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Journal of the Optical Society of America B

Picosecond time-scale phase-related optical pulses: measurement of sodium optical coherence decay by observation of incoherent fluorescence

John T. Fourkas, William L. Wilson, Gerhard Wäckerle, Amy E. Frost, M. D. Fayer

Published Oct 1, 1989• 67 citations
Publisher

Abstract

A technique for producing phase-related picosecond optical pulses is described. A sequence of two phase-related pulses, well separated in time, is applied to the D1 line in Na vapor. When the phase between the two pulses is scanned, oscillations in the incoherent fluorescence are observed. This demonstrates the well-defined phase relationship between the pulses. Measuring the amplitude of the oscillations as a function of pulse separation yields the Na optical coherence decay. A beat arising from the ground-state hyperfine splitting is observed, and the results agree well with calculation.

DOI
10.1364/josab.6.001905

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A technique for producing phase-related picosecond optical pulses is described. A sequence of two phase-related pulses, well separated in time, is applied to the D1 line in Na vapor. When the phase between the two pulses is scanned, oscillations in the incoherent fluorescence are observed. This demonstrates the well-defined phase relationship between the pulses. Measuring the amplitude of the oscillations as a function of pulse separation yields the Na optical coherence decay. A beat arising from the ground-state hyperfine splitting is observed, and the results agree well with calculation.
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Several groups1–4 have recently proposed a simple and powerful optical technique for obtaining ultrahigh time resolution, which is limited only by the coherence time of the excitation light rather than by the overall pulse duration. The present work reports an application of this technique to study the dephasing rates of the Na and Rb resonance lines perturbed by several atmospheres of argon gas. By comparison with traditional measurements, we demonstrate that relaxation processes occurring on time scales much shorter than the excitation pulse duration can indeed be accurately and unambiguously measured using the present technique.
The phase relaxation of the resonance fluorescence from sodium vapor was studied by measuring the contrast of interference between the delayed fluorescence pulses. It is shown that the phase relaxation time can be measured by a conventional cw mode-locked pico-second pulsed laser without any sophisticated apparatus to actively lock the phases of exciting laser pulses. Using a modified Michelson type interferometer, and a homemade software based on 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 phase relaxation time is explained by the change in the spread of atomic velocity distribution with temperature (Doppler broadening mechanism). Analytical calculation indicates that when the laser bandwidth is much broader than the linewidth of the fluorescence, as in the present case, the envelope of the 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.
levels of atomic sodium using ultrashort laser pulses in a stepwise excitation scheme. Two broadband femtosecond laser pulses are used to excite the 6S level from the 3S ground level in sodium vapor. The first one is simultaneously resonant with the D1 and D2 lines of the 3S→3P transition, and the second one with the 3P→6S transition. The delay between the two pulses is scanned in order to probe the dephasing between the two excited intermediate levels. Quantum beating is observed by monitoring the fluorescence signals at 330 nm and 515 nm, namely transition 4P→3S and 6S→3P, respectively. Signals are compared to a semi-classical model that reproduces the oscillations with a very good agreement.
The method of Synchronized Quantum Beat Spectroscopy has been improved by introducing the amplitude modulation on the excitation pulse train. By using an amplitude-modulated train of picosecond pulses from a cw mode-locked dye laser, we obtain resonance enhancement of the substate coherence (quantum beat) at f 0 = n f p + n ' f m ( n , n ' : integer, f 0 : sublevel splitting, f p : pulse repetition frequency, f m : modulation frequency), which can be detected by monitoring the optical anisotropy due to the substate coherence. Resonances can be obtained in the frequency domain by sweeping f m . Pressure shift coefficients of the hyperfine splitting in Na 3 2 S 1/2 are measured to be 82, 61, -1 and -47 Hz/Torr for He, N 2 , Ar and Kr, respectively, within an accuracy of ±3 Hz/Torr.