Evaluation of gases'molecular abundance ratio by fiber-optic laser-induced breakdown spectroscopy with a metal-assisted method

A metal-assisted method is proposed for the evaluation of gases’molecular abundance ratio in fiber-optic laser-induced breakdown spectroscopy(FO-LIBS).This method can reduce the laser ablation energy and make gas composition identification possible.The principle comes from the collision between the detected gases and the plasma produced by the laser ablation of the metal substrate.The interparticle collision in the plasma plume leads to gas molecules dissociating and sparking,which can be used to determine the gas composition.The quantitative relationship between spectral line intensity and molecular abundance ratio was developed over a large molecular abundance ratio range.The influence of laser ablation energy and substrate material on gas quantitative calibration measurement is also analyzed.The proposed metal-assisted method makes the measurement of gases’molecular abundance ratios possible with an FO-LIBS system.

Study on the Two-Dimensional Density Distribution for Gas Plasmas Driven by Laser Pulse

We perform an experimental study of two-dimensional（2D） electron density profiles of the laser-induced plasma plumes in air by ordinarily laboratorial interferometry. The electron density distributions measured show a feature of hollow core. To illustrate the feature, we present a theoretical investigation by using dynamics analysis. In the simulation, the propagation of laser pulse with the evolution of electron density is utilized to evaluate ionization of air target for the plasma-formation stage. In the plasma-expansion stage, a simple adiabatic fluid dynamics is used to calculate the evolution of plasma outward expansion. The simulations show good agreements with experimental results, and demonstrate an effective way of determining 2D density profiles of the laser-induced plasma plume in gas.

Enhancing precision in fs-laser material processing by simultaneous spatial and temporal focusing

In recent years,femtosecond(fs)-lasers have evolved into a versatile tool for high precision micromachining of transparent materials because nonlinear absorption in the focus can result in refractive index modifications or material disruptions.However,when high pulse energies or low numerical apertures are required,nonlinear side effects such as self-focusing,filamentation or white light generation can decrease the modification quality.In this paper,we apply simultaneous spatial and temporal focusing(SSTF)to overcome these limitations.The main advantage of SSTF is that the ultrashort pulse is only formed at the focal plane,thereby confining the intensity distribution strongly to the focal volume and suppressing detrimental nonlinear side effects.Thus,we investigate the optical breakdown within a water cell by pump-probe shadowgraphy,comparing conventional focusing and SSTF under equivalent focusing conditions.The plasma formation is well confined for low pulse energies,2 mJ,but higher pulse energies lead to the filamentation and break-up of the disruptions for conventional focusing,thereby decreasing the modification quality.In contrast,plasma induced by SSTF stays well confined to the focal plane,even for high pulse energies up to 8 mJ,preventing extended filaments,side branches or break-up of the disruptions.Furthermore,while conventional focusing leads to broadband supercontinuum generation,only marginal spectral broadening is observed using SSTF.These experimental findings are in excellent agreement with numerical simulations of the nonlinear pulse propagation and interaction processes.Therefore,SSTF appears to be a powerful tool to control the processing of transparent materials,e.g.,for precise ophthalmic fs-surgery.