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

Practical realization of a passive coherent population trapping frequency standard

Jaques Vanier, M. W. Levine, S. Kendig, D. Janssen, Chris Everson, M. Delaney

Published Apr 12, 2005• 15 citations
Publisher

Abstract

The paper describes the practical realization and characteristics of a small, totally autonomous, passive frequency standard based on the coherent population trapping (CPT) phenomenon. A description of the frequency standard, including physical characteristics, block diagrams of the optical control CPT resonance system and of the digital control electronics is given. The characteristics of the frequency standard relative to general functionality and frequency stability are given, as measured on several units.

DOI
10.1109/freq.2004.1418434

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Journal of the Optical Society of America B•Nov 1, 2006•23 citations
The paper describes the practical realization and characteristics of a small, totally autonomous, passive frequency standard based on the coherent population trapping (CPT) phenomenon. A description of the frequency standard, including physical characteristics, block diagrams of the optical control CPT resonance system and of the digital control electronics is given. The characteristics of the frequency standard relative to general functionality and frequency stability are given, as measured on several units.
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 examine in this paper the spectral characteristics of a coherent population trapping (CPT) maser in order to evaluate the frequency stability achievable by this kind of passive frequency standard. The peculiar features of the coherent microwave emission associated with the coherent population trapping phenomenon are theoretically examined taking into account the impact of the various noise contributions on the physical principle of operation. The theoretical predictions are then compared with the experimental results obtained with a laboratory prototype of 87Rb^{87}\mathrm{R}\mathrm{b}87Rb CPT maser. The measured frequency stability is mainly limited in the short term by the system thermal noise and in the medium term by the thermal drift and turns out to be in agreement with the computations within a factor of 2.
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.