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.