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

Study of the Laser Linewidth Influence on "lin ¿ lin" Coherent Population Trapping

Evelina Breschi, G. Mileti, Georgy A. Kazakov, Boris G. Matisov, Roland Lammegger, Laurentius Windholz

Published May 1, 2007• 4 citations
Publisher

Abstract

We investigated, theoretically and experimentally, the influence of the laser spectrum (in particular the linewidth) on the coherent population trapping (CPT) resonance in87Rb buffer gas cell. We investigated the case of linearly polarized laser fields. The experiments have been performed by using either a current-modulated vertical cavity surface emitting laser (VCSEL) or a phase-locked-laser (PLL) system. A model based on the density matrix approach has been used for the data interpretation. We found that, under experimental conditions suitable for compact clock applications, the fact that VCSELs have a broader spectrum than PLLs does not influence significantly the CPT signal amplitude. We demonstrate that a not negligible role is played by the destructive interference between different CPT excitation channels and propose further investigations to evaluate this contribution with respect to the other causes. Finally, we discuss the application of this scheme in compact atomic clocks.

DOI
10.1109/freq.2007.4319146

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We evaluate the use of Coherent Population Trapping (CPT) excited with parallely polarized laser fields in vapour cell atomic clocks. We study the resonance shape, the discriminator slope and signal-to-noise ratio as a function of relevant parameters such as the laser frequency detuning and the applied magnetic field. We show that a stability of 6⋅10−126 \cdot 10^{-12}6⋅10−12 τ−1/2τ^{-1/2}τ−1/2 may be reached in a compact system using a modulated VCSEL.
We investigate experimentally and theoretically the coherent population trapping (CPT) effect occurring in 87Rb^{87}\text{R}\text{b}87Rb D1{D}_{1}D1​ line due to the interaction with linearly polarized laser light ($\text{lin}\ensuremath{\parallel}\text{lin}$ CPT). In this configuration, the coherence is strongly influenced by the structure of the excited state; consequently, the quantum interference between dark states is an essential feature of this interaction scheme. We study the $\text{lin}\ensuremath{\parallel}\text{lin}$ CPT resonance as a function of the laser optical detuning. The comparison between experimental and theoretical results allows us to quantify the contribution from different dark states to the total signal. Based on these results, we investigate the signal depending on both the pressure broadening of the optical transition and the laser linewidth, and we find in which conditions the laser linewidth does not degrade the $\text{lin}\ensuremath{\parallel}\text{lin}$ CPT resonance.
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A method based on crossed polarizers to observe high-contrast coherent population trapping (CPT) resonance has been developed. Because crossed polarizers have a simple optical system, our method is suitable for chip-scale atomic clocks (CSACs). In CPT, the Faraday rotation in a linearly polarized light field (lin||lin) was calculated using two pairs of Λ-system models; the spectrum of the Faraday rotation is also estimated. After measuring the contrast and linewidth with the crossed-polarizer method, a comparison of the theoretical model and experimental data showed they were in good agreement. Moreover, the experimental results showed that a high contrast (88.4%) and narrow linewidth (1.15 kHz) resonance could be observed using a Cs gas cell and D1-line verticalcavity surface-emitting laser (VCSEL).