The principle of fiber optical current transformer and the function of λ/4 wave plate are described in this paper. We propose and demonstrate three kinds of main fabrication errors of λ/4 wave plate, including fiber material error, axial angle error and length interception error. In order to reduce the errors above, an elliptical core polarization-maintaining fiber, which is relatively temperature-insensitive and has a long beat length 20 mm, is used as the wave plate fiber. And we build a set of all-fiber λ/4 wave plate fabrication platform utilizing polarization analyzer and optical microscope. Therefore, the excellent performance λ/4 wave plate (DOP<98%, PER<0.2dB) was obtained with the fabrication platform. Experimental results of DC current measurement (100A˜3000A) and temperature test (-40°C˜85°C) show that the measurement accuracy of prototype with the fabricated λ/4 wave plate is better than 0.2S, which verified that the analysis of fabrication error and optimization technique of λ/4 wave plate in this paper can improves the performance of FOCT effectively
A fiber optic current sensor utilizing orbital angular momentum (OAM) beam is proposed in this paper. The superposition principle of composite OAM beam is deduced, and the current sensing process is derived by Jones matrix. The current is measured by detecting the rotation angle of petal-like patterns formed after the composite OAM beam through the polarizer. The reflective structure of the sensor doubles the rotation angle, which improves the measuring sensitivity and the reciprocity of the system. Through simulation analysis, we verified that the rotation angle changes linearly with the increase of current, and the sensitivity of the proposed sensor is 0.1254°/A. Finally, on this basis, the angle recognition method is optimized, and the final measurement error is less than ±0.2% in the range of 50A-1500A. .
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