Observations of the light intensity variations across the partially transmitting face of a laser crystal operated near threshold are reported. The intensity variation patterns are suggestive of the modes of a cylindrical microwave cavity and are believed to be closely related to the transverse modes of the optical resonator. Preliminary polarization studies indicated that patterns with only a twofold symmetry axis are linearly polarized, whereas others are unpolarized. The pattern sizes range between approximately 170 and 850 μ. The divergence of the emerging beam (several milliradians) decreased with increasing resonator length. Fabry-Perot analysis showed only one excited longitudinal mode for a 1-cm long resonator, but the number increased with the resonator length.
V. Evtuhov and J. K. Neeland, "Observations Relating to the Transverse and Longitudinal Modes of a Ruby Laser," Appl. Opt. 1, 517-520 (1962)