We present a stress-induced ultrahigh-birefringence fine-diameter polarization-maintaining fiber (PMF). The proposed PMF is composed of an elliptical-core, with four elliptical side-holes distributed around it, and two symmetrical circular/bow-tie stress-applying parts (SAPs) creating a fiber with both geometric and stress birefringence. The influence of the air-hole dimensions and positions on the properties of the proposed PMF, including modal birefringence, effective mode area, and nonlinearity, is comprehensively investigated via numerical simulations. This characterization reveals that this novel design with appropriate parameters for the airholes is capable of ultrahigh birefringence of 1.27 x 10-3 in Panda-type PMFs and 1.28 x 10-3 in Bow-Tie PMFs at a wavelength of 1550 nm. This result shows a three-fold increase relative to the birefringence of a conventional Panda-type PMF. The effective mode area is ideal for sensing applications and there is a significant improvement in the nonlinearity. Remarkably, the fiber still exhibits excellent reflection spectra characteristics after being inscribed into the grating. The fabrication process of the novel PMF is briefly introduced. The geometric dimensions of the side-holes in the proposed PMF are shown to be deviation-tolerant within the existing fiber manufacturing facility restrictions, and thus are promising for practical fabrication. In summary, the proposed PMF with improved polarization-maintaining ability is a promising candidate for wide use in highprecision optical fiber sensors and fiber communication systems, with the specifications of the fine-diameter PMF conforming to the trend of miniaturization in current fiber optic sensors.
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