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IEEE Transactions on Instrumentation and Measurement

Atomic Clock Using Microwave Pulse-Coherent Techniques

Maurice Arditi, T. R. Carver

Published Jun 1, 1964• 42 citations
Publisher

Abstract

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.

DOI
10.1109/tim.1964.4313389

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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.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.
We describe a method to stabilize the amplitude of the interrogating microwave field in compact atomic clocks working in a Ramsey approach. In this technique, we take advantage of the pulsed regime to use the atoms themselves as microwave amplitude discriminators. Specifically, in addition to the dependence on the microwave detuning, the atomic signal after the Ramsey interrogation acquires a dependence on the microwave pulse area (amplitude times duration) that can be exploited to implement an active stabilization of the microwave field amplitude, in a similar way in which the Ramsey clock signal is used to lock the local oscillator frequency to the atomic reference. This stabilization results in a reduced sensitivity of the clock frequency to microwave amplitude fluctuations that are transferred to the atoms through the cavity-pulling effect. The proposed technique is then effective to improve the clock stability and drift on medium and long term. We demonstrate the method for a vapor-cell clock working with a hot sample of atoms but it can be extended to cold-atom compact clocks.
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.
We demonstrate a high-performance coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique in the pulsed regime, allowing the detection of high-contrast and narrow Ramsey-CPT fringes. The impact of several experimental parameters onto the clock resonance and short-term fractional frequency stability, including the laser power, the cell temperature, and the Ramsey sequence parameters, has been investigated. We observe and explain the existence of a slight dependence on laser power of the central Ramsey-CPT fringe line-width in the pulsed regime. We report also that the central fringe line-width is commonly narrower than the expected Ramsey line-width given by 1/(2TR), with TR the free-evolution time, for short values of TR. The clock demonstrates a short-term fractional frequency stability at the level of 2.3×10−13 τ−1/2 up to 100 s averaging time, mainly limited by the laser amplitude modulation noise. Comparable performances are obtained in the conventional continuous wave regime, with the use of an additional laser power stabilization setup. The pulsed interaction allows to reduce significantly the clock frequency sensitivity to laser power variations, especially for high values of TR. This pulsed CPT clock, ranking among the best microwave vapor cell atomic frequency standards, could find applications in telecommunication, instrumentation, defense or satellite-based navigation systems.