Fig. 1. Spatial distributions of (a) nitrogen ion density induced by the filament and (b), (c) the photon density of air lasers (with and without the Kerr effect, respectively) for different external focusing conditions: (a1), (b1), (c1) f = 0.2 m; (a2), (b2), (c2) f = 1 m; (a3), (b3), (c3) f = 34 m.

Fig. 2. Far-field angular distribution of the air laser for (a) f = 0.2 m, (b) f = 1 m, and (c) f = 34 m. (d) Divergence angle of the air laser as a function of the external focusing length.

Fig. 3. Energy of the air laser along the propagation direction for external focus of (a) f = 0.2 m, (b) f = 1 m, and (c) f = 34 m. (d) Air laser energy as a function of the external focal length.