Improved defocus stability in laser-induced breakdown spectroscopy via microlens array beam homogenization

Abstract

The significant signal fluctuation in laser-induced breakdown spectroscopy (LIBS) severely limits its quantitative analysis accuracy. Although beam homogenization has proven effective in mitigating these fluctuations by producing uniform ablation conditions, conventional diffractive optical elements (DOEs) are constrained by their requirement for single-transverse-mode lasers and exhibit strong sensitivity to defocus, restricting their applicability in practical LIBS environments. To address these limitations, we propose a beam homogenization strategy using a microlens array (MLA). We demonstrate that the MLA efficiently transforms a multimode laser beam (M² = 5) into a top-hat-like profile with a periodic array distribution, thereby redistributing energy more uniformly across the focal region. This transformation suppresses air breakdown caused by localized energy concentration at the focus. Plasma imaging confirms the suppression of air breakdown and reveals highly symmetric plasma evolution within both positive and negative defocus ranges. As a result, the depth of focus (DOF) is extended from 3–3.25 mm to 7 mm, leading to improved defocus tolerance and enhanced spectral stability. In inclined surface scanning experiments on micro-alloyed steel, this DOF extension reduced the spectral signal's relative standard deviation (RSD) from 35–50% to around 20%, while increasing the signal intensity by a factor of 1.2 to 2. Furthermore, in calibration experiments simulating surface undulations under varying defocus distances, the root mean square error (RMSE) for Cu I 324.75 nm was substantially reduced from 1.836% to 0.687%. These results confirm that MLA-based beam homogenization is a robust, mode-insensitive, and highly effective solution for significantly improving the reliability and quantitative accuracy of LIBS on uneven surfaces.