Self-started unidirectional operation of a fibre ring soliton laser without an isolator

We demonstrate self-started mode-locking in an erbium-doped fibre ring laser by using the nonlinear polarization rotation mode-locking technique but without an isolator in cavity. We show that, due to the intrinsic effective nonlinearity discrimination of the mode-locked pulse propagating along different cavity directions, the soliton operation of the laser is always unidirectional, and its features have no difference to that of unidirectional lasers with an isolator in cavity.