Focal-ratio degradation (FRD) mitigation in a multimode fibre link using mode-selective photonic lanterns

We present a new way to mitigate focal-ratio degradation (FRD) when using optical fibres to transport multimode light. Our approach exploits a custom multicore fibre (MCF) with six dissimilar cores that are single-mode at ∼1550 nm wavelength and minimally coupled over 7 m. We fabricated adiabatic mode-selective photonic lanterns (PLs) at each end of the MCF to create a fibre link with multimode ports, the PLs coupling each spatial mode of the multimode ports to a specific core of the MCF and vice versa. The PL-MCF-PL link exhibits superior FRD behaviour compared to a conventional multimode fibre that also supports six modes, because it inhibits the transfer of light from lower order modes to higher order modes. These results open up a potential powerful new approach to mitigate FRD in multimode fibre links, with particular applications in astronomical instruments.