Formation of plasma channels in ambient air by axicon-focused femtosecond laser pulse

In this paper, we investigate the formation of plasma channels in ambient air by axicon-focused femtosecond laser pulse experimentally. Unlike the smooth on-axis radial intensity profiles around the focal region generated by an ideal axicon, in practice, the base angle of axicon plays a determining role on the on-axis intensity profile: when the axicon base angle is 0.5°, an evident periodic structure can be observed, but a more smoothly on-axis intensity profile can be generated by an axicon with the base angle is 5°. The distance between two adjacent peaks and the full width at half maximum (FWHM) of each peak are decreased monotonously along the propagation axis. The relative intensity ratio of all three lines from the second positive band system of N2 is inconsistent with our previous work and the ratio is always positive along the propagation axis. This will be helpful to further air-lasing actions applications such as lightning control and remote sensing.