SELF-FOCUSING OF ELLIPTIC GAUSSIAN LASER-BEAMS IN SATURABLE NONLINEARMEDIA

A semi analytical treatment of elliptic Gaussian laser beam propagatio n in a saturable nonlinear medium is presented. Two values of critical electric field (P(cr1), P(cr2); P(cr1) < P(cr2)) are found for self t rapping and these values increase with increasing ellipticity. Below a minimum value of effective beam radius it is not possible to self tra p the beam and this minimum value increases with increasing ellipticit y. It is further shown that, unlike Kerr medium, collapse of the laser beam never takes place in this case. When p < p(cr1) the beam defocus es. When p lies between p(cr1) and P(cr2) (i.e. P(cr1) < P < P(cr2)) t he medium behaves as an oscillatory waveguide; the beam first diverges , then converges and so on. For p > p(cr2), the beam first diverges, t hen converges, then diverges and so on.