Spectral stability improvement through wide fields of view collection optics in laser-induced breakdown spectroscopy applications

Abstract

The shot-to-shot fluctuation of the line intensity is a significant challenge in LIBS applications. LIBS spectra are obtained by spatially integrating the inhomogeneous plasma emission with the spatial window of the collection optics. The field of view, the radial extent of the spatial window, plays a crucial role in LIBS spectra collection by determining the size of the plasma emission sampling region. However, the impact of the field of view on LIBS spectra remains to be fully understood. In this study, we provide collection models and field of view calculation formulae for three types of light collection optics used in LIBS: bare fiber, imaging, and Köhler. Different collection optics with varying fields of view gradients were designed to investigate the impact of the field of view on spectral signals. Using ray tracing algorithms, we simulated their respective collection geometries. Simulation results revealed an asymmetric diamond-shaped high-efficiency region for imaging, while bare fiber and Köhler exhibited a trapezoidal high-efficiency region with reduced chromatic aberration effects. Additionally, LIBS experiments were conducted with these collection optics. The experimental results demonstrated that the wide field of view collection optics can reduce shot-to-shot fluctuation of line intensity by 30–50% to around 5% compared with the narrow field of view collection optics. Moreover, the novel wide field of view collection method using Köhler optics obtains signals with low shot-to-shot fluctuation and the highest intensity by providing uniform and suitable fields of view across wide wavelength ranges.