Shanghai Institute of Optical Machinery has made progress in atmospheric filament formation with high repetition frequency femtosecond laser

Recently, the research team of the State Key Laboratory of High Field Laser Physics of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences found that the higher the laser repetition rate, the higher the mercerization intensity in the process of high repetition frequency MJ-magnitude femtosecond laser filament atmosphere, and proposed a physical image of the cumulative effect of femtosecond laser filament atmosphere pulses based on "low density holes". The paper is in High Power Laser Science and Engineering.

With the rapid development of kHz and even 100 kHz heavy frequency femtosecond laser technology, high heavy frequency femtosecond laser atmospheric filaments provide unprecedented opportunities for laser processing, communication through fog, laser cloud rain, laser mine guiding and other applications. Due to the photothermal relaxation process of air molecules in the order of millisecond, the pulse accumulation effect of high repetition frequency femtosecond intense laser is inevitable in the process of high repetition frequency laser filament formation. Understanding the influence of pulse accumulation effect on high repetition frequency laser filament formation is the key to further develop new applications of laser atmospheric filament formation. Focusing on the above key issues, the research team used femtosecond laser with a refrequency of up to 100kHz and pulse energy of 0.4mJ to carry out atmospheric filament formation research, and found that the higher the refrequency, the longer the high-light filament, the weaker the single-pulse fluorescence, the stronger the third harmonic induced by the light filament, and the significantly reduced the induced high-voltage discharge breakdown threshold. A physical mechanism by which high repetition frequency generates low density air channels through pulse accumulation effect is proposed [Advanced Photonics Research 4, 2200338 (2023)].

FIG. 1 Theoretical calculation results of spatial evolution of atmospheric mercerization intensity of 100Hz and 1000Hz refrequency femtosecond laser at different pulse energies: (a) 0.1mJ, (b) 0.2mJ, (c) 0.7mJ and (d) 1.2mJ.

In this work, the researchers numerically calculated the filament formation process of a single femtosecond laser pulse, obtained the spatial distribution of the plasma density of the optical filament, calculated the composite heat of the plasma according to the plasma density, combined with the heat conduction equation to obtain the "low density hole" induced by the optical filament at different repetition frequencies. By modifying the correlation coefficient of the numerical simulation equation of femtosecond laser pulse nonlinear transmission through the "low density hole", the filamentary results of laser pulse with different refrequency are obtained. By measuring the fluorescence of nitrogen molecules and nitrogen ions induced by filament, the light intensity inside the filament is characterized. The theoretical expectation is confirmed experimentally, and the change law of the light intensity inside the filament induced by femtosecond laser pulses with different repetition frequencies is successfully explained, which provides a reliable scientific basis for further understanding of the high repetition frequency femtosecond laser atmospheric filament and developing new applications.

FIG. 2 Experimental results of the change of the average nonlinear transmission intensity of 100Hz and 1000Hz refrequency laser with the laser pulse energy (a) and the change of the internal light intensity of the optical filament with the laser pulse energy (b) when the laser pulse energy is 1.2 mJ. (c) and (d) are the corresponding numerical simulation results. (e) Distribution of air molecular density in low density regions at different repetition frequencies.

Source: Shanghai Institute of Optics and Machinery, Chinese Academy of Sciences