1. Optics for Gaussian beams

Optical instruments modify the geometry of beams by changing their orientation and intensity in accordance with the equations for tracing light rays [E 4 042] . These rules also apply to Gaussian beams. First, they are applied to the passage through a thin biconvex lens to establish the basic formulas. Then, the generalization to paraxial optics is made by introducing the matrix formalism, taking into account the fact that these Gaussian beams travel along the optical axis with low divergence