Based on the dual-resonance principle around the dispersion turning point, a scheme of chiral long-period fiber gratings (CLPGs) formed by twisting a high-birefringence (Hi-Bi) fiber is herein proposed to realise ultra-broadband flat-top circular polarizers. The coupling bandwidth is approximately seven times larger than that of traditional CLPGs. In addition, by introducing chirp characteristics in these CLPGs, an ultra-broadband flat-top circular polarizer with ∼200 nm@3 dB was conveniently achieved. Subsequently, by optimising the chirped CLPGs, a circular polarizer with a bandwidth extinction ratio of approximately 30 dB and a high level of ∼100 nm at 1 dB was realised. It was shown that the mode-controlling performances of the CLPGs can be remarkably improved, which has significant applications in light-field regulation. Finally, for the first time, it was proved that the CLPG cannot generate a vortex beam.

Circularly polarized light play an important role in various areas. Compared with the circular polarizer generated in free space, the all-fiber circular polarizer based on double helix chiral long period fiber grating (CLPG) has the advantages of higher conversion efficiency, compatibility, and lower insertion loss, etc. However, circular polarizers of CLPGs have the problems of narrow bandwidth, low purity, and uneven working band, which seriously limit the performance and become a new challenge in this field. In this work, we proposed a method to efficiently generate an ultra-broadband flat-top circular polarizer by double-helix CLPGs. The mechanism of realizing the ultra-broadband and flat circular polarizer of the CLPG—which was eight times larger than that of traditional CLPGs—was elucidated. Meanwhile, a broadband and flatness circular polarizer mode coupling theory for CLPGs was proposed and established.

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