Optical micro-lens array for laser plasma generation in spectrochemical analysis

Abstract

A Q-switched Nd:YAG laser beam is focused by an optical micro-lens array to produce a laser plasma at a target. The spatial intensity distribution in the focal plane is an array of closely-spaced high intensity spots. At a laser output energy of ∼200 mJ (wavelength 1064 nm, pulse duration ∼8 ns, repetition rate 10 Hz), a laser-induced air breakdown is observed in ∼47 spots inside an area of ∼9 mm diameter at the focal plane. For solid targets, a laser plasma is produced with a micro-lens array for spectrochemical analysis, e.g., laser-induced breakdown spectroscopy. Emission spectra of the laser plasma plume as well as pulse-to-pulse measurements with a simultaneous spectrometer are taken from a rotating slag sample with inhomogeneous distribution of species such as SiO₂ or CaO. For the micro-lens configuration, the variation of the spectral line emission, e.g., of Si and Ca lines, is reduced in comparison with a single lens focusing showing the averaging effect of the micro-lens array produced plasma. A more representative analysis of the average concentration of inhomogeneous samples can be expected without elaborate and time-consuming sequential scanning of extended sample areas. Calibration curves for SiO₂ and Fe₂O₃ are taken with samples of silicate glasses.