Far-field spectra for (a) 21st, (b) 25th, and (c) 29th order. The black solid line is extracted from the spectrogram in Fig. 2(a) without propagation effect included. The propagation effect is taken into account in 1-mm-thick (red dashed line), 2-mm-thick (green dot-dashed line), and 3-mm-thick (blue dotted line) gas target centered at the beam focus with the same parameters used in Fig. 2(a).