Mode Counting in Multimode Anti-Resonant Hollow Core Fibres

Multimode optical fibres are widely used to couple to intense multimode sources, increasing their throughput efficiency and lowering damage thresholds compared to single-mode fibres. As a result, there has been recent interest in multimode hollow core fibres which can enable even higher damage thresholds and operate at challenging wavelengths such as the ultraviolet or mid-infrared. However, the strong mode-dependent loss means the effective number of guided modes depends on the fibre length as well as its input coupling conditions.