cladding waveguides FBG using femtosecond lasers and the precise measurement of curvature vectors

This paper presents a cladding waveguide optical fiber Bragg grating (CLWFBG) fabricated using femtosecond (fs) laser micromachining. Due to the presence of the cladding waveguide, the FBG can be inscribed at the edge of the cladding, far from the neutral axis of the fiber. When the fiber Bragg grating (FBG) is offset from the neutral axis, its effective refractive index and grating period change due to bending, enabling curvature measurement. Moreover, because the sensor structure disrupts the circular symmetry of the fiber, the wavelength response of the CLWFBG differs for different bending directions, thus allowing one-dimensional vector curvature measurement. Experiments show that the bending response of the CLWFBG is similar in the ranges of 0° to 180° and 180° to 360°, so the sensor can only achieve vector measurements in the ranges of 0°-180° or 180°-360°, with a sensitivity of 150 pm/m-1 at 90° and -163 pm/m-1 at 270°. The experimental results indicate that the sensor has high sensitivity, a large measurement range, and good linearity. The sensor has broad application prospects in industrial, architectural, and structural health monitoring fields.