Compact diode-pumped Nd:YAG laser for remote analysis of low-alloy steels by laser-induced breakdown spectroscopy

Abstract

A low weight diode-pumped Nd:YAG laser (400 g, 1064 nm, 5 ns, 130 mJ per pulse) was developed for a compact laser-induced breakdown spectroscopy (LIBS) system to be installed on a robotized arm. Fiber optics delivery vs. conventional LIBS were compared for C, Si, Mn and Cr analysis in low-alloy steels. Fiber optics transformed the multimode laser beam to a flat-top beam with an improved fluence profile stability, resulting in shallow and more reproducible craters. A fast imaging study revealed that plasma generated by fiber optic pulses was plane-shaped, more uniform and dissipated two-fold faster compared with the plasma induced by direct laser beam focusing. Greater peak fluence for conventional LIBS provided plasma with 20–100 times more intensive emission due to the greater ablated mass, higher temperature and electron density. Improved reproducibility of shot-to-shot measurements was observed for plasma induced by fiber optic pulses, due to more stable ablation. The analytical capabilities of LIBS were compared for fiber optics vs. conventional LIBS in terms of calibration curve linearity, limits of detection and the root mean square error of the cross-validation procedure. Limits of detection for Si, Cr and Mn were always better for direct laser beam focusing; however, more importantly, the conventional LIBS system provided quantitative analysis for carbon in low-alloy steels (0.025–0.5% wt) with acceptable detection limits (55 ppm) while fiber optic pulses produced too-low intensity plasma.