Diffraction and self-focusing during amplification of high-power light pulses. I. Development of diffraction and self-focusing in an amplifier

It is shown that the diffraction and self-focusing of laser radiation plays the dominant role in several types of characteristic damage observed in cylindrical active elements during the amplification of high-power light pulses. An analytic treatment is given of the Fresnel diffraction of a laser beam passing through apertures of different shape that are encountered in practice. An analysis is also made of the development of small perturbations of a laser beam which eventually damage the active element because they become self-focused against the background of a strong field. It is shown that "seed" perturbations may be waves diffracted by aperatures or secondary waves which appear due to the scattering of the principal wave by local inhomogeneities in the active element.