  A theoretical description in terms of the coherence propagation is given for
self-focussing. The concept of coherence length is defined in terms of free,
self-focussing propagation giving results in accordance with well known
experimental criteria for the laser. Extension of the method is given for an
Atom Laser showing good results in agreement with recents numerical results of
Trippenbach et. al.: J. Phys. B:At. Mol. Opt. Phys. 33, 47-54 (2000).
