Fig. 1
Three regimes of spin orbit interaction. Conceptual illustration of the effect of SOI, with OAM depicted as a spiral (with handedness giving sign, and the number of arms the value of L) and SAM as circular black arrows. Colors are used to distinguish different modes and do not indicate the frequency of the light beam. a In the scalar approximation (negligible SOI), two OAM states of the same |L| will propagate with the same effective index, regardless of polarization. b In fiber engineered for OAM-based data transmission, due to SOI, states with OAM and SAM parallel and antiparallel have different effective and group indices. c In strongly-SOI-enhanced fibers, superpositions of free space SAM and OAM states with specific relative amplitudes and phases propagate through the fiber as a single mode. These modes are defined by a total AM and can be obtained as the product of a given OAM phase distribution and a space-variant elliptical polarization state (as in mode 4), but they are neither OAM nor SAM eigenstates, and would not be propagation-invariant modes of free space
