A laser device comprising: a rectangular enclosure having an array of periodic gain elements distributed in a lateral direction in the rectangular enclosure with a period P gain, the lateral direction being perpendicular to a longitudinal direction along which the radiation of free space wavelength λ is amplified, the rectangular enclosure having a front exit side and a back side that are normal to the longitudinal direction, and two lateral sides that are parallel to the longitudinal direction; two parallel lateral mirrors (1, 2) situated at the lateral sides of the rectangular enclosure; a back mirror situated at the back side of the rectangular enclosure, the back mirror together with the two lateral mirrors forming a U-Shaped mirror configuration; a semi-transparent mirror at an input-output boundary situated at the front exit side opposite to the back mirror for closing an inside optical path, such that the laser amplification is produced along the inside optical path, by back-and-forth traveling between the back mirror and the semi-transparent mirror; and the semi-transparent mirror allows the laser field distribution to exit the rectangular enclosure; the laser device is characterized in that: the lateral mirrors reflect with phase shift π the laser field component which propagates in the lateral direction, and allow the formation of lateral stationary sinusoidal waves having nodes at the lateral mirrors; the lateral stationary sinusoidal waves include a preferred mode which has its field intensity peaks at the gain element locations; the preferred mode consists of laterally propagating wave which have a wavelength Λ q, twice the gain elements distribution period P gain; the preferred mode is an out of phase mode due to each intensity peak coupled with a gain element being out of phase with its adjacent peaks coupled with corresponding gain elements; the laterally propagating waves arriving at the input-output boundary exiting the rectangular enclosure in two diffraction limited beams, inclined at an inclination angle θ relative to the boundary normal given by the relation sin θ = ±λ/Λ q; wherein the device further comprises two external mirrors (6, 7) configured to rearrange the two beams in parallel and close to each other, such that the beam parameter product of the resulting beam can be only three times larger than that of a single diffraction limited beam. a rectangular enclosure having an array of periodic gain elements distributed in a lateral direction in the rectangular enclosure with a period P gain, the lateral direction being perpendicular to a longitudinal direction along which the radiation of free space wavelength λ is amplified, the rectangular enclosure having a front exit side and a back side that are normal to the longitudinal direction, and two lateral sides that are parallel to the longitudinal direction; two parallel lateral mirrors (1, 2) situated at the lateral sides of the rectangular enclosure; a back mirror situated at the back side of the rectangular enclosure, the back mirror together with the two lateral mirrors forming a U-Shaped mirror configuration; a semi-transparent mirror at an input-output boundary situated at the front exit side opposite to the back mirror for closing an inside optical path, such that the laser amplification is produced along the inside optical path, by back-and-forth traveling between the back mirror and the semi-transparent mirror; and the semi-transparent mirror allows the laser field distribution to exit the rectangular enclosure; the lateral mirrors reflect with phase shift π the laser field component which propagates in the lateral direction, and allow the formation of lateral stationary sinusoidal waves having nodes at the lateral mirrors; the lateral stationary sinusoidal waves include a preferred mode which has its field intensity peaks at the gain element locations; the preferred mode consists of laterally propagating wave which have a wavelength Λ q, twice the gain elements distribution period P gain; the preferred mode is an out of phase mode due to each intensity peak coupled with a gain element being out of phase with its adjacent peaks coupled with corresponding gain elements; the laterally propagating waves arriving at the input-output boundary exiting the rectangular enclosure in two diffraction limited beams, inclined at an inclination angle θ relative to the boundary normal given by the relation sin θ = ±λ/Λ q; wherein the device further comprises two external mirrors (6, 7) configured to rearrange the two beams in parallel and close to each other, such that the beam parameter product of the resulting beam can be only three times larger than that of a single diffraction limited beam.