DOES THE NONLINEAR SCHRODINGER-EQUATION CORRECTLY DESCRIBE BEAM PROPAGATION

The parabolic equation (nonlinear Schrodinger equation) that appears in problems of stationary nonlinear beam propagation (self-focusing) is reconsidered. It is shown that an additional term, which involves changes of the propagation constant along the propagation direction, should be taken into account. The physical consequences of this departure from the standard approximation, which uses the parabolic equation, are discussed. A numerical simulation showing the difference between the new approach and the standard nonlinear Schrodinger equation is given as an example.