108.96 nm Broadband Generation of Second-Order OAM Beam Using an Angular Modulated Cascading LPFG

Broadband generation of orbital angular momentum (OAM) beam in fiber has a lot of applications in optical communications and fiber sensing. However, the efficient broadband generation of second-order OAM beam using long-period fiber grating (LPFG) is still a great challenge due to the low intermodal overlap. In this paper, we achieved a 108.96 nm efficient broadband generation of second-order OAM beams with a conversion efficiency over 95% in experiment using an angular modulated cascading LPFG with low insertion loss. By changing the refractive index modulation area in certain angle, not only the coupling between fundamental mode and second-order mode in fiber can be greatly enhanced, but also second-order even and odd modes are generated with approximately equal magnitudes, which is very beneficial to high-purity OAM beams generation. Finally, by choosing proper cascading parameters and an adaptive refractive index modulation fabrication strategy, we achieved a LPFG mode converter with a conversion efficiency over 95% in a 108.96 nm from 1520.96 nm to 1629.92 nm. The insertion loss of the LPFG is measured only similar to 0.48 dB.