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In this paper, an all multimode fiber electric current sensor with fiber Fabry-perot interferometer is presented. As shown in Fig. 1, our system mainly consists of a laser diode, a multimode gradient input fiber, a multimode gradient Fabry-Perot interferometer, a multimode output fiber and a detector as well as a processing unit. The interferometer is wrapped and fastened on a magnetostrictive amorph metal ring, the cable of electric current to be measured goes through the ring. The magnetic field of electric current changes the radius of the ring therefore the cavity length of the fiber Fabry-Perot interferometer. The output of the interferometer that is the number of interference fringes in one period of AC current depends on the strength of the magnetic field produced by the electric current to be measured. So, by counting the number of fringes, the electric current can be determined. The fiber Fabry-Perot interferometer is mostly made of single mode fiber in order to keep the coherence of light. R. Kist et al. [ 1] made the Fabry-Perot interferometer with multimode gradient fiber and used the single mode fiber as the input fiber, and got good results. But we also get good results with all of the fibers used in our system which are multimode gradient. There are two reasons to enable the system to work well: one is, in Fabry-Perot interferometer, the multimode fiber is bent, because of larger bend loss of high mode, the high modes are filtered, there are only several low modes; the other is because of energy distribution in modes, the output of interferometer is mainly determined by the interference of several modes. Additionally, the coherence of light source and the coupling efficiency have effect on the output of the interferometer.
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