Ultra-Broadband OAM Mode Generator Based on a Phase-Modulated Helical Grating Working at a High Radial-Order of Cladding Mode

In this study, an ultra-broadband orbital angular momentum (OAM) mode generator based on a phase-modulated helical long period fiber grating (HLPG) working at highradial order cladding mode is firstly proposed and numerically demonstrated. The proposed phase-only modulation method can considerably facilitate the HLPG’s fabrication by eliminating the grating’s apodization, which is extremely difficult to realize in the screw-type grating. Meanwhile, the proposed method provides more flexibilities in the design of HLPGs. As a typical example, a first-order OAM (OAM −1) mode generator with a bandwidth of ~63 nm, a conversion efficiency of ~100%, and purity of ~ 91% has been numerically demonstrated. Moreover, an OAM −1 mode generator with a bandwidth of ~469 nm, a conversion efficiency of ~100%, and purity of ~ 91% has been successfully obtained, covering all bands of fiber communication, i.e., o riginal- (O-), extended- (E-), short- (S-), conventional- (C-), long- (L-), and ultralong- (U-) wavelength bands, which, based on our knowledge, are the best results reported to date among those of the HLPG-based OAM mode generators. The proposed component may find potential applications in the OAM mode division multiplexing (MDM) system combined with the wavelength division multiplexing technique.