Core-to-Cladding Ratio-Optimized L-Band Coupled 12-Core Fibre Amplifier with the Highest Power Conversion Efficiency

We reveal that the core-to-cladding ratio (R
cc
) dependence on the power conversion efficiency (PCE) in cladding-pumped multi-core fibre (MCF) amplifiers differs depending on the operating bandwidth. We obtain the highest PCE from the reported cladding-pumped L-band coupled-MCF amplifiers, 5%, with a fabricated R
cc
-optimized 12-core amplifier.