Double-cone structure of an optical beam in a nonlinear medium

A double-cone structure for an optical beam in an amplifying medium with appreciable multiphoton absorption and Kerr nonlinearity is predicted and investigated. An analysis is made of the width of the principal maximum and of the contrast of the ring structure (which divides the double-cone structure of the optical beam into outer and inner regions), and also of the distance at which this is formed as a function of the parameters of the active medium. Attention is drawn to the possibility of measuring multiphoton absorption and Kerr nonlinearity constants by observing the double-cone structure. It is also noted that high-order cylindrical guided light waves, which are not subject to self-focusing even at powers substantially higher than the critical value, may be excited in the adjacent medium.