As an important physical parameter, curvature can reflect the structure and shape of the measured object and is a crucial parameter for assessing the degree of deformation of an object. Curvature sensors can be used not only for monitoring the health of bridges and buildings but also play a significant role in aerospace and medical fields. Therefore, curvature sensors have a promising development outlook and significant research importance. Multi-core optical fibers are used for making curvature sensors due to their unique spatial structure advantages. Although some multi-core optical fiber curvature sensors exhibit high sensitivity, most optical fiber curvature sensors have limited measurement ranges due to complex sensor structures or measurement methods. To extend the measurement range of sensors, a Michelson interferometer curvature sensor based on a seven-core optical fiber has been proposed. It is composed of a single-mode fiber spliced with a tapered seven-core optical fiber, featuring a simple structure. The tapered region is primarily used for coupling light between cores, which provides the potential for expanding the bending measurement range. The proposed sensor has the characteristics of a large measurement range and insensitivity to temperature, and is expected to be effective in scenarios requiring large curvature monitoring.

First, the end faces of the single-mode fiber and the seven-core fiber are polished flat. Then, the cores are spliced using a fusion splicer. Finally, the spliced fiber is placed into a CO₂ laser splicer. The transition zone length, waist zone length, and taper diameter are manually set, and the seven-core fiber is subjected to fused tapering using the CO₂ laser. After tapering, when light from the single-mode fiber reaches the taper region of the seven-core fiber, due to the reduction in the diameter of the central core, part of the light couples into the outer cores and cladding. After traveling a certain distance, the light reaches the end face of the seven-core fiber and is reflected back into the taper region due to Fresnel reflection, forming a Michelson interferometer. When the fiber bends, the inner cores are in a compressed state while the outer cores are in a stretched state, causing a change in the optical path length and resulting in a shift in the interference wavelength. Thus, curvature sensing can be achieved by monitoring the shift in the interference wavelength.

A simulation model is established using Rsoft to model the electric field distribution under taper diameters of 60, 50, and 40 μm (Fig. 3). It is observed that as the taper diameter decreases, the intensity of the light field of the outer core gradually increases. Therefore, it is inferred that the sensor has a better extinction ratio when the diameter is 40 μm. Samples with taper diameters of 60, 50, and 40 μm are fabricated, and their reflection spectra are obtained (Fig. 4). When the taper diameter is 40 μm, the interference spectrum had smoother spectral lines and a higher extinction ratio, which are consistent with the simulation analysis. Therefore, the sample with a 40 μm taper diameter is chosen for the experiments. Curvature experiments are conducted on samples with a seven-core fiber length of 15.6 cm, and the measurable curvature range is 0?11.492 m^-1. Within the linear curvature range, the sensitivity is -1.152 nm/m^-1, and the linearity is 0.99 (Fig. 7). Additionally, repeatability tests are performed on the samples (Fig. 8), and the sensor exhibits good repeatability. Experiments on samples with different lengths of seven-core fibers are also conducted (Fig. 9). Within the linear curvature range, the maximum sensitivity of the sensor is -1.683 nm/m^-1. Temperature experiments show that the sensor sensitivity to environmental temperature changes is only -8.64 pm/℃, while the curvature sensitivity is approximately 194 times that of temperature sensitivity, indicating that the sensor is insensitive to temperature.

We design a Michelson interferometer curvature sensor based on a seven-core optical fiber, constructed from splicing a single-mode fiber with a tapered seven-core fiber. The taper region is primarily used for core-to-core light coupling, which provides the possibility of expanding the bending measurement range. Experimental results show that the sensor has a curvature measurement range of 0?11.492 m^-1, with a maximum sensitivity of -1.683 nm/m^-1, and its sensitivity to environmental temperature changes is only in the pm/℃ range. This provides a solution to the problems of limited linear measurement range and cross-sensitivity to bending and temperature found in existing interferometric curvature sensors.