Wigner function and intensity moments of spatio-temporal light fields
The Wigner distribution function and its spatial-angular moments (intensity-moments) are known as efficient instruments for characterization of complex quasimonochromatic light beams and their transformations. In this paper, the generalization of the WF-based approach to spatio-temporal (ST) light fields (wave packets, short pulses) is considered. It is shown that the ST intensity moments are related with the important characteristics of the wave-packet structure, especially, with the transverse orbital angular momentum (OAM) being a specific feature of the ST optical vortices (STOV). The ST moments' transformations in a paraxial optical system obey simple and unified rules involving the ray-transfer ABCD-matrix of the system. On this base, and with simple examples of the OAM-carrying optical pulses, the schemes and mechanisms of the STOV generation and transformation are presented. Examples of non-vortex ST wave packets with the transverse OAM, their possible realizations are discussed as well as the relations between the OAM and the visible pulse rotations. The regular and unified formalism, developed in this paper, can be generalized and applied to more complex situations where the ST field propagates through inhomogeneous and random (scattering) media.