The invention relates to a laser source with an external cavity generating a useful laser beam (Fu), that comprises at least one laser diode DL (10) mounted in an extended optical cavity (30, 82) defined between two reflecting surfaces (92, 48) forming an optical path for a laser beam Fd generated by the laser diode DL, a mode selection filter (44, 86) in the optical path of the cavity for selecting a resonance mode from a range of resonance modes of the laser beam in the cavity. The extended cavity (2) includes in the optical path thereof an optical device (94) including a polariser (100) that is 100% permissive to a rectilinear polarisation of the beam Fd generated by the laser diode, the reference polarisation angle θ0being of 0 degrees, a polarisation rotator (102) of the beam at the output of the polariser (Fc1) by a predetermined rotation angle θ−1, a polarisation separator (04) of the beam (Frt) at the output of the polarisation rotator, the polarisation separator being inclined in the cavity so that its transmission axis (tt′) is oriented at an angle (−θ−1) having an opposite sign to and the same value as the polarisation angle of the beam (Frt) at the output of the rotator (102), wherein the optical device prevents any feedback of a reflection beam (Fretum) of the useful beam (Fu) towards said laser diode DL. The invention can be used in a laser for cooling atoms, in interferometry experiments, inertial sensors and atomic clocks.