Impacts of a collection system on laser-induced breakdown spectroscopy signal detection

Collecting strong enough and repeatable signals from laser-induced plasmas is the primary goal of laser-induced breakdown spectroscopy optical detection systems. Typically, the light emitted from the plasma is refracted by the lens, collected by the fiber, and measured by the spectrometer. In the present work, we established a three-dimensional model to systematically evaluate the overall emission collected from different positions of the plasma for a typical optical collection system composed of a focus lens and a collection fiber, and sensitivity analyses were further performed. In addition, experiments were conducted and partially validated the model. Results showed that for the collection system with an optical fiber located on the focal point of the collection lens, the collection efficiency distribution is almost constant within a large cylindrical-shaped area, while for that located off the focal point, there is a rhombus-shaped area with higher collection efficiency than other areas. This much higher collection efficiency area is small in size but has a large impact on the detected spectral intensity. The spatially distributed collection efficiency on the lens parameters, such as size and position, was further discussed to clarify the impacts of the collection system. Furthermore, sensitivity analyses were performed to evaluate the impact of the collection system on the signal repeatability. Based on these calculations, recommendations for the design of the collection for optimized spectral intensity and stability were proposed.