Fig. 3. Illustration of the approximation of the vortex volume. The calculated vortex boundary on the transverse plane was modeled by an ellipse, which represents the cross-section of the vortex in the transverse plane. The calculated vortex boundary and the model ellipse have the same long axis length and also the same width. The long axes of the calculated boundary and the approximated shape are parallel. The volume of the vortex is approximated by the product of the center-width w of the vortex in the laser plane view and the projected area A of the vortex ring onto a plane perpendicular to the laser sheet.