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

Possible Avenues of Improvement of the Short and Long Term Stability of Optically Pumped Passive Rubidium Frequency Standards

L.-G. Bernier, Anne Brisson, M. Têtu, J.-Y. Savard, Jaques Vanier

Published Jan 1, 1980• 2 citations
Publisher

Abstract

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.

DOI
10.1109/freq.1980.200430

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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.
We have realized and studied a rubidium atomic frequency standard based on a paraffin-coated cell, exhibiting a short-term frequency stability <3 × 10−12 τ−1/2 between τ = 1 and 100 s. Characterization of the wall-coating is performed by measuring the T1 and T2 relaxation times. Perturbations of the medium- to long-term clock stability, due to variations in the laser-intensity, laser frequency, the microwave power shift, and the shifts due to temperature variations are measured and analyzed. A method for reducing the intensity light-shift by detuning the laser frequency and the resulting improvement in clock stability is demonstrated. This work is of relevance for further improvements on Rb cell standards using anti-relaxation wall-coating technology.
The /sup 87/Rb cell frequency standard, optically pumped by a laser diode, is of great interest in the development of high short-term stability oscillators. We report experiments performed on a rubidium cell prototype aiming at evaluate the transfer of the laser noise to the hyperfine clock transition via the light shift effect, which appears to be the main contribution to the observed short term frequency stability in this kind of standard. Pulsed optical pumping will also be examined in order to strongly reduce the light shift effect and to achieve the theoretical limit never approached up to now despite the strong detected microwave signals.
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)
A pulsed optical pumping rubidium frequency standard has been constructed for the purpose of eliminating the light shift and investigation the long-term frequency stability in the absence of light shifts. The device used employs a very small physics package that occupies a total volume of only 260 cc. Initial measurements of the light shift for this device showed that the light shift was reduced by about an order of magnitude: to approximately -1.5 times 10 to the -10th power fractional frequency shift for a 30$ reduction in light intensity. Under these conditions, the square root of the Allan variance was determined to be of the order of parts in 10 to the 12th power for averaging times of 24 hrs and 48 hrs.
The present status of rubidium atom frequency standard is introduced. The factors influencing long-term stability were discussed such as the light frequency shift, collision frequency shift and microwave power frequency shift. When pumping light spectral function has small change, it will cause light shift. The method of reducing light shift was to control the slope between the light shift curve and collision shift curve until they were equal in value but opposite in direction. For reducing collision frequency shift, the rubidium absorbs cell with special glass was adopted to weaken the chemical reaction in the experiment. We believed that the new reason based on AC Zeeman effect was the main one to influence microwave power frequency shift, so the traditional explanation was not the main factor. In the experiment, microwave power frequency shift is eliminated through the circuit improvement. Therefore, better long-term stability can be obtained. The performance of rubidium atom frequency standard is greatly improved
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.