Tuning a grating-controlled CO2 laser by means of a scanning slit

The use of diffraction gratings as tuning devices in laser resonators is well known and has been demonstrated abundantly for CO2 lasers. As the grating rotates, the lines of the infrared spectrum of the CO2 molecule reach threshold and oscillate one by one. Here it is shown that it is possible to obtain the same spectrum by moving an aperture inside the cavity perpendicularly to the laser axis while keeping the grating stationary. The conditions for obtaining a well-resolved spectrum are formulated for both the rotating grating and the moving aperture case.