Investigating the Threshold Conditions of Air Breakdown with Mode-Locked Q-Switched Laser Pulses, and the Temporal Dynamics of Induced Plasma with Self-Scattering Phenomenon

Abstract

A Q-switched Nd:YAG laser with mode-locked modulations is utilized to explore the laser-induced air breakdown. The various modulation depths of the mode-locking within the Q-switched pulse can be utilized to investigate the threshold conditions. With the GHz high-speed detectors to accurately measure the temporal pulse shape pulse by pulse, it is verified that the air breakdown threshold is crucially determined by the peak-power density instead of the energy density from the statistic results, especially for mode-locked Q-switched lasers. The stability of the system for laser-induced breakdown can be evaluated by threshold width through fitting the statistical result. Otherwise, by measuring the temporal characteristics of the excitation pulse and the induced plasma, it is further found that the plasma radiation displays a few-nanoseconds time delay to the excitation pulse and shows a decaying tail to be 10 times longer than the plasma build-up time. Moreover, the incident laser pulse is observed to be self-scattered by the air breakdown, and a rapidly modulated scattering rate is found with a slight delay time to the excitation mode-locked subpulse modulations.

The influence of more concentrated energy distribution formed by an aspheric lens with eliminated spherical aberration on laser-induced air breakdown

“…The definition of the air breakdown threshold is the laser power at which 50% of the laser pulses induce evident energy absorption and air breakdown, according to previous studies. 17,20,30 The scattering laser energy is measured. The results show that the scattering laser energy is of micro-joule order, so it is ignored.…”

“…The trends of the resulting curve and air breakdown thresholds are consistent with previous studies. 17,30…”

“…Since the temporal and spatial distribution of the laser exhibits an approximately Gaussian-like distribution, the particles in the air are ionized when the peak point reaches the laser power density threshold. 30 The aspheric lens forms a smaller beam radius near the focusing spot, a more concentrated laser energy distribution, and a higher energy density. Therefore, the air breakdown threshold can be reached at lower laser energy.…”