In Situ Acoustic Monitoring of Focused Femtosecond Pulse Duration in Air

ABSTRACT The monitoring of the acoustic amplitude of an air breakdown at the focus of the femtosecond laser pulse was used to determine the pulse duration using a computer microphone. Pulse duration was tuned using the chirp settings of a femtosecond laser and measured by an autocorrelator before the focusing optics, where the fs pulses were compressed down to fs (‐Photonics compression unit). The amplitude of the acoustic signal scaled with the pulse duration as in the range from 50 fs to 2 ps. The acoustic signal showed dependence on the pulse energy , which was the same for the shortest (50 fs) and longer (250 fs) pulses widely used in laser machining and 3D nano/microprinting. This scaling is consistent with the electron–ion scattering defined by the electron temperature in the air breakdown plasma . The demonstrated principle that the sound of air breakdown can be used to measure the pulse duration (50 fs–5 ps) is promising for femtosecond laser pulses at different wavelengths, where second‐harmonic autocorrelators are not available.