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

The influence of laser polarization variations on CPT atomic clock signals

Michael C. Y. Huang, James C. Camparo

Published May 1, 2011• 2 citations
Publisher

Abstract

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.

DOI
10.1109/fcs.2011.5977275

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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.
Vapor cell atomic clocks exhibit reduced frequency stability for averaging time between about one hundred and a few thousand seconds. Here we report a study on the impact of the main parameters on the mid-to-long term instability of a buffer-gas vapor cell Cs clock, based on coherent population trapping (CPT). The CPT signal is observed on the Cs D1 line transmission, using a double Λ scheme and a Ramsey interrogation technique. The effects on the clock frequency of the magnetic field, the cell temperature, and the laser intensities are reported. We show in particular that the laser intensity shift is temperature dependent. Along with the laser intensity ratio and laser polarization properties, this is one of the most important parameters.
In chip-scale atomic clocks based on coherent-population-trapping (CPT), the laser of choice is a VCSEL due to its low threshold current, very high quantum efficiency, and large dynamic bandwidth. A problem with these lasers, however, is that they suffer from polarization fluctuations, which could degrade CPT signal-to-noise ratios. In previous work we found that a rapid change in laser polarization led to a multi-exponential variation of the light intensity transmitted through a resonant Rb vapor. Here, we show that this is not a systematic effect, but is a result of atomic dynamics. Specifically, we show that the change in transmitted light intensity arises from both a change in the system's trapping state and a re-establishment of the atomic coherence that is at the heart of the CPT signal.
In order to understand how stochastic processes might enter and influence coherent atomic dynamics, we have studied the behavior of a \ensuremath{\Lambda} system under periodic polarization modulation. Polarization, in addition to amplitude and phase, is a defining feature of a classical vector field. However, to date, there has been little study concerning the response of quantum systems to temporal variations in polarization, even though some lasers are known to exhibit stochastic polarization fluctuations. In our work, we square-wave modulate the polarization of a laser that induces coherent-population trapping (CPT) in 87{}^{87}87Rb. At low-modulation frequencies, we find that the amplitude of the CPT resonance increases with modulation frequency because the polarization variations limit the number of atoms confined to the system's trapping state. Surprisingly, at higher-modulation frequencies, we find that the CPT resonance splits into a doublet. We have developed an analytical theory of CPT in the presence of polarization modulation that captures the primary features of our experimental findings and shows that the doublet is a consequence of ground-state coherence modulation in the \ensuremath{\Lambda} system. The present results lay a foundation for understanding how more complicated (i.e., stochastic) temporal variations in laser polarization could influence \ensuremath{\Lambda}-system dynamics.
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 report on measurements of frequency shifts in a microwave cesium vapor cell atomic clock based on coherent population trapping (CPT). The dependence of the clock frequency on numerous experimental parameters, such as the laser power, the laser frequency, the microwave power, the cell temperature, the static magnetic field, but also the temperature of some key components, or the translation and rotation of critical wave plates and optical elements, is investigated. The stability budget of the clock frequency at one day is reported and discussed. This study constitutes a solid database for the future demonstration of a CPT-based cell clock with enhanced mid- and long-term stability performances.
Coherent population trapping (CPT) is extensively studied for future vapor cell clocks of high frequency stability. In the constructive polarization modulation CPT scheme, a bichromatic laser field with polarization and phase synchronously modulated is applied on an atomic medium. A high contrast CPT signal is observed in this so-called double-modulation configuration, due to the fact that the atomic population does not leak to the extreme Zeeman states, and that the two CPT dark states, which are produced successively by the alternate polarizations, add constructively. Here, we experimentally investigate CPT signal dynamics first in the usual configuration, a single circular polarization. The double-modulation scheme is then addressed in both cases: one pulse Rabi interaction and two pulses Ramsey interaction. The impact and the optimization of the experimental parameters involved in the time sequence are reviewed. We show that a simple seven-level model explains the experimental observations. The double-modulation scheme yields a high contrast similar to the one of other high contrast configurations like push-pull optical pumping or crossed linear polarization scheme, with a setup allowing a higher compactness. The constructive polarization modulation is attractive for atomic clock, atomic magnetometer, and high precision spectroscopy applications.