Spot size characterization of focused non-Gaussian beams

We present a new technique for the analysis of non-Gaussian laser beams which can not be described by an analytical formula. As a generalization of the beam spot area we apply and extend the definition of so called effective area (Aeff) [1] in order to avoid using the full-width at half maximum (FWHM) parameter which might be misleading for non-Gaussian beams. Furthermore, we demonstrate a practical utilization of our technique for a femtosecond soft x-ray free electron laser. The ablative imprints in poly(methyl methacrylate) - PMMA and amorphous carbon (a-C) are used to characterize the spatial beam profile and to determine the effective area. Two procedures of the effective area determination and their further utilization are presented in this paper