Research On All-fiber Optic Current Transducers Based On3×3Fiber Couplers

The rapid development of the national economy makes the growing demand ofelectric power. The ultra-high voltage grid and the smart grid are two importantdevelopment directions of the State Grid. Fiber optic current transducers, which meetthe new requirements of the grid, received widespread attentions these days.The demodulation of fiber optic current transduces by3×3directional couplers isa passive homodyne method. Since there is no need of a phase modulation signal,this technique is easy to operate and low cost. However, the random state ofpolarization (SOP) in single mode fibers is a key factor that uncontrollable, whichdegrades the stability of the system. The influence of the variation of SOP on thedemodulation signal of fiber optic current transducer by the3×3coupler is analyzedtheoretically, and a new scheme is presented. The effects of the polarizationrandomness in single mode fibers could be effectively reduced by polarization controlof the light path. The main content of the paper is as follows:Firstly, a theory model of the fiber current sensor based on3×3couplers wasestablished. The relationships between the sensitivity of the system and the SOP of theinput light and the optical properties of the sensing loop were discussed. Based on this,a new method of polarization control is presented. Taking the advantage of circulardichroism of the fiber, the birefringence and polarization coupling of light could besuppressed at the same time, thus the stability of the system could be improved.Secondly, the performances of the3×3directional couplers were analyzed indetail. By solving the coupled-mode theory, the influence of the structureunsymmetry was discussed, and verified by the software of BeamPROP. Theanalysis results show that the imperfect of the structure in practice will cause theun-uniformity of power distribution and offset of phase difference of the3×3coupler,which will affect the scale factor of the system. We also derived the polarizationcoupled-mode theory of2×2and3×3couplers. Numerical results show that the birefringence of the fiber has little impact on the output of couplers.Thirdly, the performances of a3×3coupler were measured, and the opticalproperties of the sensing loop were also measured. Based on this, we studied theperformance of a fiber optic current sensor based on the3×3coupler experimentally.The experiment results show that out schematic could control the polarizationevolution in the loop very well, thus the stability of the system could be improved.And finally, a new measuring method of the phase bias of the3×3coupler basedon magneto-optic modulation is proposed, and experimental results prove itsfeasibility.Research work of this paper provides some theoretical and experimentalfoundation of the fiber optic current sensors based on3×3couplers, which is helpfulfor the further development of this field.