A comparison is made of near- and far-field radiation patterns for two different ruby laser crystals of known and differing optical quality. It is shown that ring-width in far-field patterns is directly related to optical quality as measured using a Twyman-Green interferometer. Near-field patterns of the higher quality crystal (λ/10) that were observed using high-speed photographic equipment always appeared to be transverse modes of a cylindrical resonator whereas this was not true for the poorer quality crystal (λ/4). An approximate mode sequence during firing of the λ/10 laser rod shows a reproducible mode development (in time). A sudden switching to higher-order modes was observed and several factors are suggested which could explain this.
C. Martin Stickley, "Optical Quality and Radiation Patterns of Ruby Lasers," Appl. Opt. 2, 855-860 (1963)