  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.
