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Improvement of short-term stability of pulsed optically pumped rubidium atomic clock

I. H. Choi, S. B. Lee, Taeg Yong Kwon, S. E. Park

Published Jun 1, 2010• 1 citation
Publisher

Abstract

We present a87Rb vapor-cell atomic clock using pulsed optical pumping with an optical detection technique. The pulsed optical pumping technique can eliminate the light shifts and increase the stability of vapor-cell atomic clock. For simplicity, direct laser absorption signal is used for the detection of a clock transition. A frequency stability of about 2×10-11at 1s has been measured and short term stability is improved by reducing the pulse sequence time with a real time control system.

DOI
10.1109/cpem.2010.5545140

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We present a87Rb vapor-cell atomic clock using pulsed optical pumping with an optical detection technique. The pulsed optical pumping technique can eliminate the light shifts and increase the stability of vapor-cell atomic clock. For simplicity, direct laser absorption signal is used for the detection of a clock transition. A frequency stability of about 2×10-11at 1s has been measured and short term stability is improved by reducing the pulse sequence time with a real time control system.
In this paper, we present the performance of a vapor-cell rubidium frequency standard working in the pulsed regime, in which the clock signal is represented by a Ramsey pattern observed on an optically detected laser absorption signal. The main experimental results agree with previously reported theoretical predictions. In particular, we measured a relative frequency stability of σy(τ) - 1.6 × 10(-13)τ-1/2 for integration times, τ, up to 200 s, which represents a record in short-term stability for a vapor-cell clock. We also discuss the most important physical phenomena that contribute to this result.
In this paper we discuss how to improve both short and medium-long term frequency stability of a vapor cell frequency standard working in pulsed regime. We propose in particular a multi-step pumping technique that is able to put almost all the atoms in one of the two clock levels, increasing in this way the signal-to-noise ratio with respect to the usual optical pumping made with a single laser pulse. Moreover, we discuss the phenomena affecting the medium-long term frequency stability and how it is possible to control them, making the overall clock stability compatible with sigmay(tau) =3x10-13tau-frac12for integration times up to tau = 105s.
In order to eliminate the light shifts and to increase the stability of gas-cell atomic clocks using optical pumping of the alkali atoms, the detection of a microwave pulse-induced coherent emission is used as the correction signal to lock a crystal oscillator to the atomic transition. Also, the line width of the O-O hyperfine transition is artificially narrowed by subjecting the atoms in the cell to two phase-coherent microwave pulses. In this case the resonance line has a width characteristic of the pulse-repetition frequency which can be made several times smaller than the natural line width in the cell. Proper timing of the sequence of microwave pulses and light pulses for optical pumping is necessary to conserve phase coherence and to obtain a well-defined Ramsey pattern. Results of the tests on a laboratory model using Rubidium-87 atoms show the elimination of the light shifts and an improvement of the short-term stability of the crystal oscillator by one order of magnitude.
We present theoretical and experimental studies concerning the realization of a rubidium vapor cell frequency standard based on the pulsed optical pumping (POP) technique. This technique is extremely powerful since it avoids in principle the light shift effect and the noise conversions from the laser to the clock signal. The detection of the clock transition in the microwave domain (the so called POP maser) has been extensively investigated at INRIM and a frequency stability of 1.1 × 10-12τ-1/2 has been measured until integration times of 105 s, reaching the level of 5 × 10-15 (drift removed). The optical detection is still under evaluation but this technique may lead in effect to some advantages with respect to the microwave detection, such as a lower or negligible cavity pulling, a lower operation temperature (which implies a lower spin exchange contribution) and a higher signal-to-noise ratio. All these features may lead to a better frequency stability performance than that previously reached. Read More: http://www.worldscientific.com/doi/abs/10.1142/9789812838223_0040
The /sup 87/Rb cell frequency standard, optically pumped by a laser diode, is of great interest in the development of high short-term stability oscillators. We report experiments performed on a rubidium cell prototype aiming at evaluate the transfer of the laser noise to the hyperfine clock transition via the light shift effect, which appears to be the main contribution to the observed short term frequency stability in this kind of standard. Pulsed optical pumping will also be examined in order to strongly reduce the light shift effect and to achieve the theoretical limit never approached up to now despite the strong detected microwave signals.