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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

The pulsed optically pumped Rb frequency standards in National Time Service Center

Du Zhi-jing, Shougang Zhang, Jie Liu, Li Xiaofeng, Dandan Liu, Xue Wen-xiang

Published May 1, 2012• 2 citations
Publisher

Abstract

In this paper we report a laboratory prototype of pulsed optically pumped (POP) rubidium frequency standard. In POP operation the pumping, interrogation, and detection phases are separated in time, avoiding the coupling between microwave and optical coherence. The POP87Rb vapor-cell frequency standard was studied experimentally based on the detection of free induction microwave signal or optical absorption signal. Ramsey fringes were obtained based on87Rb in buffer gas, with FWHM of 65Hz and 150Hz for microwave and optical central Ramsey fringes respectively. The atomic quality factor was on the order of 108. With microwave detection, the achieved frequency stability was 1.2×10-12τ-1/2(τ=1~100s).

DOI
10.1109/fcs.2012.6243579

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Study of Pulsed Optically Pumped Rb Frequency Standard

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The pulsed optically pumped(POP) 87Rb vapor-cell frequency standard was studied based on the detection of free induction microwave signal or optical absorption signal.In POP operation the pumping,interrogation,and detection phases are separated in time,avoiding the coupling between microwave and optical coherence.The theoretical results show that an oscillation period doubled with respect to the classical Ramsey method which leads to a doubled atomic quality factor Q,and the reduction of optical shift and cavity pulling shift will be obtained,when the atoms are submitted to π/2 Ramsey microwave pulse.In experiment Ramsey fringes were obtained with a clock prototype based on 87Rb in buffer gas,with bandwidth of 65 Hz and 150 Hz for microwave and optical signal respectively.The frequency stability achieved was 1.2×10-12 for measurement times up to 100 s.

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A compact magnetron-type microwave cavity has previously been developed for a high performance Rb atomic clock based on conventional continuous-wave double-resonance (DR) interrogation. This magnetron-type cavity shows a homogenous microwave field distribution while having a cavity volume of only 35 cm3. So it is interesting for realizing a high performance Rb cell clock based on pulsed optical pumping (POP) technique with a reduced physics package size. In this proceeding we report on the evaluation of pulsed optical pumping with our magnetron-type cavity. Using the pulsed technique and optical detection, high-contrast Ramsey fringes were observed. The central fringe of the Ramsey signal has a linewidth of 160 Hz and contrast of 35%. Under these conditions, the shot-noise limit is estimated at a level around 2×10-14τ-1/2which shows the great potential of a POP clock based on the compact magnetron-type cavity. The Rabi oscillation result also indicates an acceptable uniform field distribution inside the cavity for a high-performance POP clock. Some optimizations of pumping and detection power for improving the contrast are also presented.

European Frequency and Time Forum•Jun 1, 2014•6 citations
In this paper we report a laboratory prototype of pulsed optically pumped (POP) rubidium frequency standard. In POP operation the pumping, interrogation, and detection phases are separated in time, avoiding the coupling between microwave and optical coherence. The POP87Rb vapor-cell frequency standard was studied experimentally based on the detection of free induction microwave signal or optical absorption signal. Ramsey fringes were obtained based on87Rb in buffer gas, with FWHM of 65Hz and 150Hz for microwave and optical central Ramsey fringes respectively. The atomic quality factor was on the order of 108. With microwave detection, the achieved frequency stability was 1.2×10-12τ-1/2(τ=1~100s).
The pulsed optically pumped(POP) 87Rb vapor-cell frequency standard was studied based on the detection of free induction microwave signal or optical absorption signal.In POP operation the pumping,interrogation,and detection phases are separated in time,avoiding the coupling between microwave and optical coherence.The theoretical results show that an oscillation period doubled with respect to the classical Ramsey method which leads to a doubled atomic quality factor Q,and the reduction of optical shift and cavity pulling shift will be obtained,when the atoms are submitted to π/2 Ramsey microwave pulse.In experiment Ramsey fringes were obtained with a clock prototype based on 87Rb in buffer gas,with bandwidth of 65 Hz and 150 Hz for microwave and optical signal respectively.The frequency stability achieved was 1.2×10-12 for measurement times up to 100 s.
In this paper we report the results we obtained with a prototype of pulsed optically pumped (POP) Rb clock. In terms of frequency stability, an Allan deviation σy(τ)=1.7 ×10-13τ-1/2has been measured, where τ is the integration time. In addition, the long term frequency drift results below 10-14/day. In the paper we also describe a technique to stabilize the amplitude of the microwave signal using the atoms themselves as discriminator. Moreover, an anomalously large temperature sensitivity of the clock frequency and a technique to keep it under control are also reported. Finally, we describe some non linear effects that modify the shape of the clock signal represented by the Ramsey central fringe.
We present a multilevel theoretical approach to describe the behavior of a pulsed optically pumped (POP) 87Rb^{87}\mathrm{Rb}87Rb frequency standard based on a microwave cavity-vapor cell arrangement. The full Zeeman manifold of the ground-state hyperfine levels is then considered, and the dynamics induced among them by relaxation processes (buffer gas, spin exchange, and cell-walls collisions) is taken into account. The model includes as well the absorption of the pumping laser along the cell and the cavity feedback on the atoms; the effects related to a nonuniform excitation of the atomic sample are also discussed. Theoretical predictions are proven with a laboratory prototype of POP passive maser in which the clock transition is excited by means of the Ramsey technique. The agreement between theory and experiment is very satisfactory from both a quantitative and qualitative point of view, in terms of the shape of the Ramsey fringes, microwave power delivered by the atoms, and short-term frequency stability. In particular, a frequency stability of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at one second has been measured, in very good agreement with the expected value. On this basis, the model has been extended to predict the short-term frequency stability of a POP frequency standard operating with optical detection. A shot-noise limited frequency stability (Allan deviation) of ${\ensuremath{\sigma}}_{y}(\ensuremath{\tau})\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\ensuremath{\tau}}^{\ensuremath{-}1∕2}$ is foreseen. This value can be degraded by microwave phase noise and laser fluctuations so that the overall predicted clock stability is ${\ensuremath{\sigma}}_{y}(\ensuremath{\tau})\ensuremath{\approx}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1∕2}$.
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
A laboratory prototype of a compact quantum frequency standard based on a rubidium vapour cell with pulsed laser pumping is developed. With significantly smaller overall dimensions (a volume of ∼10 L and a mass of ∼10 kg), the frequency standard is expected to have the short-term instability comparable with that of the commercial passive hydrogen masers and even the laboratory fountain-type atomic clocks based on cold atoms. The advantages of the technique of pulsed optical pumping and pulsed excitation of microwave resonances over the traditional techniques of double radio-optical resonance detection are substantiated. The results of the experimental study of pulsed optical pumping of a rubidium cell with a mixture of buffer Ar – Ne gases implemented at the VNIIFTRI are presented, the Ramsey resonances are obtained, and the process of optimising the contrast of the central resonance peak is described.
A compact magnetron-type microwave cavity has previously been developed for a high performance Rb atomic clock based on conventional continuous-wave double-resonance (DR) interrogation. This magnetron-type cavity shows a homogenous microwave field distribution while having a cavity volume of only 35 cm3. So it is interesting for realizing a high performance Rb cell clock based on pulsed optical pumping (POP) technique with a reduced physics package size. In this proceeding we report on the evaluation of pulsed optical pumping with our magnetron-type cavity. Using the pulsed technique and optical detection, high-contrast Ramsey fringes were observed. The central fringe of the Ramsey signal has a linewidth of 160 Hz and contrast of 35%. Under these conditions, the shot-noise limit is estimated at a level around 2×10-14τ-1/2which shows the great potential of a POP clock based on the compact magnetron-type cavity. The Rabi oscillation result also indicates an acceptable uniform field distribution inside the cavity for a high-performance POP clock. Some optimizations of pumping and detection power for improving the contrast are also presented.