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IEEE Transactions on Instrumentation and Measurement

On the use of intensity optical pumping and coherent population trapping techniques in the implementation of atomic frequency standards

Jaques Vanier, M. W. Levine, D. Janssen, M.J. Delaney

Published Jun 1, 2003• 56 citations
Publisher

Abstract

The paper summarizes the relative advantages and disadvantages of coherent population trapping (CPT) or intensity optical pumping (IOP) for the implementation of a passive atomic frequency standard using the isotope /sup 87/Rb. This paper outlines the basic principles common to both CPT and IOP when using laser optical pumping, and makes explicit their similarities and their differences. This paper describes experimental results obtained in the same cell on the characteristics of the CPT and IOP /sup 87/Rb-hyperfine-resonance line. The measurements showed that the signal contrast is larger in CPT than in IOP for the same resulting line width; the light shift is smaller in CPT than in IOP, and is easier to control; in principle, a passive frequency standard based on CPT has a smaller size than that based on IOP, due to the absence of a microwave cavity. Conclusions on overall expectations for the future of such frequency standards are drawn.

DOI
10.1109/tim.2003.814687

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IEEE Transactions on Instrumentation and Measurement•Nov 22, 2005•40 citations
The paper summarizes the relative advantages and disadvantages of coherent population trapping (CPT) or intensity optical pumping (IOP) for the implementation of a passive atomic frequency standard using the isotope /sup 87/Rb. This paper outlines the basic principles common to both CPT and IOP when using laser optical pumping, and makes explicit their similarities and their differences. This paper describes experimental results obtained in the same cell on the characteristics of the CPT and IOP /sup 87/Rb-hyperfine-resonance line. The measurements showed that the signal contrast is larger in CPT than in IOP for the same resulting line width; the light shift is smaller in CPT than in IOP, and is easier to control; in principle, a passive frequency standard based on CPT has a smaller size than that based on IOP, due to the absence of a microwave cavity. Conclusions on overall expectations for the future of such frequency standards are drawn.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.
This paper provides an overview of the efforts accomplished during the last few decades in the attempt of implementing a Rb-cell frequency standard using optical-microwave double resonance and laser optical pumping. The results of a new analysis based on a three-level model are summarized. Pumping rates, line widths, light shifts, cell radiation transmission and resonance signal size relative to background are calculated making use of the coherence property of the pumping laser. The expected frequency stability is calculated. The results obtained by several research groups are reviewed, compared to the analysis, and evaluated in the context of the implementation of a practical frequency standard. It is concluded that compromises must be made between the goal of obtaining a frequency stability close to the shot noise limit and that of minimizing complexity. Two directions that address such compromises are outlined.
This letter presents for the first time a systematic study of an atomic frequency standard based on coherent population trapping (CPT) effect in87Rb vapour optically pumped with a multifrequency laser radiation formed with sidebands of a single-frequency laser whose injection current is modulated at a subharmonic frequency (1/2-1/6) relative to the frequency of hyperfine ground state splitting. Measured are the CPT resonance parameters and the atomic clock short-term stability at different subharmonic frequencies of microwave modulation. The possibility of CPT resonance light shift suppression is experimentally demonstrated at modulation frequencies between 1/3 and 1/5 of the resonant frequency for continuous wave optical pumping of the87Rb vapour D1 line. Analyzed are the capabilities and potential of reduced energy consumption of atomic clocks when CPT resonance in rubidium vapour is excited at subharmonic modulation frequencies.
The coherent population trapping phenomenon (CPT) has created interesting avenues in the practical realization of atomic frequency standards using laser optical pumping. In practice two different types of standards can be implemented: a passive clock based on the dark state created in the ensemble by the CPT phenomenon and a maser based on the ground state oscillating magnetization induced by the same phenomenon. The paper describes the basic physics involved in the approach and summarizes the progress made thus far in the practical implementation of such frequency standards.
We have studied the effects of the intensity difference of the two laser fields on the performance of the coherent population trapping (CPT) atomic clocks. Our experimental results show that there is a ratio range for the intensity of the two laser fields, between 1:2 and 2:1, in which the CPT resonance has larger amplitude and narrower line width. If we can adjust the ratio close to the center (1: 1), the signal quality (ratio of amplitude to line width) of CPT resonance will be efficiently improved. Our results also show that using full hyperfine frequency to modulate the laser has its advantages.