Orthogonal long-period fiber grating for directly exciting the orbital angular momentum

An orthogonal long-period fiber grating (OLPFG) is proposed and demonstrated for directly exciting the orbital angular momentum (OAM), without the need for other devices. This grating was produced using CO₂ laser exposure in the orthogonal direction. A helical phase was then optically induced in the OLPFG, with a chirality determined by the structure of the OLPFG. In this study, ±1-order OAM resonances were respectively observed in OLPFGs with a different orthogonal direction. The conversion efficiency of OAM mode in this process was 99%, and the purity was higher than 98%. In addition, incident light in any polarization state was observed to excite OAM with the same polarization.