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

Effects of the intensity difference of two laser fields on coherent population trapping clocks

Zhong Wang, Ke Deng, Dingwu He, Xinyuan Liu, Lu Liu, Tao Guo, Xuzong Chen

Published May 1, 2008• 2 citations
Publisher

Abstract

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.

DOI
10.1109/freq.2008.4623084

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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.
This work demonstrates the CPT (Coherent Population Trapping) signal of Cs atomic clock can be affected by magnetic field theoretically. The amplitude and the width of the resonance line of the CPT signal are the functions of the magnetic intensity. When both the coherent lasers have a specific rate of intensity the amplitude of the CPT signal keeps the most stable. Compared with four lambda models in theory, research shows that the scheme of the translation from ground states |Fg=4, mF=0> and |Fg=3, mF=0> to excited state |Fe=4, mF=-1> is the most suitable for Cs CPT atomic clock.
Spectroscopy studies of coherent population trapping (CPT) with constructive polarization modulation are presented. In these studies, the effects of modulation frequency and laser intensity on the CPT contrast of the clock transition are investigated. We experimentally show that no CPT transition with ΔmF=2 is excited. Moreover, the constructive polarization modulation CPT signals are observed in both cases, with excited levels F'=3 and F'=4 in the Dl line of Cesium. Our studies show that this scheme, with potential to implement a compact and high performance clock, could also be applied to high pressure buffer gas cell, e.g., chip scale atomic clock.
We theoretically study the frequency shift in atomic clocks caused by the line-shape asymmetry of coherent population trapping (CPT) resonance in a bichromatic laser field. This asymmetry arises due to the inequality of the resonant spectral components and nonzero one-photon detuning. The line-shape-asymmetry-induced shift depends on the intensity of the resonant fields, which leads to a degradation in long-term stability due to fluctuations of the laser field parameters. A frequency stabilization based on the harmonic modulation of two-photon detuning is considered. It is shown that the use of a high-frequency modulation (compared to the CPT resonance width, i.e., the Pound-Drever-Hall regime) makes it possible to significantly suppress this shift (by one to two orders of magnitude).
A lin⊥lin quasi-bichromatic light can be employed to realize a double-Λ coherent population trapping (CPT) resonance configuration, thereby enabling a CPT atomic clock with improved performance. A method for generating such light is proposed, in which two lasers with different linewidths are used to achieve a microwave frequency separation of several gigahertz. The effects of the optical injection ratio from the master extended-cavity diode laser into the slave vertical-cavity surface-emitting laser and the microwave power modulating the slave laser on the locking range were systematically investigated. The influence of both the injection ratio and microwave power on the contrast, linewidth, and the ratio of contrast to linewidth of the CPT signal is also studied. To enhance the trapping efficiency, single-pass and multi-pass interactions between the lin⊥lin quasi-bichromatic light and 87Rb atoms are investigated, with near-purely coherent states achieved under multi-pass interactions. Our scheme offers a promising solution for high-performance atomic clock applications.
The Coherent-Population-Trapping (CPT) vapor-cell atomic clock represents a clear break from previous vapor-cell clock technology. As such it has advantages that may result in significant frequency stability improvements. However, along with the technology's benefits come new effects that may limit the CPT clock's capabilities, and one such effect is laser polarization noise. Here, we present our first results looking at the response of a CPT signal to modulation of a laser's polarization. Our results show that the CPT clock signal undergoes a splitting when the polarization modulation is rapid, suggesting that various Fourier components of a field's stochastic polarization fluctuations will affect the CPT clock signal in qualitatively different ways.
Coherent population trapping (CPT) has been demonstrated as an interesting technique for miniature atomic frequency references [1,2] and quantum information. It is based on the coupling of the two hyperfine ground states of an alkali atom – namely cesium (133Cs) for atomic clocks – through excitation to a common atomic level by two phase-coherent laser fields nearly resonant with the atomic transitions. The frequency difference between the two laser fields is tuned at the atomic frequency splitting in the microwave range, equal to 9.192 GHz for 133Cs atoms. Outputs powers in the mW range and narrow-linewidth emission (<500 kHz) are required for the two laser beams.