Acoustic measurement of the critical power for self-focusing of femtosecond lasers in air

Femtosecond laser filamentation, characterized by unique nonlinear optical phenomena, has a wide range of applications, including air lasing, air waveguides, and free-space optical communication. The critical power for self-focusing is a key parameter in this process; however, existing measurement methods are often limited by factors such as versatility and practicality. In this study, we introduce a newly developed acoustic technique using a cell phone to determine the critical power for self-focusing of femtosecond Gaussian and vortex beams in air. By analyzing the peak intensity and spatial integral of acoustic signals along the laser propagation path as a function of incident laser energy, we successfully measured the critical powers for self-focusing of both laser beam types. The technique offers several advantages, including simplicity, cost-effectiveness, high sensitivity, immunity to background light, and robustness in noisy environments. Our results suggest that acoustic diagnostics can be a viable alternative to traditional optical methods for studying laser filamentation, particularly in challenging environments.