Several papers reported that the mode effective refractive index (neff) of vortex modes (such as OAM+1 mode) would be changed with the variation of different pitches (or twist rates) in the twisted optical fibers, but does neff of the degenerate vector modes (such as HE21 mode) change also? I am not very clear whether twisting just changes the phase difference between the degenerate vector modes instead of the vector modes themseleves.