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