Ultra-broadband conversion of OAM mode near the dispersion turning point in helical fiber gratings

Abstract

Based on the dual-resonance principle around the dispersion turning point (DTP), for the first time, an ultra-broadband orbital angular momentum (OAM) mode converter formed by the helical long-period fiber grating (HLPG) is proposed. The converter used for delivering the OAM operation with 3-dB bandwidth of 287 nm, which is about 7 times of general OAM converters and has only one mode. Furthermore, by chirping the HLPG working around DTP, a flat-top broadband OAM mode converter with bandwidth of ∼182 nm@3 dB is conveniently achieved. The flatness of spectrum can be increased by apodizing and optimizing the length of the chirped HLPG. Subsequently, we significantly developed a flat-top broadband rejection filter with >30 dB bandwidth of a high level of ∼100 nm@1 dB by double-cascading the HLPG. It is shown that the performances of the OAM mode converter and the flat-top broadband rejection filter can be remarkably improved by accomplishing the DTP in the mode phase-matching for the HLPG.