In $3\text{D}+t$ numerical simulations, we propose an experiment wherein a mixture of gases is used for the continuous transition from $\mathsf{X}$- to $\mathsf{O}$-shaped angle-wavelength spectrum of a femtosecond infrared filament. The mixture consists of transparent nitrogen at 40 bar pressure and $85{}^{\circ }\mathrm{C}$ temperature as well as absorbing water vapor at 0–0.36 bar, with the rovibrational resonance at a wavelength slightly longer than the $1.3\mu \mathrm{m}$ center of the pump pulse driving the filament. The continuous increase in the water vapor density results in the gas mixture dispersion changing from normal to anomalous and transformation of the short-wavelength conical emission onto on-axis propagating radiation.

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• Laser spectroscopy
• Nonlinear optics
• Ultrafast optics