Abstract: A laser has a slab in which the beam F is propagated in zig-zag manner between two opposing faces which have the property of ensuring total reflection. For this purpose each of these faces is covered with a solid layer of a good heat conducting material and having a refractive index below that of the slab. Its thickness is preferably greater than three times the operating wavelength of the laser. Thus, a cooling fluid can be in contact with the solid layer to effectively cool the slab. Pumped energy sources are preferably provided opposite the layers.