Circularity-preserving fiber is primarily used in key sensitive components in systems such as fiber-optic current transformers (FCTs). It maintains circular polarization and is resistant to environmental disturbances.High-birefringence fiber preforms are used and rotated at high speed during the drawing process. This specialized process produces two eigenmodes of elliptically polarized light. Its inherent birefringence effectively resists stress induced by external conditions (such as humidity, bending, and vibration), thereby effectively maintaining circular polarization properties. Circularitypreserving fiber not only maintains circular polarization but also exhibits exceptional resistance to environmental disturbances, ensuring high operating accuracy despite temperature fluctuations and vibration.