Confining light in air- and hollow-core microstructured fibers has many advantages such as low nonlinearity, low scattering, low dispersion, low loss, and low delay that are expected to break through the limits of fiber optic gyroscopes caused by the backscattering, Faraday effect, Kerr effect, and Shupe effect. They have become a hot topic in the research on a new generation of high-precision fiber optic gyroscopes. At small bending radii (on the order of millimeters), the optical fiber generates large stress and torsion owing to deformation, making it difficult to measure the bending loss. Therefore, there are few experimental reports on the bending loss of hollow-core photonic bandgap fiber (HC-PBGF) under an extremely small bending radius. In addition, in the winding process of the fiber optic gyro ring, to overcome the stress during bending and make the optical fibers compact, it is often necessary to apply a certain tension that would also increase bending loss. Therefore, it is of great significance for the development of high-precision fiber optic gyroscopes to research the bending loss of hollow-core microstructure fibers.

Hollow-core microstructured fibers are mainly divided into HC-PBGFs and hollow-core anti-resonant fibers (HC-ARFs). In this study, these two hollow-core microstructured fibers with excellent performance are drawn using the stack and draw technology. The finite element method combined with a perfect matching layer is used to simulate the scattering and confinement losses of two hollow-core fibers in straight and bent states. In the bending loss measurement, the light of the supercontinuum source is drawn out through the pigtail of the single-mode fiber and coupled into the hollow-core fiber through the fusion splicer. Optical fibers are wound onto cylinders with different bending radii, and the output of the optical fibers is connected to the spectrometer. The bending loss is determined by the difference in the spectra before and after bending. Furthermore, the HC-PBGF is tightly wound on the gyro ring skeleton by applying different tensions in turn with a fiber optic gyro winding machine, and the bending loss is measured using the truncation method.

Two types of hollow-core microstructured fibers propagating mainly at a wavelength of 1550 nm are successfully fabricated. The duty ratio of the HC-PBGF is as high as 97.6%, its core diameter is 32.8 μm, and the thickness of the lattice wall is between 40 nm and 75 nm. As for the fabricated HC-ARF, the core diameter, wall thickness of the cladding, and inner diameter of the cladding tube are 34.5 μm, 553 nm, and 23.8 μm, respectively. In terms of the simulated results, the bending loss of the HC-PBGF is 2-3 orders of magnitude lower than that of the HC-ARF at small bending radii (Fig. 4). The confinement loss of the PBGF begins to increase sharply when the bending radius is reduced to 1 cm, and the mode field of the fundamental mode shifts severely, but no coupling exists between the wall dielectric mode and the core fundamental mode. For the HC-ARF, the core and cladding modes achieve phase matching when the bending radius is 3.5 cm and the energy of the core leaks into the cladding. The experimental results show that in the wavelength of 1624 nm, the bending losses of the PBGF under bending radii of 5 cm, 3 cm, and 1 cm are 1.38 dB/km, 11.74 dB/km, and 23.29 dB/km, respectively. When the bending radius is less than 1 cm, the bending loss increases significantly. For example, at an extremely small bending radius of 0.25 cm, the bending loss at 1624 nm rapidly increases to 231.58 dB/km (Fig. 6). However, the bending loss of the HC-ARF is 2-3 orders of magnitude higher than that of the HC-PBGF (Fig. 7). At the wavelength of 1550 nm, after applying stresses of 0.1 N, 0.5 N, and 0.9 N using the fiber gyro loop winding machine, the ring losses increase by 3.69 dB/km, 16.43 dB/km, and 70.15 dB/km, respectively.

This study proves that the HC-PBGF has a better bending resistance than the HC-ARF. By using the bipolar symmetric winding method, the bending loss of the HC-PBGF at an extremely tight bending radius of 0.25 cm is successfully measured,which is 231.58 dB/km @1624 nm. This is the current lowest bend loss reported for such a tight bending radius. Furthermore, the losses of the HC-PBGF loop under different winding tensions are measured for the first time oriented to the application of a fiber optic gyro. The results show that the losses of the HC-PBGF loop increase significantly as the winding tension increases. Therefore, the winding of the HC-PBGF loop should be performed under small tensions. The research results are of great significance for promoting the application of hollow-core microstructured fibers in fiber optic gyros.