A review for optical fiber bending sensors is presented. The article mainly focuses on the measurement methods of the structure bending. Firstly, the different optical fiber bending sensors are summarized, including measurement methods, sensing structure, measurement principles, and the performance. Alternatively, this paper compares th. A review for optical fiber bending sensors is presented. The article mainly focuses on the measurement methods of the structure bending. Firstly, the different optical fiber bending sensors are summarized, including measurement methods, sensing structure, measurement principles, and the performance. Alternatively, this paper compares the performance of different sensing structure and discusses the advantages and disadvantages of various sensors. Finally, the existing problems of the bending measurement methods and the future prospects of different bending sensors are also discussed.••Optical fiber bending sensorCurvature measurementDirection measurementSensing structureBending deformation measurement plays an important role in the fields of mechanical engineering and structural health monitoring such as mechanical bending angle measurement, human posture detection, bridge and road construction, and structural deformation,,,. Compared to other electrical sensors, optical fiber bending sensor has advantages of small size, anti-electromagnetic interference, corrosion-resistant, high sensitivity and low maintenance costs,,,,,, so it is widely used in the bending measurement.In general, according to the principle of modulation, the optical fiber bending sensor can be divided into several categories as follows: intensity modulation,, wavelength modulation, frequency modulation,. In 1980, American researchers Fields J N and Cole J H proposed the fiber micro-bend sensor for the first time. In recent years, many optical fiber curvature sensors based on different optical devices have been proposed, such as multi-core fiber (MCF), long period fiber gratings (LPFGs) [18. There are two different experimental setups for curvature measurements in most bending sensing experiments.The first type is shown in Fig. 1. The input light is launched from light source such as amplified spontaneous emission (ASE) and broadband light source (BBS) to the sensor and then the experiment data is recorded by using the optical spectrum analyzer (OSA). The sensing probe is stacked on the surface of the steel ruler, and the steel ruler is fixed on the adjustable platform. The lifting platform F is contacted with the steel ruler, and the sensing probe can be bent to different curvature with the lifting platform F moving up and down. The curvature expression could be calculated by the geometric relationship (1)C=1R=2dd2+S2where C is the curvature, R is the bending curvature radius, S is half of the distance between the adjustable platforms, and d is the movement distance of the lifting platform.3.1. Measurement methods based on modal interferenceAs shown in Fig. 3, the input light propagated in the first SMF (input SMF) is launched into the section A (section A can be a section of multi-core fiber, a lateral offset splicing joint or a fiber taper), then multiple modes are excited and propagated in the fiber core and cladding of section B (section B can be MCF, SMF, PCF). The different modes produce the interference in the section C (section C can be a piece of multi-core fiber, a lateral offset splicing joint, or a fiber taper). Finally, the light is re-coupled into the second SMF (output SMF). Due to the elastic deformation and the elastic effect of the optical fiber, the effective refractive indices difference between the two modes will change as the sensing structure bending, and the spectrum will shift as a result,. The effective refractive index difference has a function relationship with the curvature and the wavelength, so the curvature can be obtained according to analyzing the changes of interference spectrum.3.2. Measurement methods based on FBG.