Plane and spherical waves are simple solutions of the Helmholtz wave equation, as discussed in Chap. 3 . However, planewaves have infinite extent and do accurately represent beams emitted by light sources like lasers. Laser beams are typically well collimated and have a finite extent. The wavefront normals of these beams form a small angle with the direction of propagation giving them the characteristics of paraxial rays described in Chap. 3 . Therefore, a more precise description of optical beams is needed. This is achieved through an approximate solution of the Helmholtz wave equation known as the paraxial approximation, and the corresponding paraxial wave equation. This approximate analytic solution of the paraxial wave equation is known as the Gaussian Beam, characterized by a Gaussian intensity distribution across the beam.