Determining the Spot Size and Gaussian Distribution Coefficient of Pulsed Laser Beams Using KAPTON Films

The laser beam spot size at the focal point of a pulsed laser is a critical parameter in laser materials processing, but accurate determination of the spot size is very difficult, particularly for high power pulsed lasers. In this study, effective methods were developed for such determination, by interpretation of measured hole diameters produced in Kapton film using pulsed laser beams with multimode or Gaussian-like mode structures. Analysis of the hole-making process for multimode distributed laser beams showed that the measured hole diameter, Dh, varied linearly with the inverse of pulse energy, 1/Ep. By plotting experimental data on graphs of Dh versus 1/Ep, inexpensive but reliable estimates of the spot size of multimode pulsed laser beams were obtained. Further analysis showed that plots of Dh2 versus ln(Ep) could provide estimates of the distribution coefficient of Gaussian-distributed laser beams. These values were found to be in good agreement with focussed beam dimensions of a pulsed Nd:YAG laser determined with a rotating-wire type laser beam analyzer (LBA).