Abstract: A FIR Laser cavity comprising a pair of concentric tubes of heatproof dielectric, a pair of nozzles intended for circulating within the space between said pair of tubes a refrigerating and thermally stabilizing fluid; a pair of laser heads disposed at the two ends of said tubular assembly which heads are provided with optical elements; a differential thread sleeve mounted concentrically with said tubes and intended for obtaining the longitudinal micropositioning of one of said heads with respect to one another; two series of three screws which are mounted with a starlike symmetry on said heads and intended for allowing a fine adjustment of the optics carried by said heads along two axes perpendicular to one another and with respect to the geometrical axes of said tubes by means of the elastic deformation of an O ring which is interposed between each head and the tubular body and an output optics carrier and comprising either a reflector and a window for the hermetic seal, or by the reflector alone, which is g