Analysis of the Influence of Stress on Fiber Birefringence by Inner and Outer Coatings of Polarization-maintaining Fiber
The Panda-type polarization-maintaining fiber,as a typical stress-type fiber relied on stress birefringence,is utilized for maintaining the polarization state of the transmitted light.An ideal polarization performance is achieved by increasing refractive index difference along two orthogonal axes resulting from stress formed into the fiber core.The Panda-type polarization-maintaining fiber has been widely used in fiber optic gyroscopes,telecommunications,fiber optic sensors,and high-speed optical communication systems owing to its advantages of high polarization extinction ratio,low polarization mode dispersion,and low insertion loss.Currently,improving the birefringence of Panda-type fiber is a significant research direction.Various studies have focused on changing the core shape of the fiber,such as using elliptical,leaf-shaped,and square-shaped cores to enhance birefringence,but with limited effects.An alternative method is to change the shape of stress regions to improve the birefringence of the fiber,such as Knot-type polarization-maintaining fibers and elliptical cladding polarization-maintaining fibers.Remarkably,the Knot-type polarization-maintaining fiber exhibits the best polarization-maintaining performance due to its larger effective stress regions.The most commonly used fiber has a cladding diameter of 80 µm to achieve miniaturization of fiber coils.However,for high-precision satellite positioning,unmanned aerial vehicles,and automotive navigation,research on 60 µm thin-diameter polarization-maintaining fiber is urgently needed.As the cladding diameter decreases,the study of coating thickness becomes challenging because thinner coating layers are difficult to maintain the excellent transmission performance of the fiber.In this paper,the COMSOL finite element analysis software is utilized to propose a method to enhance the birefringence of Panda-type fiber by adjusting the material properties of the stress regions.By changing thermal expansion coefficients of materials from 2×10-6 K-1 to 7×10-6 K-1,the Young's modulus from 2×1010 Pa to 12×1010 Pa,and the Poisson's ratio from 0.1 to 0.5,the impact of the stress regions on effective refractive indices of the fast and slow axes of the fiber core is enhanced,and thus improving the birefringence of the fiber.For the miniaturization of fiber coils,this study simulates the effect of reducing the outer coating diameter from 165 µm to 135 µm on the fiber's transmission performance.Furthermore,a complete physical model of a 32-layer,82-turn fiber coil is built,where point loads applied to the boundaries of each turn of the fiber is used to simulate the real internal stress during fiber winding.The stress of each turn in the fiber core is then extracted as the basis for judging the output error of the fiber coil.To reduce the error caused by winding tension,the study discovers an optimal ratio of thickness between the inner and outer coatings by analyzing different material properties and effects.This improved thickness ratio reveals an excellent suppression effect of winding tension by approximately 10%compared to the original fiber.The simulation calculates the Young's modulus and Poisson's ratio of the double-coatings,with the inner coating's Young's modulus varying from 1.56 MPa to 15.6 MPa and the outer coating's Young's modulus varying from 1 GPa to 4.68 GPa.Both the inner and outer coatings have Poisson's ratios ranging from 0.25 to 0.45,and the conclusion is drawn that the material properties of the coatings also have a significant effect on suppressing winding tension.In summary,this paper proposes methods to enhance the birefringence of Panda-type polarization-maintaining fiber by changing the structure and material parameters of the stress regions.Additionally,it demonstrates that reducing the coating thickness of the fiber effectively enhances birefringence performance under low-temperature environments.Finally,to reduce the error caused by fiber winding tension,it suggests optimizing the thickness ratio of the fiber coatings and the material parameters of the coatings.
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