The effect of coupling between core and cladding modes in twisted microstructured optical fibers

We provide a detailed explanation of the effect of coupling in helical microstructured optical fibers (MOFs) based on decomposition of core and cladding modes into multiple angular harmonics, each carrying integer orbital angular momentum and spin. The obtained results show that in twisted hexagonal MOFs, the fundamental mode characterized by spin s may couple only to the cladding modes composed of harmonics carrying the total angular momentum J= 6K+ s, where K is an integer number. Strongest couplings are observed to the cladding modes composed primarily of the harmonics with the same spin as the fundamental mode. However, there is also a second type of coupling to the modes composed primarily of the harmonics with opposite spin, which we identify for the first time in case of twisted MOFs. These couplings result in polarization dependent narrow maxima in loss characteristics of the fundamental modes. Furthermore, we explain how the harmonic of opposite spins influence the ellipticity angle distribution in the cladding modes.