Role of symmetry in mode coupling in twisted microstructured optical fibers.

We have studied the effect of symmetry on coupling between core and cladding modes in helical microstructured fibers, which gives rise to resonant loss peaks observed in the fiber transmission spectra. We demonstrate that the selection rules for orbital and spin angular momenta of coupled modes, proposed first for twisted conventional fibers, are universal characteristics and correctly identify the coupled cladding modes in helical microstructured optical fibers (MOFs). Moreover, we show for the first time (to our knowledge) the effect of coupling between modes of opposite polarization handedness in twisted MOFs.