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European Frequency and Time Forum

Laser-pumped high-performance compact gas-cell Rb standard with < 3×10−13 τ−1/2 stability

Thejesh Bandi, C. Affolderbach, G. Mileti, Claudio Eligio Calosso, Camillo Stefanucci, Francesco Merli, Anja K. Skrivervik

Published Apr 1, 2012• 1 citation
Publisher

Abstract

We demonstrate a high-performance laser-pumped rubidium-cell atomic frequency standard exhibiting a short-term stability of-13τ-1/2. The key components of this standard are a Rb vapor cell of 25mm diameter and a newly developed compact microwave resonator cavity. The overall volume of the clock's physics package - including the frequency-stabilized laser head and physics package - is <; 3 liters, which shows the potential of this compact clock for future portable applications.

DOI
10.1109/eftf.2012.6502431

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We demonstrate a high-performance laser-pumped rubidium-cell atomic frequency standard exhibiting a short-term stability of-13τ-1/2. The key components of this standard are a Rb vapor cell of 25mm diameter and a newly developed compact microwave resonator cavity. The overall volume of the clock's physics package - including the frequency-stabilized laser head and physics package - is <; 3 liters, which shows the potential of this compact clock for future portable applications.
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.
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 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 present our studies on a compact high-performance continuous wave (CW) double-resonance (DR) rubidium frequency standard in view of future portable applications. Our clock exhibits a short-term stability of 1.4 × 10(-13) τ(-1/2), consistent with the short-term noise budget for an optimized DR signal. The metrological studies on the medium- to longterm stability of our Rb standard with measured stabilities are presented. The dependence of microwave power shift on light intensity, and the possibility to suppress the microwave power shift is demonstrated. The instabilities arising from the vapor cell geometric effect are evaluated, and are found to act on two different time scales (fast and slow stem effects). The resulting medium- to long-term stability limit is around 5.5 × 10(-14). Further required improvements, particularly focusing on medium- to long-term clock performance, are discussed.
This thesis presents experimental studies on continuous-wave (CW) laser-microwave double-resonance (DR) spectroscopy and metrology in rubidium (87Rb) vapor cells in view of new high-performance, compact Rb-cell atomic clocks. The Rb vapor cell is confined inside a magnetron-type cavity microwave resonator (MWR). The CW DR spectroscopy involves two resonant electromagnetic fields that are operated simultaneously to interrogate the atoms - the optical field to polarize the atoms by optical pumping, and the microwave field to drive the ground-state hyperfine clock transition that serves as an atomic frequency reference. Details on characterization of compact laser heads and microwave synthesizers used in this work are presented. The vapor cell standards are useful in our everyday lives for applications ranging from telecommunications, navigation, metrology etc. In view of improving the performances of the Rb cell standards, two different clock approaches were studied in detail: The first one is on use of vapor cells (1.4 cm3) whose inner walls are coated with anti-relaxation material. A first ever wall-coated cell clock was demonstrated with good short-term frequency stability. The medium- to long-term stability was found to be limited by the temperature coefficient (TC) of the coating material itself. This work gave the insight to important features to be considered in future anti-relaxation coating materials for atomic standards. The second novel approach involves using a bigger cell (12 cm3) within a newly developed, improved MWR towards a high-performance atomic standard. Adopting a larger cell gives a higher atomic Q-factor signal that improves the short-term clock stability. In this approach, the cell was filled with 87Rb and buffer gases. With this clock, we demonstrate the state-of-the-art short-term stability of <1.4×10−13 τ−1/2. Metrological quantitative measurements on parameters influencing the medium- to long-term stability were studied in view of next generation satellite navigation systems that demand a stability level of <1×10−14 at 10 4 s (equivalent to <1 ns/day). The potential of short-term stability and understanding of the limiting factors on medium- to long-term time scales demonstrated in this study pave the way for future work towards the commercialization of high-performance Rb atomic clocks for a variety of applications.
This paper studies a miniature low power consumption laser-pumped atom vapour cell clock scheme. Pumping 87 Rb with a vertical cavity surface emitting laser diode pump and locking the laser frequency on a Doppler-broadened spectral line, it records a 5 × 10 −11 τ −1/2 (τ < 500 s) frequency stability with a table-top system in a primary experiment. The study reveals that the evaluated scheme is at the level of 2.7 watts power consumption, 90 cm 3 volume and 10 −12 τ −1/2 short-term frequency stability.