Importance of Topological Charge Preservation in Vectorial Modulational Instability in Chiral Three-Core PCF

Circular-symmetric optical fibres are ideal for studying nonlinear optical processes involving optical vortices [1] . Chiral photonic crystal fibres (PCFs), fabricated either by post-processing techniques or by spinning the preform during fibre drawing, have been shown to support circularly polarized helical Bloch modes (HBMs), which are superpositions of multiple vortices with topological charges ℓ ( m ) = ℓ (0) + mN , where N is the number of azimuthal periods and m the order of the m -th azimuthal Bloch harmonic [2] . Here we report on the importance of the preservation of topological charge in polarisation modulational instability (PMI) between circularly polarised modes in a circularly birefringent chiral PCF with a 3-fold rotationally symmetric core.