A chiral photonic crystal fiber sensing coil for decreasing the polarization error in a fiber optic current sensor
Introduction
Fiber optic current sensor (FOCS) has been extensively used for electric power transmission as alternatives to electronic current transformer because of the advantages of good insulating property, high reliability, broadband frequency domain and good transient characteristic [1], [2]. Many studies are devoted to the problems of the accuracy of current measurement, sensitivity, and environmental suitability in FOCSs [2], [3], [4], [5]. In particular, the accuracy of the current measurement is one of the hot research topics in this field. Many factors affect the accuracy of the current measurement [6], [7], [8], in which the residual linear birefringence in the sensing coil is one of the main reasons decreasing the accuracy of the current measurement [8].
In order to solve the problem, we can consider introducing more circular birefringence or reducing the linear birefringence to improve the accuracy of the current measurement. A. H. Rose et al. proposed an annealing fiber to eliminate the linear birefringence caused by fiber bending [9], but it is fragile. S. X. Short et al. presented a loop helix sensing coil [8], whose circular birefringence is introduced by winding it in a helix. However, the geometry-induced circular birefringence is orders of magnitude smaller compared with stress-induced circular birefringence. Therefore, many studies have been implemented on the screwed silicon-based fiber (SSBF) as sensing coils, whose preforms comprising an on-axis core rod and two off-axis stress rods are spun in its hot state [10] and then the effective circular birefringence induced by the off-axis whirling stress filament is enhanced. However, the screw pitch of the SSBF may change due to the non-uniform drawing, which leads to the residual linear birefringence remaining in fiber and then degrade the stability of the circular polarization.
In recent years, it has been proved that the circular-polarization-maintaining characteristic also exists in chiral photonic crystal fiber (CPCF) in theory [11], [12], [13], [14] which has advantages of low cost, good toughness, controllable dispersion, and small bending loss [15], [16]. CPCFs with perfect symmetry structure have pure circular-polarization-maintaining characteristics. However, the deformations of the air holes from the imperfect fabrication process will introduce linear birefringence, which deteriorates the circular-polarization-maintaining property of the fiber [17]. The research shows that the closer the air hole is to the fiber core, the greater the influence of the deformations on the circular polarization degree of the fiber is [17], [18], [19]. The circular polarization characteristic will deteriorate, which lead to the increase of the polarization error to use the CPCF as a sensing coil in a FOCS. Hence, it is necessary to optimize the parameters of the CPCF to make up for the influence caused by the deformations of the air holes.
In this paper, we propose a refractive index-guided CPCF with regular hexagon structure as the sensing coil in FOCS. We use the two-dimensional chiral plane-wave expansion method to analyze the proposed CPCF, whose circular-polarization-maintaining characteristic is affected by the diameter variation of the innermost air holes. Then we optimize parameters of CPCF including lattice constant, air filling ratio and chiral parameter. Comparing with the SSBF, the optimized CPCF has a better circular-polarization-maintaining performance. Finally, it is indicated that the polarization error decreases when using the CPCF as the sensing coil. The proposed method provides a new idea of improving the accuracy of current measurement for a FOCS in theory.
Section snippets
Theoretical analysis of CPCF
In this paper, the hexagonal refractive index-guided CPCF is selected as the sensing coil in the FOCS with the circular-polarization-maintaining characteristic, and its structure is shown in Fig. 1.
Fig. 1(a) shows the schematic diagram of the cross section of refractive index-guided CPCF. The cross section is made up of six-layer regular hexagonal air holes in chiral background, in which and d denote the lattice constant and air holes’ diameter, respectively. In this paper, the griseofulvin
The comparison of circular polarization degree between CPCF and SSBF
The better the circular polarization characteristic of the sensing coil is, the smaller the polarization error is, and then the higher the accuracy of the current measurement will be. In order to be proved that it is more advantageous for decreasing the ratio error of current measurement to use the CPCF sensing coil than to use the SSBF sensing coil in the FOCS, it is necessary to compare the circular polarization degree of them. The circular polarization degree of CPCF has been calculated with
Conclusion
The CPCF was designed as a sensing coil in a FOCS, the influence of the diameter of the innermost air holes on the circular polarization degree in CPCF was analyzed and the parameters lattice constant, air filling ratio and specific rotation were designed as , 0.42 and , respectively. It can be guaranteed the circular polarization degree of the CPCF is larger than or equal to 0.99 when the diameter fluctuation of the air holes is 10%, which is superior to in
CRediT authorship contribution statement
Hongze Gao: Writing - original draft, Validation, Supervision, Formal analysis, Investigation, Resources. Guochen Wang: Project administration, Funding acquisition. Wei Gao: Conceptualization. She Li: Methodology, Software, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (51909048), China Postdoctoral Science Foundation (CPSF) (2019T120260, 2018M631920) and Heilongjiang Province Postdoctoral Foundation (HPPF) (LBH-Z17091, LBH-TZ1015).
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