I have aligned a CO2 laser using HeNe laser with 2 ZnSe brewster windows, an output mirror and a back mirror,  and the alignment is almost right. Then, I fired the CO2 laser and I got nearly 28 W as an average power. In addition, I have to use a diffraction grating to tune the lasing wavelength in a single wavelength. What is the best procedure to replace the back mirror (full reflection mirror) with the diffraction grating as an alignment process?