A Detailed Investigation Into Birefringence Characteristics of a 20/400-µm Polarization Maintaining Large Mode Area Double Clad Fiber Depending on Material and Geometric Properties

This work systematically investigates the primary factors affecting birefringence in a 20/400 µm polarization-maintaining large-mode-area double clad fiber (PLMA DCF), which is extensively used in high-power laser applications. The fiber structure comprises a pure SiO₂ cladding, a GeO₂-doped core, and B₂O₃-doped stress-applying parts (SAPs). The finite element method (FEM) was utilized to analyze the complex relationships between material characteristics and geometric attributes. Initially, birefringence was evaluated by contrasting ideal circular cross-sections with those of elliptical geometry. The effects of SAP location and diameter on both the distribution of stress and resulting birefringence were quantitatively assessed. Furthermore, the influence of material variation was studied, focusing on the impact of SAP numerical aperture (NA) as it relates to B₂O₃ concentration. To mitigate deformation stemming from thermal expansion mismatch, the introduction of a Ge-Al-co-doped cap encasing the core was explored. This cap substantially lessened stress disparities between the core and adjacent materials, thereby enhancing mechanical reliability, while maintaining effective birefringence. The results offer actionable guidance for the design optimization of PLMA DCF systems.