Sub-ppb NaCl aerosol detection at a distance of 30 meters by femtosecond laser induced plasma spectroscopy.

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 acoustic signal emitted from the optical filament is increased by 69.5 times, and the detection of limit of sodium element in 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 decreasing the sodium chloride aerosol's detection of limit.

Detection of 1.4 µg/m3 Na+ in aerosol at a 30 m distance using 1 kHz femtosecond laser filamentation in air.

An optimized remote material detection scheme based on the laser filament-induced plasma spectroscopy and light detection and ranging (FIPS-LIDAR) is proposed in this work. The elemental composition and concentration of aerosol are measured by FIPS-LIDAR. By focusing the femtosecond laser with a large aperture (Φ41 cm) concave mirror and coaxial fluorescence collection scheme, the remote detection of aerosol in air at µg/m3 level has been realized at a distance of 30 m. The limit of detection for Na+ in aerosol droplets is 8 ppm (3 µg/m3 in air), which is the lowest detection limit that has been reported using millijoule femtosecond laser pulse (4.4 mJ). Furthermore, using spectral preprocessing and optimization of the proposed significance of peak (SOP) algorithm, feature peak signals are extracted from weak signals and the limit of detection can be further decreased to 1.4 µg/m3.

Giant enhancement of acoustic and fluorescence emission from an off-axis reflective femtosecond laser filamentation system.

Femtosecond laser filamentation propagating tens of meters to several kilometers with high intensity in the atmosphere has been demonstrated as a powerful tool for remote sensing. In contrast to the refractive systems, the reflective optical systems possess a variety of advantages including broad bandwidth, large aperture, light weight and low energy loss. However, astigmatic aberration is inevitably introduced by off-axis reflective mirrors. It can greatly affect the filament quality, which is critical for exciting and detecting the fluorescence of target molecules. Here we elaborately design a free-form phase plate to correct the astigmatism in off-axis reflective optical systems. It is demonstrated that the free-form surface exhibits excellent performance, significantly reducing the astigmatic difference from 44 cm to 4 cm and increasing the maximum acoustic intensity by a factor of 53. In addition, extremely strong nitrogen fluorescence spectra have been detected. These results indicate that the free-form phase plate can effectively compensate for astigmatic aberration in off-axis reflective system, providing a guiding significance for the optimal control of filamentation and remote sensing.