Contents I. Introduction
Achieving accurate measurements of curvature is crucial for deformation measurement, robot mechanical control, and structural health monitoring [1], [2], [3] of many large buildings, such as bridges, dams, and electricity generating stations. Currently, the main types of sensors used for curvature measurement include capacitive sensors, resistive sensors, magnetic sensors, and optical fiber sensors. Among them, optical fiber sensors have been rapidly growing and developing due to their superior features in anti-electromagnetic interference, fast response time, easy monitoring from a distance, distributed sensing, and so on [4], [5], [6], [7], [8]. They have been widely used in the fields of environmental protection, food monitoring, pesticide residues, mineral surveys, and medical diagnosis [9], [10], [11], [12], [13]. So far, many works have widely proposed and developed a variety of curvature sensors based on different optical fibers. There are different types of fiber grating that are used for curvature measurements, such as fiber Bragg grating (FBG) [14], long-period fiber grating (LPFG) [15], [16], tilted FBG (TFBG) [17], and other different types of fiber grating. However, the complicated and expensive manufacturing process of the grating, especially the tilted grating, hinders the application. Meanwhile, many single mode fiber (SMF)-based secondary processing structures have been designed and fabricated, including tapered, biased, and S- or Z-shaped structures [18], [19], [20], [21], which have the defect of poor mechanical properties due to the special production techniques. Furthermore, with the development of optical fiber preparation technology, sensors based on special microstructured optical fibers have been developed. Hollow core fiber (HCF) [22], polarization-preserving fiber (PMF) [23], seven-core fiber [24], and photonic crystal fiber (PCF) [25] are some examples, and the complex manufacturing processes as well as the higher cost are also problems for these structures. According to the above comparison, the main problems of optical fiber sensors can be summarized as follows: the preparation process of some optical fiber sensors is complicated and expensive, which is not conducive to a wide range of promotion. Second, some of the structures are not sensitive enough for curvature measurement, and some sensors can only reach sensitivity values in the order of pm. Additionally, the relationship between the output response of the bending sensor and the change in curvature cannot be linear over a wide measurement range; in other words, the range of curvature change that the sensor can measure is not wide enough. The sensor proposed in this article is simple to be prepared and has excellent sensitivity to curvature measurements, especially reaching a high sensitivity of −40.2971 nm/ in the range of 7.6577–8.3535 , which is difficult to achieve by the reported structures. In addition, this output response maintains a linear relationship with curvature in the relatively large range compared to the reported articles.
