A Ytterbium-doped fiber laser in a unidirectional ring cavity containing a polarizer placed between two in-line polarization controllers is presented. Depending on the equivalent saturableabsorber the laser operates in continuous, Q-switched, Q-switched mode-locked or CW modelocked regimes. The passive method exposed here allowed us to choose the operating regime of the fiber laser by rotating the two polarization controllers and adjusting the pump power. As there is no active element inside the cavity, this laser potentially has more advantages than conventional pulsed fibre laser systems. Furthermore, the operation processes are analyzed theoretically.