Dual-core fiber curvature sensor based on femtosecond laser processing

• A novel dual-core fiber curvature sensor structure based on femtosecond laser processing was proposed. • Recognition of one-dimensional bending direction is achieved by introducing optical path difference. • The sensitivity and FSR of the sensing structure are independent and adjustable. • The difference in equivalent refractive index at the fiber core solves the problem of insensitivity to bending. In this paper, a curvature sensor based on femtosecond laser processing of two-core fiber is proposed. The sensor uses the interference between two core modes of the two-core fiber to realize curvature sensing. According to the two-beam interference formula, the curvature sensing sensitivity and the free spectral range of the sensing structure are theoretically independent and adjustable, consequently it is more flexible. The sensor comprises single-mode fiber, no-core fiber, and two-core fiber. Femtosecond laser fabricates waveguides in no-core fiber to guide light from single-mode fiber into a two-core fiber. In order to construct the optical path difference, the end face of the two-core fiber is processed into a stepped shape using the femtosecond laser. The entire fiber structure constitutes the MI. The curvature tests of this sensor are carried out. The experimental results demonstrate that the sensor can identify a one-dimensional bending direction. Curvature is applied to the two-core fiber of different lengths. It is found that the longer the length of the bending part, the higher the sensor’s sensitivity. When the bending part of the two-core fiber is 4 cm, in the curvature range of 0 to 0.241 m −1 , the curvature sensitivity of the sensor bending in different directions can reach −60.4 nm/m −1 and +60.1 nm/m −1 , while maintaining high linearity.