Efficient generation of second to fourth order OAM beams based on long period fiber gratings written by preset twist

In this paper, an efficient all-fiber method is proposed to generate high-order OAM beams by employing a preset twisted LPFG in a few-mode fiber. In the experiment, the conversion from fundamental mode to second-order mode, third-order mode and fourth-order mode was achieved. The conversion efficiencies were 99.47%, 99.55% and 99.69%, and the insertion losses were 0.27 dB, 0.96 dB and 2.4 dB, respectively. This method has a great guiding significance to generate higher-order mode with high efficiency and low insertion loss.