Sub-ppb aerosol detection at a distance of 30 meters by millijoule femtosecond laser pulse filamentation in air

TL;DR

This study demonstrates sub-ppb aerosol detection at 30 meters using femtosecond laser filamentation, with a novel off-axis focusing method that significantly enhances detection sensitivity and reduces astigmatism.

Contribution

It introduces a non-symmetrical beam focusing technique that improves laser filament intensity and aerosol detection limits in air over long distances.

Findings

Detection limit of sodium chloride aerosol reduced to 0.32 ppb

Optical filament intensity increased by 69.5 times

Astigmatism significantly decreased with off-axis focusing

Abstract

In this work, sub-ppb aerosol detection is achieved by femtosecond laser filament with a single pulse energy of 4 mJ at a distance of 30 m. A concave mirror with an open aperture of 41.4 cm is employed in an off-axis optical system to focus the femtosecond laser beam and collect the fluorescence of NaCl aerosol. The simulation and experimental results show that the astigmatism can be greatly reduced when femtosecond laser beam is incident non-symmetrically on the concave mirror. Compared with the case that femtosecond laser strikes at the center of the concave mirror, the intensity of the optical filament is increased by 69.5 times, and the detection of limit of sodium chloride aerosol is reduced by 86%, which is down to 0.32 ppb. The improved excitation scheme in this work utilizes the nonsymmetrical beam spot on the concave mirror to compensate the non-symmetry induced by the off-axis setup, reducing the astigmatism of the focusing laser beam and improving the aerosol's detection of limit.