High sensitivity strain fiber MZI with circular ring structure based on sensing length changes

Fiber Mach-Zehnder interferometer (MZI) strain sensors have wide applications in various fields with strain measurement needs. However, their strain sensitivity is limited by the variation range of refractive index differences between the fiber core and cladding, and cannot be further improved. Based on this, this article proposes a high-sensitivity fiber strain sensor that utilizes a circular ring structure to achieve changes in the length of the sensing area. The sensor is formed by bending a single-mode fiber into a circular ring and fixing the intersecting part of the fiber in a spiral winding manner. When the optical fiber at both ends of the circular ring structure is subjected to axial strain, the shape of the circular ring region gradually changes to an ellipse due to the presence of the spiral structure, resulting in changes in the coupling positions of the fiber core and cladding modes, thereby significantly changing the length of the sensing area and achieving high-sensitivity strain sensing. The experimental results indicate that the strain sensitivity of the circular ring structure MZI can reach 91.55 pm/με, and the strain detection range is 0-400 με. This article proposes a new approach for strain sensitization of the MZI strain sensor, which can significantly increase sensitivity by changing the length of the sensing area. It is expected to be applied in fields such as aerospace with high sensitivity strain detection requirements.