Deformation of the space–time structure of a laser pulse as a result of two-photon absorption

Paraxial equations for the intensity and eikonal of a light wave, and also a kinetic equation for the difference between the level populations are used to analyze deformation of the space–time structure of a laser pulse occurring in a medium exhibiting two-photon absorption under noncoherent interaction conditions. The analysis is made using time and space compression functions, and the results of a numerical solution of two-dimensional equations for the propagation of radiation. A determination is made of the principal laws governing the changes in the time and space distributions of the intensity of radiation under quasisteady and transient interaction conditions, and also in the case of a small signal. It is shown that two-photon absorption results in a significant inhomogeneity of the space–time structure of a pulse. The results are given of experimental investigations of changes in the time, space, and energy characteristics of a neodymium laser pulse undergoing two-photon absorption in gallium arsenide.