Uncertainty Evaluation in Knife-Edge Laser Spot-Size Measurements for Industrial Applications.

Accurate laser spot-size measurements are important in many scientific and industrial applications: the knife-edge technique is commonly used to measure the laser beam dimension due to the relative ease, low cost, and reliability of the measurement. We propose an original use of a numerical method to evaluate the standard uncertainty of the measured laser spot-size in knife-edge measurements and we apply it to the analysis of experimental data from two different laser sources. The method allows obtaining the sensitivities of the spot-size uncertainty to the uncertainties of relevant input quantities such as the measured optical power and the knife-edge displacement. Since the indirect measurement of laser spot-size by the knife-edge technique is non-linear, analytical calculation of the uncertainty sensitivities is not possible and they are evaluated numerically based on experimental data and specific input noise/uncertainty conditions. By our analysis, calculation of specific sensitivities for two different experimental conditions is done and then used to estimate the standard uncertainty of the measured parameter: the standard radius of a Gaussian-beam laser spot. More generally, the method proposed allows evaluating the effects of different uncertainty contributions on the laser spot-size uncertainty estimation. This can then suggest proper design of the measurement experiment with the goal of reducing the final uncertainty. Accurate measurements, and of known and tailorable accuracy, of the laser parameters are of interest in different industrial applications where precise beam shaping is important.