Self Focussing as a Coherence Propagation Phenomena. An Application to Calculate the Coherence Length for an Atom Laser

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)