A design of experiment approach for investigating active fiber lasers: Influence of the adjustable parameters in the cleaning process

The use of active fiber lasers for cleaning applications in the field of cultural heritage is in rapid expansion. One of the main potentials of this new technology lies in the possibility to combine a broader range of parameters compared to lamp-pumped lasers, enabling different types of interactions with the materials. In addition, the possibility to choose from a variety of scan shapes and sizes modifies the spatial distribution of the energy on the surface and accelerates the cleaning process. In this study, a Design of Experiment (DoE) approach was combined to Principal Component Analysis (PCA) to investigate the influence of repetition rate, pulse duration, power and scan speed in the cleaning process. The tests were performed on mock-ups replicating the Roman fresco technique coated both with an organic layer, a strappo glue, and an inorganic layer, a limewash. A series of responses were selected to evaluate the transformations occurred on the surface after laser irradiation: the changes in surface roughness and color before and after the treatment, and the assessment of the alteration percentage induced on the materials.The DoE methodology was implemented to define an experimental domain, allowing the evaluation of both the influence and the interactions of the investigated parameters across the entire domain with a preliminary settled and reduced number of tests. The selected responses were analyzed through Principal Component Analysis (PCA), which revealed the underlying correlations among them and permitted the selection of a reduced, yet representative, subset of responses for analysis. Through this integrated approach it was possible to identify the key parameters controlling the two different cleanings, to distinguish those suitable for optimization and to categorize the possible surface alterations that can occur when the parameters are not properly adjusted. Finally, the present study also highlights the differences in the removal of inorganic and organic materials.