Orbital Angular Momentum Mode Converter Based on Helical Long Period Fiber Grating Inscribed by Hydrogen–Oxygen Flame

A high-efficiency grating fabrication method was, for the first time, demonstrated to inscribe helical long period fiber gratings (H-LPFGs) in small numbers by means of twisting a standard single-mode fiber (SMF) during hydrogen-oxygen flame heating and then cutting the helical fiber into in series of sections. Each section of the helical fiber was a desired LPFG whose resonant wavelength, i.e., grating pitch, can be changed by adjusting the twist rate of the helical fiber. The H-LPFG inscribed in a standard SMF could be used to generate orbital angular momentum (OAM) modes, i.e., OAM+1 mode, with a purity of 91% and a conversion efficiency of 87% within a large wavelength range from more than the cutoff wavelength of an SMF, which is highly advantageous to all-fiber optical communications based on the OAM mode-division multiplexing technique.