The advantages of photonic bandgap fiber (PBF) in terms of temperature, radiation, magnetic field, and other aspects of environmental adaptability make it an important development direction of fiber optic gyroscope technology. PBF has attracted extensive attention from research institutions worldwide. However, the fiber loss of seven-core PBF which is suitable for fiber-optic gyroscopes is large, and the PBF cannot meet the low-loss application requirements of fiber-optic gyroscopes for fibers.

The PBF loss is mainly caused by the coupling between the fundamental mode and surface modes of the core wall, and the scattering loss caused by the roughness of the inner wall of the fiber core. In this study, an isolated anti-resonant core photonic bandgap fiber (IAC-PBF) is developed, in which the fiber structure can isolate the core from the cladding, thereby the coupling between the fundamental mode and surface mode is suppressed through the anti-resonant effect of the core wall. An IAC-PBF structural model is established and the loss of the fiber is calculated using the F parameter method. The optimized fiber structure is obtained by scanning the structural parameters. The mode characteristics of the fiber and the loss reduction principle are determined by mode coupling analysis, and it is verified that the proposed fiber structure has low theoretical loss. Finally, the fiber is fabricated using the stacking-drawing method. Although the core size is small and the bandgap is offset, the feasibility of low-loss fiber is proved.

The IAC-PBF structure is proposed in this article (Fig. 1). Its cladding structure is the same as that of 19-cell PBF, and the photonic crystal structure is still formed through the periodic arrangement of hexagonal air holes to generate the photonic bandgap effect to prevent light leakage from the cladding. A hexagonal anti-resonance layer is used to isolate the core and cladding layers spatially. This reduces the mode coupling and scattering loss. The mode field diameter is approximately 8 μm, and the fiber loss is less than 3.5 dB/km, which is achievable with the current fiber fabrication process (Fig. 2). The reason for the increase in loss caused by structural changes is studied based on the relationship between the mode coupling and the structure of the PBF (Fig. 4). The simulation results show that the fundamental mode couples with the surface mode of the core wall when the core wall is thin. When the thickness of the core wall is greater than 0.4 μm, the surface mode of the core wall is coupled with the higher-order mode, and the mode refractive index is far from that of the fundamental mode, ensuring the decoupling of the fundamental mode from the surface mode and higher-order mode. For the bending loss, when the bending radius is greater than 4 mm, the bending loss of the IAC-PBF varies within 0.3 dB/km, maintaining the excellent bending characteristics of the PBF (Fig. 5). The IAC-PBF is fabricated using the stacking-drawing method (Fig. 7). The feasibility and loss reduction effect of the proposed fiber structure are verified by fiber testing and theoretical analysis, laying a foundation for the subsequent development of long-distance PBFs with a small mode field and low loss (Fig. 8). Theoretical simulation and experimental results show that IAC-PBF exhibits small fiber loss, a small mode field diameter, and bending resistance.

Compared with anti-resonance fiber, the PBF has relatively large loss but a small mode field and bending resistance, making it an ideal fiber for a high-stability fiber-optic gyroscope. In this study, an isolated anti-resonance core photonic bandgap fiber is designed. In this configuration, the core and cladding are spatially isolated, and the coupling between the fundamental and cladding surface modes is significantly reduced. An anti-resonance core is used to enhance the confinement of the fundamental mode, compress the fundamental mode field, and reduce the scattering loss. At the same time, the core structure of the IAC-PBF is formed by a stack of independent capillaries, unlike the traditional bandgap fiber which is formed by the capillary surrounding the fiber core. In the process of drawing, it is easy to control the fiber core thickness and reduce the roughness of the core wall, which can significantly reduce the fiber loss. The theoretical analysis results show that the loss of the PBF with a mode field diameter of approximately 8 μm can be reduced to less than 3.5 dB/km. The structure of the fiber is basically reproduced, but the difference in the quartz wall thickness causes a shift in the bandgap. The minimum loss of the fiber is approximately 25 dB/km @ 1200 nm.