  We consider an Yb-doped double-clad fiber laser in a unidirectional ring
cavity containing a polarizer placed between two half-wave plates. Depending on
the orientation of the phase plates, the laser operates in continuous,
Q-switch, mode-lock or unstable self-pulsing regime. An experimental study of
the stability of the mode locking regime is realized versus the orientation of
the half-wave plates. A model for the stability of self-mode-locking and cw
operation is developed starting from two coupled nonlinear Schrodinger
equations in a gain medium. The model is reduced to a master equation in which
the coefficients are explicitly dependent on the orientation angles of the
phase plates. Analytical solutions are given together with their stability
versus the angles.
