Abstract: Laser device emitting a far infrared wave beam having a mode confining for charged particles. The laser devices of this kind generally emit circular polarized wave beams in which the phase shift of the radioelectric oscillation at a point having a given azimuth with respect to the beam axis is proportional to the azimuth. They comprise a quarter wave plate for converting a linear-polarized wave to a circular polarized wave and a non-confining to confining mode converter. The known converters use birefringent materials but there is no birefringent material available in the far infrared range. The present non-confining to confining mode converter uses dielectric parallel sided plates having a metal coated face forming a mirror.