Measurement of focused beam waist of UV pulse laser and material processing threshold

A method is introduced for measurement of the beam waist from a UV pulse laser on its focal plane and the processing thresholds of silicon and copper. The processing threshold is defined as the threshold of material removal. It is hard to measure the beam waist of a focused UV pulse directly due to its discontinuity, high intensity, and small size. The punched radius, important for micromachining, depends on the processing threshold of the material, when the processing parameters are set. Based on the formulas of laser beam profile, the relation among the pulse power, beam waist, processing threshold, and the punched radius is obtained. In our experiment, we punch the test sample at the focal plane with different powers by adjusting the attenuator. The pulse power and the punched radius can be measured. The linear regression of logarithm pulse power versus square of punched radius yields the values of beam waist and processing threshold.