Bending Stability of Polarization in Hollow-Core Photonic Bandgap Fibers

Hollow-core photonic bandgap fibers (HC-PBGFs) exhibit exceptional environmental stability, making them as prime candidates for next-generation, high-precision fiber optic gyroscopes (FOGs). High fiber birefringence is non-negotiable for maintaining gyroscope accuracy. However, the coil winding process introduces a challenge: bending shifts the core mode away from the center, altering its overlap with the core wall, which consequently destabilizes the birefringence. This study systematically investigates how bending impacts birefringence in two standard PM HC-PBGF designs, 7-cell and 19-cell fibers. We explore the effects of varying core geometries (specifically, different fundamental mode-surface mode anti-crossing states), bending radius, and bending directions. Using the birefringence change in Panda polarization-maintaining fiber at a 2 cm bending radius (measured at 0.46×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup>) as our benchmark, we find that the variations in birefringence of HC-PBGFs arise from surface mode drift. Near the anti-crossing point, the 7-cell HC-PBGF exhibits a striking maximum change of 4.15×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup>, being an order of magnitude greater than Panda fiber. The 19-cell fiber follows with a maximum of 0.77×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup>, roughly double that of Panda fiber. Crucially, away from this anti-crossing point, the birefringence change plummets below 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup>, demonstrating stability two orders of magnitude superior to Panda fiber. These insights chart a clear course for designing low bending sensitivity fibers, providing essential theoretical and data-driven guidance for developing highly stable hollow-core fibers tailored for FOG applications.