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Mode conversion from LP11 to LP02 based on in-line mode orientation control and antisymmetric refractive-index perturbation in a few-mode fibre

We present intra-fibre mode conversion from the antisymmetric LP11 mode to the circularly symmetric LP02 mode using a force-induced, long-period fibre grating accompanied by an in-line polarization controller. The field orientation of the incoming LP11 mode is aligned to the antisymmetric perturbation in the fibre grating to activate resonant mode coupling. Mode conversion is achieved by applying pressure with a spatial period of 430 μm along an interaction length of 50 mm. We verify the mode conversion process by observing the far-field patterns of the fibre output and comparing the transmission power with and without a high-numerical aperture, narrow-core fibre. The intra-fibre mode conversion system with erasable and reconfigurable natures exhibits an insertion loss of 0.48 dB and a 3-dB bandwidth of 52 nm.