Direct Measurement of Radial Fluence Distribution Inside a Femtosecond Laser Filament Core

Modulation and direct measurement of the radial fluence distribution inside a single filament core (especially less than 100 μm in diameter) is crucial to filament-based applications. We report direct measurements of the radial fluence distribution inside a femtosecond laser filament core and its evolution via the filament-induced ablation method. The radial fluence distributions were modulated by manipulating the input pulse diffraction through an iris. Compared with using a traditionally circular iris, a stellate iris substantially suppressed the diffraction effect, and laser fluence, as well as the plasma density inside the filament core, was considerably increased. The radial fluence inside filament cores was also quantitatively measured via the filament drilling diaphragms approach. Furthermore, numerical simulations were performed to support the experimental results by solving nonlinear Schrödinger equations. In addition to being beneficial in understanding the filamentation process and its control, the results of this study can also be valuable for filament-based applications.