We propose that more than one spatial fundamental Gaussian mode may occur i
n concentric and confocal resonators under persisting nonlinear effect. Ext
ensively studying the influence of the resonator's parameters in a Kerr-len
s mode-locked resonator, pitchfork bifurcation results in more symmetrical
configurations, and saddle-node bifurcation appears as the symmetry being b
roken. From the properties of bifurcation, we suggest that the equal-arm an
d near-confocal resonator is suitable for the emergence of bistability in K
err-lens mode-locked lasers. (C) 1999 Elsevier Science B.V. All rights rese
rved.