Verdet Constant of the "Active Medium" in a Laser Cavity

It is shown that the frequency of the intensity modulation observed when the output of a gas laser in a homogeneous axial magnetic field is veiwed through a polarizer is simply related to the Verdet constant of the "active medium." This method is used to determine the Verdet constant of "active" neon at 0.633 μ. A value of 5.9×${10}^{-7\mathrm{}}$ rad/cm-Oe is obtained. Theoretical expressions for the Verdet constant of a dilute monatomic gas at a frequency close to the center of a Doppler-broadened line are derived for the three allowed transitions, $\Delta J=0\mathrm{}$ and $\Delta J=\pm 1$. The results of the experiment and the theory are used to estimate the threshold values of the absorption coefficient and the population inversion density in the present case.