A laser for use in all fields of measurement using a heterodyne effect delivers two waves at different frequencies, having mutually orthogonal linear polarizations. It comprises a lasing medium placed in a laser resonating cavity defined by mirrors and a birefringent crystal placed on the path of the two waves. The lasing medium is located in the resonating cavity in a zone where the two waves are separated spatially. Two quarter wave plates, one of which has neutral lines at substantially 45.degree. from the polarization axes of the birefringent means, are placed in the common path of the two waves so as to give a helical polarization to the two waves between the plates.