Abstract

We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally, in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside the ${{\rm LP}_{11}}$ group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside the ${{\rm LP}_{11}}$ group. We employ two output OAM modes ${l} = + 1$ and ${l} = - 1$ as resultant linear combinations of degenerate ${{\rm LP}_{11 \rm a}}$ and ${{\rm LP}_{11 \rm b}}$ modes inside the ${{\rm LP}_{11}}$ group of a ${\sim}{0.6}{\text -}{\rm km}$ GI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying data stream. We measure the transmission matrix (TM) in the OAM basis within ${{\rm LP}_{11}}$ group, which is a subset of the full LP TM of the FMF-based system. An inverse TM is subsequently implemented before the receiver by a spatial light modulator to mitigate the intra-modal-group power coupling. With AO mitigation, the experimental results for ${l} = + 1$ and ${l} = - 1$ modes show, respectively, that (i) intra-modal-group crosstalk is reduced by ${\gt}{5.8}\;{\rm dB}$ and ${\gt}{5.6}\;{\rm dB}$ and (ii) near-error-free bit-error-rate performance is achieved with a penalty of ${\sim}{0.6}\;{\rm dB}$ and ${\sim}{3.8}\;{\rm dB}$, respectively.

© 2020 Optical Society of America

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