Energy limit for linear-to-nonlinear femtosecond laser pulse focusing in air

TL;DR

This study investigates the effects of optical nonlinearity on femtosecond laser pulse focusing in air, identifying optimal focusing parameters to minimize distortions and maximize laser intensity for micromachining applications.

Contribution

It experimentally and numerically determines the laser pulse energy and aperture thresholds that minimize distortions during nonlinear focusing in air.

Findings

Minimal pulse distortions occur within NA=0.002 to 0.005 for 1 mJ pulses.

Threshold energy for nonlinear effects decreases with increasing numerical aperture.

Optimal focusing conditions enhance laser intensity and beam quality for micromachining.

Abstract

Propagation of a tightly focused high-power ultrashort laser pulse in an optical medium is usually substantially influenced by the medium optical nonlinearity that can noticeably affect the laser pulse parameters around the nonlinear focus and lead to unavoidable and often undesirable spatial distortions of the focal waist. We present the results of our experimental study and numerical simulations on a femtosecond Ti-Sapphire laser pulse propagation in air under different spatial focusing. We concentrated our study on spectral-angular and spatial pulse transformations under different focusing regimes, from linear to nonlinear one, when pulse filamentation occurs. For the first time to the best of our knowledge, we found the laser pulse numerical apertures range,namely, from NA=0.002 to 0.005 (for laser pulse energy of 1 mJ), where the laser pulse distortions both in frequency-angular spectrum and pulse spatial shape are minimal. By means of the numerical simulations, we found the threshold pulse energy and peak power in a wide range of focusing conditions, within which a transition between the linear and strongly nonlinear laser pulse focusing in air takes place. This energy limit is shown to decrease with pulse numerical aperture enhancement. Our findings identify the laser pulse numerical apertures and energy adequate for getting a maximum laser intensity with a good beam quality around the focal point suitable for various laser micropatterning and micromachining technologies.