The principles of use of gratings as laser wavelength-selective end reflectors are reviewed. A useful output beam can be derived from a grating’s zeroth-order reflection. This beam moves when the grating is rotated to select various laser wavelengths, but can be made stationary by the addition of auxiliary mirrors. The grating-mirror combination has been applied to a CO2 laser in the in and to a dye laser in the visible.
T. M. Hard, "Laser Wavelength Selection and Output Coupling by a Grating," Appl. Opt. 9, 1825-1830 (1970)