Fig. 2: Spectral evolution and spectrogram analysis of the pulse propagating in the HCF in Fig. 1b, c.

Simulated evolution dynamics of the 10 fs, 26 μJ, 766 nm pulse (N = 3.9) (a–d) and the 18 fs, 64 μJ, 781 nm pulse (N = 7.8) (e–h), with the same simulation parameters as Fig. 1b, c. a, e Simulated spectral evolution of the pulse as a function of fibre length. b–d, f–h Spectrogram analysis of the pulse at the indicated positions in the HCF. I – Narrow-band DW emission, II – asymmetric soliton splitting phenomenon, III – spectral broadening of the UV DW through XPM effect. Through using the ideal filters with transmission windows covering 400–4000 nm and 100–400 nm, the temporal profiles of the pump pulse and DW were obtained, respectively, which were plotted as red lines and blue shadows. The black lines indicate the pulse spectra at different fibre positions. The purple lines indicate the phase-matching wavelength of DW emission, and \(\varDelta {{{\rm{n}}}}\) represents the nonlinear refractive index caused by the pump pulse.