# Optical Fiber Current Sensors Based on FBG and Magnetotstrictive Composite Materials
## 3 Optical Fiber Current Sensor Based on Magnetostirctive Composite Materials
### Sensor Structure  
In 2005, [PERSON] et al. [13] proposed an optical fiber current sensor based on magnetostrictive material. The proposed sensor uses 40 g cylindrical rod magnetostrictive material, as shown in Figure 7. Magnetostirctive materials have unipolar characteristics. To avoid the rectified output of the sensor, a direct current (DC) bias coil is introduced into the sensor. At the same time, mechanical devices are added to the sensor to provide preloading stress. It is found that the output of the sensor is non-linear when the preloading  
Figure 5: Role of permanent magnets. Figure 6: Sensor system diagram. stress is low. When the preloading stress is high, the linearity of the sensor will increase. Although the mechanical device improves the linearity of the sensor, it also increases the complexity of the sensor structure, and the addition of a DC bias coil leads to the sensor being active. In 2013, [PERSON] et al. [14] proposed an optical fiber current sensor for measuring AC. The proposed sensor uses 144 g annular solid magnetostrictive material, as shown in Figure 8. The stainless-steel compression ring exerts a certain pressure on the magnetostrictive material, which makes the relationship between the strain produced by the magnetostrictive material and the applied magnetic field approximate to a quadratic function. At the same time, the linearity of the sensor is better, and the measurement error is smaller under the action of preloading stress. The proposed sensor has fast transient response, but the sensor does not introduce DC bias device, which leads to a frequency-doubling of the output of the sensor. In 2015, [PERSON] and [PERSON] [15] proposed a new and compact optical fiber current sensor. The new sensor is composed of 74 g solid rectangular magnetostrictive material, magnetic conductive material and permanent magnet, as shown in Figure 9. The magnetostrictive material is Terfenol-D alloy, the magnetic conductive material is made of ordinary ferrosilicon, and the permanent magnet is made of NdFeB. The purpose of adding permanent magnets to the magnetic circuit is to provide a certain constant magnetic field, so that the sensor can work in a linear region and obtain a complete AC cycle. The magnetic  
Figure 8: Optical fiber current sensor based on 144 g annular solid magnetostrictive material. Figure 7: Optical fiber current sensor based on 40 g cylindrical rod magnetostrictive material. field acting on magnetostrictive material consists of an AC magnetic field generated by wire and a constant magnetic field generated by a permanent magnet. The author uses permanent magnets instead of DC drive coils, so the sensor is passive. Although the proposed new sensor eliminates the frequency-doubling phenomenon of magnetostrictive materials, it still does not solve the disadvantage of Terfenol-D alloy being expensive and fragile. In 2019, [PERSON] et al. [16] proposed a new ring optical fiber current sensor based on magnetostrictive material. As shown in Figure 10, the proposed sensor is composed of magnetostrictive material, magnetically permeable material, and a permanent magnet. Iron bars (99.5% Fe, relative permeability 500) are introduced into the magnetic circuit, which not only reduces the mass of TD, but also increases the magnetic flux density per gram of TD. To reduce the mass of TD greatly, the author uses finite element simulation (FEM) to guide the structure design of the proposed optical fiber current sensor. Compared with the sensor based on annular magnetostrictive material, the shape of magnetostrictive material of the proposed sensor is 1/6 annular, the mass is only 5.6 g, and the sensitivity is higher. The structure proposed by the author not only reduces the consumption of TD and the cost of the sensor, but also improves the sensitivity of the sensor. However, TD still has the disadvantage that it is difficult to form. Subsequently, [PERSON] et al.