Self-focusing below the critical power in gas-filled hollow-core PCF

Summary form only given. Nonlinear optics in fibres has for decades been carried out predominantly in step-index glass cores that support just one transverse mode, or if the core supports higher-order modes (HOMs), careful launch alignment is used to minimize their excitation in practice. In gas-filled hollow-core fibres, however, single-mode propagation ceases to be valid at high intensities, even under perfect launch conditions. This is because self-focusing can be understood as being caused by transfer of energy from the fundamental mode to HOMs through nonlinear mode coupling. We report that photoionization of the gas inside a hollow-core photonic crystal fibre (PCF) leads to breakdown of single-mode behaviour well below the critical power for Kerr-effect self-focusing. The underlying nonlinear mode coupling mechanism is intermodal four-wave mixing (iFWM), phase-matched through plasmadriven changes in dispersion. In this particular case, the pump and anti-Stokes signals are in the fundamental LP01-like mode, while the Stokes signal is in the LP
₀₂
-like mode.