Beat frequency between two axial modes of a He–Ne laser with internal mirrors and its dependence on cavity Q

Differences of the beat frequencies of a He-Ne laser in two-frequency operation up to 1.8 MHz in the two different states of polarization are observed. These differences of the beat frequencies are explained by the dependence of the beat frequency on the cavity Q, using Lamb's theory. A method of measuring the cavity Q of a passive laser resonator as a function of the direction of polarization of the external exciting radiation is described. The linear dependence of the difference of the beat frequencies DeltafII,I and DeltafI,II from the difference of the reciprocal values of the cavity Q factors 1/Q(II) and 1/Q(I), derived from Lamb's theory for the simple case of midtuning, is proved qualitatively by experiments.