The wave fronts of the focused laser beam shown are moving with the speed of light. In the focus the wave fronts get plane. This location is a preferred part of the beam for interferometrical applications.
All previous illustrations shown have been calculated using the assumptions of
H. Kogelnik, T. Li: ‘Laser Beams and Resonators’, IEEE vol. 54 1312-1329, 1966. 5, 26
