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European Quantum Electronics Conference (EQEC)

A compact, high-performance laser-pumped rubidium atomic frequency standard

C. Affolderbach, G. Mileti, Fabien Droz

Published Jan 18, 2006• 1 citation
Publisher

Abstract

The paper presents the realisation of a high-performance vapour-cell atomic frequency standard demonstrator based on laser optical pumping. The development of such devices aims to fully exploit the advantages of using a narrow-band diode laser pump light source instead of the discharge lamps commonly used in Rb gas-cell clocks. These advantages include improved control over the pump light spectrum, higher selectivity of the optical pumping, and increased signal contrast, and thus show the potential for the realisation of Rb atomic clocks with improved short-term frequency stability.

DOI
10.1109/eqec.2005.1567365

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Natarajan D. Bhaskar

It is experimentally demonstrated that, using diode lasers for optical pumping and RF /spl pi/-pulses for population transfer, nearly all of the ground state Rb atoms in a vapor cell can be concentrated into one of the two M/sub F/ = 0 sublevels. The fractional population difference /spl Delta/ between the two M/sub F/ = 0 levels can be made as high as 1. This should result in a very significant improvement (about a factor of 100) in the S/N over the conventional discharge-lamp-pumped rubidium frequency standards (RFSs). This would lead to a substantial improvement in the short-term stability of the RFS. This is a pulsed population transfer scheme. The microwave interrogation takes place in the absence of strong pumping light. This has the potential for eliminating the light shift and thereby improving the long-term frequency stability of a an RFS.>

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The paper presents the realisation of a high-performance vapour-cell atomic frequency standard demonstrator based on laser optical pumping. The development of such devices aims to fully exploit the advantages of using a narrow-band diode laser pump light source instead of the discharge lamps commonly used in Rb gas-cell clocks. These advantages include improved control over the pump light spectrum, higher selectivity of the optical pumping, and increased signal contrast, and thus show the potential for the realisation of Rb atomic clocks with improved short-term frequency stability.
We present our development activities on compact Rubidium gas-cell atomic frequency standards, for use in space-borne and ground-based applications. We experimentally demonstrate a high-performance laser optically-pumped Rb clock for space applications such as telecommunications, science missions, and satellite navigation systems (e.g. GALILEO). Using a stabilised laser source and optimized gas cells, we reach clock stabilities as low as 1.5·10-12 τ-1/2 up to 103 s and 4·10-14 at 104 s. The results demonstrate the feasibility of a laser-pumped Rb clock reaching < 1·10-12 τ-1/2 in a compact device (<2 liters, 2 kg, 20 W), given optimization of the implemented techniques. A second activity concerns more radically miniaturized gas-cell clocks, aiming for low power consumption and a total volume around 1 cm3 , at the expense of relaxed frequency stability. Here miniaturized “chip-scale” vapour cells and use of coherent laser interrogation techniques are at the heart of the investigations.
We report our investigations on a compact high-performance rubidium (Rb) vapourcell clock based on microwave-optical double-resonance (DR).These studies are done in both DR continuous-wave (CW) and Ramsey schemes using the same Physics Package (PP), with the same Rb vapour cell and a magnetron-type cavity with only 45 cm 3 external volume.In the CW-DR scheme, we demonstrate a DR signal with a contrast of 26% and a linewidth of 334 Hz; in Ramsey-DR mode Ramsey signals with higher contrast up to 35% and a linewidth of 160 Hz have been demonstrated.Short-term stabilities of 1.4×10 -13 τ -1/2 and 2.4×10 -13 τ -1/2 are measured for CW-DR and Ramsey-DR schemes, respectively.In the Ramsey-DR operation, thanks to the separation of light and microwave interactions in time, the light-shift effect has been suppressed which allows improving the long-term clock stability as compared to CW-DR operation.Implementations in miniature atomic clocks are considered. IntroductionSince the first invention of lamp-pumped rubidium (Rb) clocks in the 1960's [1,2], the demand for a stable, reliable, and compact Rb frequency standard with low power consumption has been growing, for various applications fields such as industry, telecommunication, navigation, space applications (e.g.GPS [3], GALILEO [4] and COMPASS [5]).Recently, thanks to the semiconductor laser technology, laser-pumped Rb clocks have been considered in several studies, for their better optical pumping efficiency results in short-term frequency stabilities at least one order of magnitude better than their lamp-pumped counterparts [6][7][8][9].This opened up also several new schemes for vapour-cell atomic clocks, such as the continuous-wave (CW) Rb clock [9], coherent population trapping (CPT) Cs clock [10] and pulsed optical pumping (POP) Rb clock [11].In this paper we report our studies on high-performance Rb frequency standards based on the laser-microwave double-resonance (DR) scheme, using both CW and Ramsey interrogation.Both CW-DR and Ramsey-DR schemes require two resonant electromagnetic fields of light and microwave.In the case of CW-DR, the laser optically pumps the vapour of the Rb atoms by creating a polarization of the atomic ground-state; simultaneously the microwave field, generated from a quartz oscillator and is near-resonant with the 87 Rb hyperfine clock transition (energy levels 5 2 S 1/2 │F g =1, m F =0˃ ↔│F g =2, m F =0˃), is applied.The opacity of the Rb vapour to the transmitted light signal as a function of microwave frequency is a measure of the atomic ground state polarization known as CW-DR signal [9].In CW-DR, the steps of optical pumping, microwave interrogation and optical detection are applied simultaneously (figure 1(b)).In the case of Ramsey-DR, these three sequences are separated in time, see figure 1(c): first, during optical pumping a strong laser pulse creates a significant atom population imbalance between
We discuss the basic physical principles of laser optical pumping double-resonance spectroscopy, which form the basis of state-of-the-art vapour-cell atomic frequency standards using laser optical pumping of Rb atoms. The main effects limiting the frequency stability of Rb vapour-cell atomic clocks are identified, and their impact on the development of high-performance frequency standars and their transfer from research laboratories to industry and space is discussed. As examples, the impact of the AC stark effect and the realated issue of laser frequency stabilisation are dealt with in more detail. The main features of the present state-of-the-art Rb atomic frequency standards will be illustrated using the example of the development of atomic clocks for satellite navigation and positioning systems (GPS, GLONAS, GALILEO, etc.) as well as some directions for further improvements that could overcome present day limitations. Such compact Rb clocks find their applications in, for example, telecommunications, local timekeeping and synchronisation, and space applications like satellite navigation and science missions. An overview of other, alternative clock schemes is given and critical issues for future developments towards further performance improvement or device miniaturisation in the field of vapour-cell atomic clocks are discussed.
It is experimentally demonstrated that, using diode lasers for optical pumping and RF /spl pi/-pulses for population transfer, nearly all of the ground state Rb atoms in a vapor cell can be concentrated into one of the two M/sub F/ = 0 sublevels. The fractional population difference /spl Delta/ between the two M/sub F/ = 0 levels can be made as high as 1. This should result in a very significant improvement (about a factor of 100) in the S/N over the conventional discharge-lamp-pumped rubidium frequency standards (RFSs). This would lead to a substantial improvement in the short-term stability of the RFS. This is a pulsed population transfer scheme. The microwave interrogation takes place in the absence of strong pumping light. This has the potential for eliminating the light shift and thereby improving the long-term frequency stability of a an RFS.>