We experimentally demonstrate the possibility of adiabatic conversion of LP11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the values of effective indices, the LP11 modes are selectively converted to right-/left-handed circularly polarized vortex modes HE21 with a total angular momentum of ±2 and to quasi-TE01/TM01 modes with a total angular momentum of 0. The proposed conversion method has a purely topological origin, therefore, it is broadband in nature, in contrast to the methods based on resonant effects, and can be applied as an all-fiber broadband source of vortex beams.
Funding: Narodowe Centrum Nauki (DEC-2016/22/A/ST7/00089, Maestro 8).
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 σ may couple only to the cladding modes composed of harmonics carrying the total angular momentum J=6K+σ, 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.
Using transformation optics formalism, we numerically analyzed the effect of resonant couplings between core and cladding modes in a helical core fiber with large core offset. Our study revealed much richer spectrum of resonant couplings than predicted by perturbative methods in fibers with small core offsets. Moreover, we demonstrated that the resonant couplings to lower order cladding modes predicted by perturbative methods are in fact prohibited for larger core offsets. Finally, we demonstrated the existence of spectrally broad polarization sensitive couplings to the cladding modes suggesting that helical core fibers with large core offsets may be used as broadband circular polarizers.
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