小 纤 芯 单 偏 振 空 芯 反 谐 振 光 纤(特 邀)

Objective Polarization effects represent a fundamental challenge that can severely degrade the performance of advanced photonic systems which are inherently polarization sensitive. Key applications such as high-precision fiber-optic sensors (such as gyroscopes, hydrophones) and fiber lasers are particularly susceptible to impairments like polarization crosstalk and polarization mode dispersion. The implementation of single-polarization fibers offers a direct and effective solution to mitigate these effects by permitting the propagation of only one stable polarization state, thereby enhancing system stability. Within the diverse family of specialty optical fibers, hollow-core anti-resonant fibers (HC-ARFs) have recently emerged as a promising platform for single-polarization guidance. Their appeal stems from exceptional properties including ultra-low nonlinearity, a high threshold for optical damage, extensive design flexibility, and the potential for broad transmission bandwidths. Despite these advantages, a significant drawback of most reported single-polarization HC-ARFs is their relatively large core diameter. This attribute leads to several practical limitations: heightened sensitivity to macro-bending, which translates to a large minimum bending radius; and substantial mode-field mismatch when attempting direct coupling to solid-core single-mode fibers (SMFs) with a core diameter of 8.2 μm. These limitations inherently restrict the deployment of such fibers in scenarios demanding miniaturization, flexibility, and easy integration with existing fiber infrastructure. To address these challenges, this study proposes a novel small-core (10 μm diameter), single-polarization, bend-resistant HC-ARF based on a semi-elliptical cladding structure. Compared to previously reported designs, the proposed fiber maintains robust single-polarization operation while achieving a substantially reduced minimum bending radius and significantly lower mode-mismatch coupling loss when directly coupled to solid-core SMFs with a core diameter of 8.2 μm. Methods The inner cladding of the designed small-core single-polarization HC-ARF incorporates four key structural elements. The primary structure begins with four half-elliptical silica tubes (Half-Elliptical Tubes 1) arranged symmetrically around the core, defining a compact core region while effectively expanding the internal cladding space. Building upon this foundation, a second set of four smaller half-elliptical tubes (Half-Elliptical Tubes 2) are positioned concentrically within the first set, in order to strengthen optical field confinement, thereby reducing confinement loss. The third and most critical element for achieving single-polarization operation is the intentional introduction of structural asymmetry. This is accomplished by placing an elliptical tube incorporating a glass rod along the x-direction, while placing a double-nested elliptical tube along the y-direction within the respective Half-Elliptical Tubes 2. Finally, four circular tubes are strategically located on the outer cladding wall at ±45° and ±135° angles relative to the x-axis to further improve bending resistance. The finite element method is employed to systematically investigate the influence of various structural parameters on fiber performance, such as the confinement loss, birefringence, and the polarization extinction ratio (PER), facilitating the optimization of their values. Following the structural optimization, the transmission characteristics of the proposed fiber across different wavelengths are analyzed to evaluate its operational bandwidth. The coupling loss arising from mode-field mismatch during direct coupling to a solid-core SMF with a core diameter of 8.2 μm at 1550 nm is calculated. Furthermore, the bending characteristics are examined for different bending directions to determine the minimum sustainable bend radius. Results and Discussions The proposed small-core single-polarization HC-ARF demonstrates excellent performance. The calculated mode-field at 1550 nm is optimally matched to that of a solid-core SMF with a core diameter of 8.2 μm, resulting in a remarkably low mode-mismatch coupling loss of only 0.074 dB. Over the wavelength range of 1508 nm to 1555 nm, the fundamental-mode transmission loss remains below 10 dB/km, while both the PER and the higher-order mode extinction ratio (HOMER) exceed 100. Bending characterization reveals that single-mode, single-polarization operation with a transmission loss under 10 dB/km is maintained for bending radii exceeding 2.26 cm (x-axis bend), 1.6 cm (y-axis bend), and 1.86 cm (45° relative to x-axis bend), respectively. The variation in bending performance with direction is attributed to the anisotropic cladding structure, and the overall results represent a significant improvement over conventional large-core HC-ARFs. Conclusions In this study, a small-core single-polarization HC-ARF based on semi-elliptical cladding structures is proposed. The introduced structural asymmetry, via an elliptical tube with a glass rod along the x-direction and a nested elliptical tube along the y-direction, induces differential coupling of the two orthogonal fundamental polarization modes to the cladding modes, resulting in a high polarization extinction ratio. Simulation results confirm that at the 1550 nm operating wavelength, the fiber achieves a fundamental-mode transmission loss of 5.68 dB/km, a birefringence of 1.29×10^-4, and a PER exceeding 120. The low mode-mismatch coupling loss of 0.074 dB with solid-core SMF (core diameter of 8.2 μm) and the broad polarization-filtering bandwidth of 47 nm (1508 ‒ 1555 nm), where fundamental-mode loss stays below 10 dB/km and PER remains above 100, highlight its practical potential. The robust bending performance, maintaining single-polarization operation with low loss for radii above 2.26 cm, indicates excellent suitability for polarization-sensitive, miniaturized, and flexible applications such as fiber optic gyroscopes and hydrophones.