Raising the saturation point of fluorescence emitted by air optical filament via π phase plate

Femtosecond laser filament induced breakdown spectroscopy is a promising technique that can be used in the remote sensing of air pollutants. Raising the laser energy saturation point below which the laser filament fluorescence increases exponentially with the incident laser energy will significantly improve the sensing sensitivity and detection distance. In this work, a saturation point raising scheme is proposed in which a quarter-circle π phase plate is used to form four optical filaments in air. The extrapolation based on the experimental results infers that the employment of the quarter-circle π phase plate and its induced multiple filaments can lead to the increase of the laser energy saturation point by four times, which is proportional to the number of sub-regions in the phase plate.