Re-evaluation of the peak intensity inside a femtosecond laser filament in air

Using a simple experimental method, we determine the fluence distribution inside a single femtosecond laser filament in air. This method exploits the capacity of a filament to drill small circular diaphragms through a thin metallic foil. With a complementary measurement of the pulse duration, the filament intensity can be deduced. We find a peak intensity I ≥ 1.4 × 10¹⁴ W cm^-2 in a filament generated with a weakly converging beam, of numerical aperture NA = 0.01 or NA = 0.005. This value is significantly higher than the usually quoted one. Measurement of the transmitted energy through a diaphragm of fixed diameter as a function of incident laser energy reveals a clamping of the fluence and peak intensity.