Hooting modes in a CO2 waveguide laser

CO₂ waveguide lasers `hoot´ when, instead of the solitary resonator mode expected from a homogeneously broadened gas laser, two or more resonator transverse modes laser simultaneously on a common laser line. The unwanted mode or modes, even if very faint and present only occasionally, can impede heterodyne measurements where a clean frequency spectrum is required. The authors report measurements of transverse-mode beat frequencies from a small RF-excited CO₂ laser with a hybrid Al/Al₂O₃ square-bore waveguide and plane mirrors; these frequencies alter with mirror tilt by up to 70 MHz mrad^-1. A minimum beat frequency is seen when the laser is well assigned. The authors suggest that a model of the misaligned resonator may help to identify the faint modes responsible for hooting