Diffraction-Beam Optics of Filamentation: I–Formalism of Diffraction Beams and Light Tubes

The concept of nonstationary diffraction-beam optics of high-power femtosecond laser pulses is presented. According to the concept, the power of a beam propagates along specific light structures—diffraction- beam tubes. These tubes do not intersect and do not exchange energy, but changes in their shape and cross sections during propagation show the effect of physical processes that occur with radiation in the medium. The nonstationary theory is supplemented with evolutionary equations for time-averaged diffraction rays and effective squared radii of diffraction tubes.