Variation with cavity Q of the beat frequency between axial modes of a gas laser

Investigations at 0.633 mu m and 1.15 mu m in the He-Ne laser show that beat frequencies at about c/2l and 2c/2l between neighbouring axial modes are found to shift linearly with inserted cavity loss. These effects are explained using Lamb theory. Information is gained concerning the stability of mode distributions. The effect is also investigated in the argon ion laser at 0.488 mu m.