# Optical Fiber Current Sensors Based on FBG and Magnetotstrictive Composite Materials
## 1 Introduction  
With the rapid development of intelligent power grid technology, optical fiber current sensors play an important role in power measurement, monitoring, and protection. Optical fiber current sensors have the following inherent characteristics: anti-electromagnetic interference, electrical isolation, small size, and light weigh. At the same time, the measurement object of the optical fiber current sensor is a magnetic field generated by the current, not the current itself, which avoids the danger of high voltage measurement [1].  
Optical fiber sensors are of particular interest for applications in high-voltage environments of the electric power industry [2]. At present, optical fiber current sensors reported in the literature mainly include Faraday magneto-optical effect, Rogowski coil photoelectric hybrid, and magnetostrictive effect combined with optical fiber. In the current sensor based on Faraday magneto-optical effect, accuracy problems caused by linear birefringence in optical fiber, the verdet constant drift caused by temperature, and aging problems ofthe sensor head have always been the biggest obstacles to the practical application of this kind of current sensor; the Rogowski coil photoelectric hybrid current transformer is still affected by environmental electromagnetic field, high-voltage circuit current supply, and other factors in practical application; and the magnetostrictive current sensor has wide current response frequency band and high sensitivity, and can realize distributed measurement with the cooperation of demodulation instruments. The cost for synchronous monitoring of multi-point current is relatively low, and it has good industrial application prospects [3].  
Giant magnetostrictive materials (GMM) have the advantages of small size, fast response speed, high precision, and easy integration [4]. Terfenol-D (TD) alloy is a new type of rare-earth giant magnetostrictive material, which has high magnetostrictive properties and is widely used as a sensitive element of the current sensor. However, Terfenol-D alloy is expensive, fragile, and difficult to form into a specific shape. This can be solved by combining TD particles with resin to form giant magnetostrictive powder composite (GMPC) [5]. Magnetostrictive composites also have other excellent properties, such as enhanced tensile strength and low eddy current loss. Preloading also can be reduced or even avoided due to the pre-stress generated during curing. In addition, because of the low content of magnetostrictive materials and a simplified process, the cost of composite materials is low; at the same time, the magnetostriction of magnetostrictive composites can compete with the magnetostrictive alloy.  
Optical fiber current sensors based on magnetostrictive composite material have the advantages of high sensitivity, high precision, wide working range, low response time, and low cost. In this paper, the application of magnetostrictive composite materials in optical fiber current sensors is summarized, mainly from the working principle of the sensor, the structure of the sensor, and the modification of magnetostrictive composite materials. The structure of the sensor is mainly studied according to the shape of magnetostrictive composite material, the shape of the magnetic permeable material, and the number of permanent magnets. The improvement of magnetostrictive composites is mainly studied from aspects of particle volume fraction, binder type, particle size, orientation magnetic field, and molding pressure.