Dispersion relations measured at the D ₂ resonance of ⁸⁵ Rb

We describe an experiment to be carried out by advanced students as part of their laboratory practice. Its aims are (i) to measure simultaneously the frequency dependence of both the refractive index n and the extinction coefficient of an atomic gas interacting with a resonant laser beam and (ii) to examine that n and satisfy the Kramers-Kronig relations. The medium to be probed consists of a dilute 85 Rb vapour exposed to collisions with a buffer gas. The D 2 resonance line is thereby broadened to such an extent that its hyperfine components merge into single Lorentzian dispersion and absorption profiles, respectively. The main features of the apparatus are a tunable laser diode, a Mach-Zehnder interferometer at nearly zero path difference, an array of photodiodes to measure the interference pattern as a function of laser frequency and Rb density in one of the interferometer arms, an absorption monitor, computer-controlled electronics to read out and analyse the data, and a video camera for visual control and adjustment of the interference pattern.