Selection of $CO_2$ laser lines by a reflecting diffraction interferometer

An analysis is made of the selection of the $CO_2$ laser emission lines by a diffraction structure consisting of a thin metal coating. A method is proposed for design calculations of a resonator containing such a structure. It is shown that the ratio of the selection sharpness to the losses experienced by the selected line can be varied within a fairly wide range by the choice of the refractive indices of the media surrounding the structure and of the relative proportion of the area occupied by the metal. It is shown that if the metal coating is characterized by a low (0.1-1%) transmission, the losses introduced in the resonator by such a reflecting interferometer are smaller than the losses in a good-quality gold mirror; moreover, the minimal thermal effects are also retained. The results are reported of an experimental study of a low-pressure $CO_2$ laser with diffraction structures in the form of silver and aluminum gratings. The experimental results are in agreement with the theory. It is shown that vibrational-rotational laser transitions can be selected without additional losses compared with a nonselective resonator. A method for experimental determination of the selector parameters is described.