LAUSR

Abstract The usual parabolic approximation used for modelling the focusing properties of a Kerr lens induced by a Gaussian laser beam, through optical Kerr effect, consists to roughly approximate a Gaussian profile by a parabola. The Kerr focal length deduced from the ABCD formalism is found to not correctly provide the focal plane position. The latter has been numerically determined from a numerical diffraction analysis, and we have empirically found that the right focal length is obtained by multiplying, by a factor equal to 3, the focal length given by the usual parabolic approximation. The expression of the focal length associated with the Kerr lens that we have obtained is in a good agreement with the expression of the focal length determined on the basis of a Zernike decomposition. In addition, it is demonstrated that the emerging beam from the Kerr lens is no longer Gaussian since its propagation factor M 2 is larger than unity. It is found that the M 2 factor increases linearly with the on axis phase shift.