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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Study of Raman-Ramsey fringes for enhanced precision in a chip scale Rb clock

G. S. Pati, K. Salit, M. S. Shahriar

Published Feb 7, 2008• 1 citation
Publisher

Abstract

We report observation of high contrast Raman-Ramsey fringes using time delayed optical pulse pairs in a rubidium vapor cell. The width of these fringes are not limited by saturation and provides a simpler means to produce narrow atomic linewidths using a thermal vapor medium for compact atomic clock applications. We also demonstrate phasescanned Raman-Ramsey fringes, with potential application to sensitive detection of trace vapors.

DOI
10.1117/12.772332

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We report observation of high contrast Raman-Ramsey fringes using time delayed optical pulse pairs in a rubidium vapor cell. The width of these fringes are not limited by saturation and provides a simpler means to produce narrow atomic linewidths using a thermal vapor medium for compact atomic clock applications. We also demonstrate phasescanned Raman-Ramsey fringes, with potential application to sensitive detection of trace vapors.
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Raman-Ramsey interference has proved a very effective technique to implement compact and high performing vapor cell frequency standards. In this paper, we theoretically characterize Raman-Ramsey resonances in an optically thick atomic vapor. Specifically, some parameters of interest for frequency standards applications, like contrast and linewidth of the central Raman-Ramsey fringe, are evaluated at different temperatures for 133Cs^{133}\mathrm{Cs}133Cs and 87Rb^{87}\mathrm{Rb}87Rb vapor cells with buffer gas. Density narrowing and broadening effects are described and explained in terms of a three-level theory where laser field propagation through the atomic medium is taken into account. Also, we investigate light shift both in low and high atomic density regimes. Light shift, which potentially degrades the medium- to long-term stability of Raman-Ramsey clocks, is composed of two contributions. The first is a pulling effect exerted by the wide Rabi profile enclosing the interference pattern on the central Raman-Ramsey fringe. The second light-shift term is strictly related to the detection time. Calculations derived from our model well describe already existing experimental results and new behaviors are predicted.
Pulsed coherent population trapping in atomic vapor provides a convenient method for generating frequency narrowed atomic resonance using Ramsey interference. Here, we present our experimental results showing high-contrast and sub-kilohertz Ramsey interference fringes produced by coherently prepared 85Rb atoms using Raman excitation formed in D1 line transitions. A test system for atomic clock has been constructed to measure light shift for both continuous and pulsed Raman excitation cases. Our measurements show that light shift measured using pulsed Raman excitation, is reduced compared to the continuous excitation case. Our ability to measure light shift is currently limited by the resolution of the frequency counter, and performance of the locking electronics used.
We present a prospective experimental study on double-resonance Ramsey spectroscopy in a micro fabricated Rb vapor cell, in view of future miniature atomic clocks with improved stability performance. Ramsey signals with a narrow central fringe width of 1.5 kHz combined with a competitive contrast of > 5 % are observed, resulting in a clock short-term stability of ≈ 1×10-11τ−1/2. The employed pulsed optical pumping (POP) scheme bears promise for reduced light-shift effects and thus for improved clock stability on medium- to long-term timescales.
The linewidth of CPT atomic resonances observed in a vapor cell are usually limited by optical saturation and collisions due to buffer gas. We have applied here the Ramsey method with optical CPT pulses generated by a phase-locked device. Fringes are then produced giving narrow atomic signals only dependent on interrogation time between the two pulses. The possibility to obtain narrow fringe widths below 100 Hz with short or long pulses is shown experimentally and analysed using the classical Schrodinger state representation.