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

On the Light Shift and Buffer Gas Shift in Passive Rubidium Frequency Standard

Jaques Vanier, R. Kunski, Peter Paulin, J.-Y. Savard, M. Têtu, Normand Cyr

Published Jan 1, 1982• 1 citation
Publisher

Abstract

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)

DOI
10.1109/freq.1982.200593

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European Frequency and Time Forum•Jun 25, 2024•0 citations
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)
The paper describes work done on the Passive Rubidium Frequency Standard in order to identify and possibly improve its frequency stability in the short and long term region. In relation to the short term region, experiments are described which permit the identification of the major sources of noise and measurements are reported. A theoretical model is developed and conclusions are drawn relative to the optimization of the various parameters. In relation to the long term region, earlier work has shown that the power shift created by inhomogeneous broadening in the presence of a buffer gas and light shift could be a cause of instabilities. To avoid that effect we have started experiments on a wall coated cell without buffer gas and we report results on the wall shift and its temperature dependence. Although that type of cell avoids the power shift, it is observed that the temperature dependence of the wall shift is relatively large. These results are discussed in terms of earlier data obtained on rubidium masers.
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.
The paper presents the measurements results of the orientational light shift of the radio-optical 0-0 resonance frequency in the rubidium vapor with isotopic pumping. The experiments were done on a laboratory quantum discriminator, the functional diagram of which contained a known element set for rubidium frequency standard on a gas cell. The discriminator was placed in the three identical pairs of Helmholtz coils, 50 cm in diameter, which created a working magnetic field (10 μT, 3 μT, and 1 μT) in the area of the gas cell. Helmholtz coils were connected to alternating voltage sources to create a rotating magnetic field relative to the normal to the direction of the pumping light. To weaken external magnetic noise and inhomogeneities of the laboratory magnetic field, the entire system was placed in a magnetic shield with a screening factor of ~ 103, which practically made it possible to exclude the effect of the residual field on the measurements accuracy. The main experimental results are formulated as follows: the orientational frequency shift of the 0-0 radio-optical resonance does not depend on the magnitude of the working magnetic field and is linearly related to the pumping light intensity; when the working magnetic field is oriented normal to the quantum discriminator optical axis, the tensor and scalar components of the light shift have equal (positive) signs, which at any temperature of the filter-cell leads to their impossible compensation; when the working magnetic field is oriented along the quantum discriminator optical axis, the tensor and scalar components of the light shift have opposite signs, which at the corresponding temperature of the filter-cell leads to their possible (partial) compensation.
The linewidth and frequency associated with the ground state hyperfine energy fo Rb 87 has been studied. A stable single mode semiconductor laser is used to optically pump Rb, in order to obtain a highly collimated, monochromatic light source. The laser is stabilized by locking its optical frequency to the Rb absorption line center. The dependence of the hyperfine frequency and resonance linewidth have been measured as a function of temperature, light intensity and microwave power; and a theoretical description of the linewidth vs. temperature data is presented. Keywords: Optical pumping; Rubidium; Frequency standard; Frequency shift; Light shift; Density matrix; Buffer gas; Linewidth; and Magnetic resonance.
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].