All-fiber flat-top orbital angular momentum mode converter realized by a SMF-based helical grating with phase modulation

An all-fiber flat-top orbital angular momentum (OAM) mode convertor with ultra-wideband is numerically demonstrated, which is based on a helical long period fiber grating (HLPG) with phase modulation. The proposed phase-modulated HLPG is formed in a single mode fiber (SMF) and works at dispersion turning point. A flat-top OAM mode convertor with a bandwidth of ∼371 nm and a conversion efficiency of ∼100% has been successfully obtained. The temperature and strain effects on the proposed OAM mode convertor have been also theoretically investigated, respectively. The results show that the proposed phase-modulated HLPG based flat-top OAM mode convertor has a certain resistance to external temperature and strain changes, which will be beneficial to reduce its packaging cost and difficulty. The proposed device may find potential applications in the OAM mode division multiplexing (MDM) system combined with the wavelength division multiplexing (WDM) technique.