  Gas-filled hollow-core fibres are a flexible platform for the manipulation of
ultrafast laser pulses through a variety of nonlinear optical effects.
Efficient high-fidelity coupling of the initial pulses is very important for
system performance. Here we study the effect of self-focusing in gas-cell
windows on the coupling of ultrafast laser pulses into hollow-core fibres using
(2+1)-dimensional numerical simulations. As expected, we find that the coupling
efficiency is degraded and the duration of the coupled pulses changed when the
entrance window is too close to the fibre entrance. The interplay of nonlinear
spatio-temporal reshaping and the linear dispersion of the window create
different results depending on the window material, pulse duration, and pulse
wavelength, with longer-wavelength beams more tolerant of high intensity in the
window. While shifting the nominal focus to compensate can restore some of the
lost coupling efficiency, it improves the pulse duration only marginally. From
our simulations we derive a simple expression for the minimum distance between
the window and the HCF entrance facet. Our results have implications for the
often space-constrained design of hollow-core-fibre systems, especially where
the input energy is not constant.
