Shanghai Institute of Optics and Fine Mechanics has made progress in the field of femtosecond laser air filamentation self focusing threshold research

Recently, the research team of the State Key Laboratory of Intense Field Laser Physics of the Chinese Academy of Sciences Shanghai Institute of Optics and Fine Mechanics has made progress in the research on the repetition rate dependent femtosecond laser air filamentation self focusing threshold. The relevant research results were published in Optics Express under the title "Pulse repetition rate effect on the critical power for self focusing of femtosecond laser in air".

Femtosecond laser filamentation is generated by the dynamic balance between Kerr self focusing effect and plasma defocusing effect, and has shown great potential for applications in fields such as ultra short pulse compression, terahertz radiation, atmospheric remote sensing, and weather control. The development of high-frequency femtosecond laser technology has brought new opportunities for optical fibers in fields such as cloud penetration, artificial lightning induction, and laser processing.

In recent research, the team focused on the atmospheric filamentation process of high-frequency femtosecond laser and discovered the characteristics and laws of high-frequency femtosecond laser filamentation. (Adv. Photon. Res. 4, 2200338 (2023)) They clarified the mechanism of the effect of pulse accumulation on light intensity (High Power Laser 11, e46 (2023)), plasma density, and temperature (Chin. Opt. Lett. 22, 013201 (2024)), and solved the bottleneck problem of automatic filamentation caused by pulse accumulation effect (Light: Sci.&Appl. 13, 42 (2024)). However, the self focusing threshold is a key parameter for evaluating whether the filamentation process occurs, and the effect of high repetition rate dependent pulse accumulation on the self focusing threshold of femtosecond laser filaments is not yet clear.

Figure 1 shows fluorescence signals induced on the filament axis at incident energies of (a) 240 µ J, (b) 280 µ J, (c) 300 µ J, and (d) 325 µ J with a repetition rate of 1 kHz. (e) Residual plots of traditional Gaussian fitting and bimodal fitting

In this work, researchers proposed a new method for determining the self focusing threshold based on bimodal fitting by utilizing the dual effects of geometric focusing and Kerr self focusing during femtosecond laser air filamentation. And through in-depth analysis of residuals, root mean square error, fitting determination coefficients, etc., it is proved that the proposed method is superior to the traditional Gaussian fitting method and can more accurately determine the self focusing threshold. The research team measured the self focusing threshold of femtosecond laser filamentation in air at 1 kHz, 500 Hz, 100 Hz, and 50 Hz repetition rates, and discovered a new pattern of lower self focusing threshold and easier filamentation of femtosecond laser pulses transmitted in air compared to lower and higher repetition rates. The numerical simulation results confirmed the accuracy of the experimental pattern. This work provides new ideas for the study of self focusing and is of great significance for a deeper understanding of the characteristics of high repetition rate femtosecond laser filaments.

Figure 2 shows the functional relationship between the peak position (bimodal fitting) of the fluorescence signal induced by the filament and the pulse energy at repetition rates of (a) 1 kHz, (b) 500 Hz, (c) 100 Hz, and (d) 50 Hz. The intersection point between the red fitting lines represents the self focusing threshold. (e) The functional relationship between self focusing threshold and repetition rate obtained using traditional Gaussian fitting (blue) and bimodal fitting (red)

Source: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences