Detection of phase relaxation in resonance fluorescence from sodium vapor by a cw mode-locked dye laser

The phase relaxation of the resonance fluorescence from sodium vapor was st udied by measuring the contrast of interference between the delayed fluores cence pulses. It is shown that the phase relaxation time can be measured by a conventional cw mode-locked pico-second pulsed laser without any sophist icated apparatus to actively lock the phases of exciting laser pulses. Usin g a modified Michelson type interferometer, and a homemade software based o n FFT algorithm, we obtained a phase correlation time as 300 ps with good S /N ratio. A local minimum observed in the decay curve is attributed to the ground state hyperfine splitting and the temperature dependence of the phas e relaxation time is explained by the change in the spread of atomic veloci ty distribution with temperature (Doppler broadening mechanism). Analytical calculation indicates that when the laser bandwidth is much broader than t he linewidth of the fluorescence, as in the present case, the envelope of t he interfering fluorescence signal can be represented by Fourier transform of the absorption spectra. The analysis based on this approximation are in good agreement with the results obtained in the experiment.