Kürzlich, a Forschung Team von dem Zustand Schlüssel Labor von Starkfeld Laser Physik, Shanghai Institut von Optik und Präzision Maschinenbau, Chinese Academy of Sciences, discovered the phenomenon that the higher the laser repetition frequency, the higher the optical intensity of the filament in the process of high-frequency mJ-scale femtosecond laser filamentation and atmospheric filamentation, and put forward a femtosecond laser atmospheric filamentation pulse accumulation effect based on the "low-density hole". A physical image of the atmospheric filamentation pulse accumulation effect based on "low-density holes" is presented. The related paper was published in Hoch Leistung Laser Wissenschaft und Technik.
With the rapid development of kHz and even 100kHz high-frequency femtosecond lasers, high-frequency femtosecond laser atmospheric filamentation provides unprecedented opportunities for laser processing, fog-piercing communication, laser cloud rain generation, laser lightning and other applications. Due to the millisecond-scale photothermal relaxation of air molecules, the pulse accumulation effect during atmospheric filamentation of high-frequency femtosecond lasers is inevitable, and an in-depth understanding of the impact of the pulse accumulation effect on the process of high-frequency laser filamentation is the key to the further development of the new applications of laser atmospheric filamentation. Focusing on the above key issues, the research team has carried out atmospheric filamentation using a femtosecond laser with a heavy frequency of up to 100 kHz and pulse energy of 0.4 mJ. The research team has found that the longer the filament is at the higher heavy frequency, the weaker the fluorescence of a single pulse is, the stronger the third harmonic induced by the filament is, and the threshold of the induced high-voltage discharge breakdown is reduced, and the physical mechanism of the high-heavy-frequency air channel at a low density is initially proposed through the pulse cumulative effect ［(A). Advanced Photonics Research 4, 2200338 (2023)].
In this work, the researchers calculated the filamentation process of a single femtosecond laser pulse by numerical simulation, obtained the spatial distribution of the filament plasma density, calculated the heat of complexity of the plasma based on the plasma density, and combined with the heat conduction equation to obtain the "low-density holes" induced by the filament at different repetition frequencies. The correlation coefficients of the numerical simulation equation for nonlinear transmission of femtosecond laser pulses are corrected by the "low-density holes" to obtain the filament-forming results of laser pulses with different repetition frequencies, and the phenomenon that the filament-forming intensity of femtosecond intense laser atmospheres increases with the increase of repetition frequency is found. By measuring the fluorescence of nitrogen molecules and nitrogen ions induced by the filament to characterize the light intensity inside the filament, the experiments confirmed the theoretical expectation and successfully explained the variation rule of light intensity inside the atmospheric filament induced by femtosecond laser pulses with different repetition frequencies, which provided a reliable scientific basis for the in-depth understanding of the atmospheric filamentation of high-repeat-frequency femtosecond lasers and the development of their new applications.
Abb. 1 Theoretische Berechnungen von der räumlichen Entwicklung von der atmosphärischen filamentbildenden Intensität von der Schwerlastfrequenz Femtosekunde Laser at 100 Hz und 10}00 Hz für andere Puls Energien: (a) 0,1 mJ, (b) 0,2 mJ, (c) 0,7 mJ und (d) 1,2 mJ.
Abb. 2 Experimentelle Ergebnisse von 100 Hz und 1000 Hz Schwerfrequenz Laser nichtlinear Transmission Durchschnitt Intensität Variation mit Laser Puls Energie (a) und Licht Intensität Variation innerhalb das Filament mit Laser Schwerefrequenz at a Laser Puls Energie von 1,2 mJ (b). (c) und (d) sind die entsprechenden numerischen Simulation Ergebnisse. (e) Dichte Verteilung von Luft Molekülen in der niedrigen Dichte Region bei verschiedenen Wiederholungen Frequenzen.
