Dispersion in the Vicinity of an Optical Resonance

The optical dispersion in the vicinity of the 3P1–1S0 transition in mercury has been examined in high resolution by a novel method employing a variable frequency light source and a Mach-Zehnder interferometer. The enhancement of instrument sensitivity through lock-in synchronous detection, particularly for small negative dispersion measurements, was demonstrated. In some cases fringe shifts as small as one ten-thousandth of a fringe could be observed. The large Faraday rotation expected near resonance was also observed and the feasibility of using it as a narrow band modulatable filter demonstrated. A derivation of the refractive index in the vicinity of the resonance using the Kramers-Kronig relations is given, and the theory is shown to be consistent with experiment. The generalization of these techniques to laser work is briefly discussed.