Fig. 1. Point source model for the optical filament.

Fig. 2. Experimental setup for the detection of acoustic emission from the femtosecond laser filament.

Fig. 3. (a) Temporal profiles of the acoustic pulses detected at different distances. (b) Dependence of acoustic pulse amplitude on the detection distance. (c) Normalized temporal profiles of the acoustic pulses detected at different distances. (d) Fourier transforms of (a). (e) Normalized spectra of the acoustic pulses detected at different distances.

Fig. 4. (a) Schematic of experimental setup. (b) Temporal profiles of the acoustic pulses detected at different angles. (c) Dependence of acoustic pulse amplitude on the detection angle. (d) Normalized temporal profiles of the acoustic pulses detected at different angles. (e) Normalized spectra of the acoustic pulses detected at different angles.

Fig. 5. (a) Temporal profile of the acoustic pulses generated by point-like plasma and (b) the femtosecond laser filament. (c) Function h(t) calculated through deconvolution of Eq. (2) with the Wiener filter.

Fig. 6. (a) Function h(t) after performing the time-to-space coordinate conversion. (b) Amplitude distribution of the acoustic pulses emitted from different positions along the filament.

Fig. 7. (a) Temporal profiles of the acoustic pulses measured at different positions along the femtosecond laser filament. (b) Normalized temporal profiles of the acoustic pulses measured at different positions along the femtosecond laser filament. The femtosecond laser filament is generated by the lens group with an effective focal length of 5 m. The single pulse energy is 5 mJ.

Fig. 8. Spatial distribution of the optical filament obtained, respectively, by the WFD method (blue line) and the traditional point-by-point acoustic evaluation method (red line).