In recent years, femtosecond laser filamentation has attracted much attention and has shown great potential in atmospheric remote sensing owing to its unique characteristics. Femtosecond laser filamentation characterization is the basis of filament regulation and application. However, the high laser intensity and electron density in the plasma channel make it challenging to measure the filament directly. Fortunately, the energy conversion effects among light, acoustic, and thermal signals during filamentation enable studies into the exploration and diagnosis of filaments using acoustic and optical methods. Owing to difference in the microscopic physical mechanism of acoustic waves and fluorescence radiation during the filamentation process, there is a difference in the quantitative relationship between the two signals and the physical parameters of the filament. However, there is still a lack of comparative research on the accuracy of the two methods. This study investigates the effect of pulse energy on the spatial distribution of filament, and the differences and similarities between acoustic and fluorescence methods for characterizing filament are systematically compared. The reasons for the differences between the two methods are theoretically analyzed.
The longitudinal plasma profile of filament induced by the propagation of an intense femtosecond laser pulse in air is measured simultaneously by employing acoustic and fluorescence methods. Acoustic emissions from the filament are detected by a directional microphone at a distance of 1 cm from the filament. To measure filament fluorescence, a convex lens (focal length f =50 mm) is employed to collect and focus the filament fluorescence onto the entrance slit of a monochromator. The dispersed fluorescence is detected by a photomultiplier tube placed at the exit slit of the monochromator. The triple standard deviation of the background noise is used as the reference for the appearance of the filament, and the differences and similarities between the acoustic and fluorescence methods in the spatial characterization of the filament are studied experimentally and theoretically.
In order to calibrate the spatial resolution of the acoustic and fluorescence detection systems, the beam is focused by a single lens with 50 mm focal length to produce a point source. By measuring the spatial distribution of such a point source, it is determined that the spatial resolution of acoustic and fluorescent methods is 0.76 cm [Fig. 2(a)] and 0.84 cm [Fig. 2(b)], respectively, which are in agreement with the results reported previously (within one centimeter). The differences and similarities between acoustic and fluorescence methods in the spatial characterization of filament are compared and analyzed. Compared with the fluorescence method, the start position of the filament measured by the acoustic method is closer to the focusing lens and the length is larger [Figs. 2(c) and (d)]. This difference increases with an increase in filament length (Figs. 3 and 4). The analysis of the physical mechanism shows that the kinetic energy of free electrons in the filament is low owing to the low intensity of the light field at the start and end of the filament, which is much less than the kinetic energy (11 eV) of electrons needed to produce N2 fluorescence through electron collision (Fig. 5).
We present a simultaneous multi-parameter measurement on filament using acoustic and fluorescence methods, and the effect of pulse energy on the spatial distribution of the filament is studied. The experimental results show that the filament length increases with increasing laser energy. Simultaneously, the start and peak positions of the filament move toward the lens. Both methods can characterize the spatial characteristics of filaments. Compared with the fluorescence method, the start position of the filament measured by the acoustic method is closer to the focusing lens, and the length is larger. A study of the physical mechanism shows that the dependence of the free electron kinetic energy in the filament on the intensity of the light field is the main reason for the difference in the characterization results. The acoustic method shows higher sensitivity to the start and end positions of the filament, which is more conducive to the experimental characterization of the weak filament.
