Effects of focusing geometries and detection methods on laser-induced breakdown spectroscopy of an aluminum target

Abstract

The effective breakdown and signal detection are the basis of laser-induced breakdown spectroscopy (LIBS) optical systems for elemental analysis. High-quality spectra can provide reliable data support for LIBS technology. In this work, the effects of focusing geometries and detection methods were investigated on LIBS analysis of an aluminum target at laser energies of 2, 10 and 20 mJ. Two focusing geometries were selected for comparison, namely focusing with a beam expander and focusing directly. The plasma signals were collected backward through a dichroic mirror. Two detection methods were employed, one to detect the plasma emission through transmission fibers at different angles and the other to couple the plasma emission directly into the entrance slit of the spectrometer. It is shown that focusing with a beam expander could increase the focusing angle and decrease the laser power density, leading to poor focusing quality, relatively weak signal intensity and signal-to-noise ratio (SNR). Focusing without a beam expander makes the ablation crater smaller and deeper, and leads to the enhancement of line intensity and SNR compared to focusing with a beam expander. In addition, detection by the entrance slit of the spectrometer directly can reflect the difference between the focusing geometries truthfully. The optical fiber endface offers a wide range of reception angles, enabling it to absorb more plasma emissions and reduce signal loss. Larger detection angles are typically preferred when detecting via optical fiber to achieve superior LIBS signals, which has been verified by plasma images. The present results suggest that focusing without a beam expander and detection by an optical fiber with a large angle make it easier to obtain high quality plasma signals when performing LIBS measurements on solid samples.