The real and imaginary parts of the complex index of refraction have been measured in the R1line of optically pumped ruby. The experiments, which employed a temperature-controlled, single-mode ruby laser as a variable-frequency monochromatic source, were performed near liquid-nitrogen temperature, where the R1 doublet structure is well resolved. R1 absorption profiles were measured for several optical-pumping conditions, ranging from zero up to a pump power that excited a population in the 2E state equal to about 60% of the population of the 4A2 ground state. For each pumping condition, the anomalous-dispersion curve was measured interferometrically. The spectral profiles of the absorption coefficient and the anomalous dispersion display a dependence on the optical-pumping conditions that is more complicated than simple saturation of the R1 transitions, but for given pumping conditions the two quantities are correlated as predicted by the Kramers-Kronig relations.
Brent L. Bean and Jerald R. Izatt, "Verification of the Kramers-Kronig relations in optically pumped ruby," J. Opt. Soc. Am. 63, 832-839 (1973)