Philip H. Jones, Onofrio M. Maragò & Giovanni Volpe
Fig. 4.6 — Gaussian beam
The intensity distribution of a Gaussian beam is (a) Gaussian in the transverse xy-plane and
(b) cylindrically symmetric around the propagation z-axis. (c) As the beam propagates along the z-axis, its phase deviates (top) with respect to that of a reference plane wave (bottom), leading to a phase shift of exactly π as z goes from −∞ to +∞ (Gouy phase shift); the shades of grey represent the phase of the beam from 0 to 2π .