A grating can be rotated to reflect one particular wavelength exactly back along the incident direction. In this 'Littrow' configuration, a sufficiently efficient grating can be used as a wavelegth-tunable mirror in a laser. This grating is directly ruled in metal to stand high powers and is blazed to maximise its efficiency in the 10 micron wavelength range.
This one was used in a CW CO2 laser with a power of about 10 W in order to select one of the many possible wavelengths corresponding to transitions between different rotational energy levels of the CO2 molecules. The mount holds the grating at roughly the correct angle, fine adjustment is made using the mirror mount screws.
The CO2 laser was then used to optically pump C2H2F2 molecules in a waveguide laser operating at 870 micrometre wavelength.
