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Light-Shifts of an Integrated Filter-Cell Rubidium Atomic Clock

James C. Camparo

Published May 25, 2015• 6 citations
OpenAlex

Abstract

Abstract : In this work, we consider the light-shift coefficients observed in rubidium (Rb) atomic frequency standards, and how the units of measurement reported for light-shift coefficients trace back to the underlying physical parameters of the light-shift. We then measure the light-shift coefficient in an integrated filter-cell, commercial Rb clock, which is similar to the Rb clock design currently flying on Galileo and BeiDou satellites. We measure the light-shift coefficient for two different rf-discharge lamps (i.e., a pure 87 Rb lamp and a lamp filled with the natural Rb isotope abundance), and show that under certain conditions it is possible to combine light-shift data. Our light-shift coefficients are consistent with those reported for the Galileo Rb clock under the assumption of a natural (or 85 Rb isotopically enriched) rf- discharge lamp for the Galileo clock.

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Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.

IEEE Journal of Quantum Electronics•Apr 1, 1987•61 citations
Abstract : In this work, we consider the light-shift coefficients observed in rubidium (Rb) atomic frequency standards, and how the units of measurement reported for light-shift coefficients trace back to the underlying physical parameters of the light-shift. We then measure the light-shift coefficient in an integrated filter-cell, commercial Rb clock, which is similar to the Rb clock design currently flying on Galileo and BeiDou satellites. We measure the light-shift coefficient for two different rf-discharge lamps (i.e., a pure 87 Rb lamp and a lamp filled with the natural Rb isotope abundance), and show that under certain conditions it is possible to combine light-shift data. Our light-shift coefficients are consistent with those reported for the Galileo Rb clock under the assumption of a natural (or 85 Rb isotopically enriched) rf- discharge lamp for the Galileo clock.
Since the 1960s, alkali-based RF discharge lamps have been used as frequency-stable light sources due to their reliable performance [1]. However, it is often reported that rubidium (Rb) lamp-based standards suffer from lamp-related frequency ageing phenomena such as light-shift effects [2], [3]. To understand this behaviour, we are characterising a commercial Rb-based RF discharge lamp over extended periods and will present an analysis of data collected so far. Electrical and optical measurements on a Rb lamp are presented here after about 40 days of monitoring. The output of the lamp was coupled into an optical fibre to observe its emitted light characteristics with an optical spectrum analyser. Optical spectra results indicated that the lamp had substantial buffer gas content. The lamp spectra exhibited various buffer gas excitation lines as well as strong Rb excitation lines of 780.0 nm (D2 line) and 794.8 nm (D1 line). Subsequently, the lamp was monitored for output optical power, temperature, and electric current dynamics in a laboratory environment. During monitoring, sudden changes in light output were observed along with longer term trends. From previous studies, we can expect that temporal changes in the light intensity of the lamps may lead to observable frequency fluctuations in an atomic clock [4].
Abstract : Results of calculation and measurements on the light shift and buffer gas shift in passive rubidium frequency standards are reported. The light shift was measured for two configurations: the separated filter and the integrated filter approaches. A phenomenological analysis, which describes well the results obtained, is presented. Measurements of the frequency shifts created by various buffer gases-pure or mixture- are reported. A nonlinear dependence of frequency versus temperature is observed. The results are compared to already published data and a simple analysis using an equation quadratic in temperature is used to explain those results. (Author)
In this paper we present an analysis and design of a micromachined Rb-85 filter for passive rubidium atomic clock. By introducing the Lorentzian shape functions into the derived output light intensity equation of the filter for a monochromatic light, a formula of light intensity in a filter with a light spectrum input is set up, and parameters in this formula involving transition coefficient, frequency shifts and linewidth are studied and determined. Based on these, a micromachined Rb-85 filter with high filtering performance is designed, in which more than 90% of a-line component in the light spectrum input from the Rb-87 lamp is filtered but b-line component is weakened by less than 3%.
In this paper, we present our studies on double resonance using atomic 87Rb in a wall-coated cell, in view of applications in a laser-pumped Rb atomic frequency standard. We study the systematic clock frequency shifts in a wall-coated cell, such as light shift, microwave power shift [11, 12] and shift due to cell temperature [13] (for the cell body, and for the Rb reservoir, also called the cell stem). We also report on our clock stability measurement using the wallcoated cell in DR scheme, and discuss the influence of the shift parameters first on the frequency stability of the clock.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.