This paper presents a linear fiber optic displacement sensor for the use over a large range based on the macro-bending loss. The sensor incorporates an extremely simple design, light source and detector. A concentric gear shaft is used to b. This paper presents a linear fiber optic displacement sensor for the use over a large range based on the macro-bending loss. The sensor incorporates an extremely simple design, light source and detector. A concentric gear shaft is used to bend the fiber and transfer the displacement. The linear relationship between the measured displacement and the bending loss of optical fiber was proved theoretically and its expression was derived. A series of calibration experiments and performance tests were conducted. The experimental results show that the sensor was characterized by a wide measurement range, a high sensitivity and a minimum displacement resolution of 0–200 mm, 0.1668 dB/mm and 0.06 mm, respectively. The stability and repeatability of the sensor was verified by repetitive testes with a m. Fiber optic displacement sensorMacro-bending lossLarge rangePerformance testsDisplacement response is the significant data to be used for evaluating the safety of a structure, which can sever as an evidence for determining the extent of aging and resistance reduction of civil structures. We can know that the common and preferred methods of displacement measurement in laboratory and field tests are displacement sensors (linear variable differential transformers, laser displacement device and GPS), and geological survey and inclinometers. However, the civil structures are often exposed to relatively harsh natural environment and other exterior loads, such as flammable gases, rain, corrosion, electromagnetic interference, periodic vehicle loads and reservoir water fluctuation,. Therefore, these terrible factors will cause that the instruments used for the measur. 2.1. Macro-bending loss in fiberIt is well known that one of the most important characteristics of the optical fiber is ease of bending. If the curvature radius of the optical fiber is too small, the transmission path of light in fiber will change, which causes the macro-bending loss and micro-bending loss of the optical fiber. The macro-bending loss generally means the additional loss produced by the bent optical fiber with a curvature radius that is much larger than its diameter; and the micro-bending loss occurs when the curvature radius of fiber is comparable to its diameter. The micro-bending loss is generally caused by the random twist of the optical fiber during the drawing process, and is relatively small and difficult to be observed. Therefore, the ai. This section presents the calibration experiment procedure followed to determine the standard coefficient and verify the sensing principle of the sensor. The instruments used in this experiment are: an optical fiber displacement sensor, a stable light source with working wavelength of 1550 nm, an optical power meter and a hot melt glue gun.The sensor is arranged on the experimental platform, and one end of the sensor is connected to the optical meter power, the other end is connected to the light source. The tooth number of the gear is 36, and the rotation angle of each tooth represents 10 º. This experiment is conducted to ensure the uniform motion of the sensor at regular intervals; therefore, the gear was rotated uniformly from 0º to 320º and 320º to 0º f.
