For instance , cylindrically symmetric gratings couple symmetric LP0m modes of the fiber .
Depending on the symmetry of the perturbation that is used to write the LPFG , modes of different symmetries may be coupled .
Cylindrically symmetric gratings couple symmetric LP0m modes of the fiber .
Depending on the symmetry of the perturbation that is used to write the LPFG , modes of different symmetries may be coupled .