  Optical fibers can support various modal forms, including vector modes,
linear polarization (LP) modes, and orbital angular momentum (OAM) modes, etc.
The modal correlation among these modes is investigated via Jones matrix,
associated with polarization and helical phase corresponding to spin angular
momentum (SAM) and OAM of light, respectively. We can generate different modal
forms by adopting superposed helical gratings (SHGs) with opposite helix
orientations. Detailed analysis and discussion on mode conversion is given as
for mode coupling in optical fibers with both low and high contrast index,
respectively. Our study may deepen the understanding for various fiber guided
modes and mode conversion among them via fiber gratings
