Determinations of optical loss in unpumped ruby laser crystals from measurements of the transmissivity vs optical path length (as changed by temperature) using an amplitude regulated, single mode, He-Ne laser beam probe are described. Excellent agreement between these measurements and the theory of Dufour and Picca was obtained for a flux-grown ruby using as-grown feedback surfaces. The essentially zero loss per pass β (apart from end reflector losses) deduced from this comparison is in excellent agreement with a previous determination of the loss in this crystal from laser threshold measurements. The reduction of the effective surface roughness by deposition of multilayer zinc sulfide-cryolite reflectors was observed in these experiments. For a high quality, 7.5-cm long, 0.05 wt% Cr2O3, 60° orientation ruby rod, β was found to be 0.03 ± 0.03. For a 90° orientation, 2.2-cm long, trumpet-shaped ruby used for continuous operation threshold studies, β was found to be about 0.14. The problems of this type of loss measurement when index of refraction variations are present are discussed, and observations connecting the crystal quality and laser mode characteristics are presented.
D. F. Nelson and K. F. Rodgers, "Measurement of Ruby Laser Cavity Losses by Fabry-Perot Resonance," Appl. Opt. 6, 421-427 (1967)