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IEEE International Conference on Nano/Micro Engineered and Molecular Systems

Analysis and design of a micromachined Rb-85 filter in passive rubidium atomic clock

Shenggui Wang, Liwei Lin, Hang Guo

Published Jan 1, 2009• 10 citations
Publisher

Abstract

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%.

DOI
10.1109/nems.2009.5068722

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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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The metal-all-dielectric optical filter in the physics package of a lamp-pumped rubidium atomic clock was investigated as a compact passive filter to condition the light emitted by the rubidium lamp. The use of such a design could enable spectral conditioning of devices using natural rubidium and reduce reliance on isotope-enriched 85 Rb vapor cells. In the case of lamp-pumped atomic clocks, the broad spectrum due to the presence of different noble gases in the rubidium lamp and their distinct emission lines in the lamp radiation could cause an increase in the optical background, a decrease in double-resonance contrast, additional optical pumping, and degradation of the microwave–optical double-resonance discriminator signal. To solve the problem described above, a one-dimensional defective metal–dielectric photonic crystal filter was designed, manufactured, and used in a rubidium atomic clock physics package. The simulation results obtained via the transfer matrix method revealed that the proposed structure can generate a wavelength-selective defect-mode passband within the photonic band gap, with efficient electric-field localization and group delay in the main defect-mode region. Conversely, our experimental results with a defective photonic crystal filter and a 600 nm long-pass optical filter, indicated that the photonic crystal filter provided higher wavelength-scale spectral selectivity, suppressed a significant fraction of the unwanted broadband lamp background, and preferentially transmitted the spectral region containing the rubidium D-line emissions. This would yield a suitable double-resonance signal for error-signal extraction compared with the case where the long-pass filter was used. The photonic-crystal thin-film filters increased the peak-to-peak double-resonance amplitude by approximately 11.3 times. They increased the zero-crossing frequency-discrimination slope by approximately 11.8 compared with that of the 600 nm long-pass filter. The equivalent RMS frequency noise of the open-loop discriminator signal decreased from about 95 Hz to about 17 Hz. However, this does not imply that thin-film photonic crystal filters can fully replace conventional rubidium atomic filter cells. Rather, this aim explores a cost-reduction approach using a natural-abundance rubidium cell and reduces the need for additional enriched 85 Rb vapor filter cells. These results suggest that photonic-crystal thin-film filters can be useful passive spectral-conditioning elements for compact lamp-pumped rubidium atomic-clock physics packages.

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[Spectroscopy study of rubidium spectrum lamps].

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Rubidium spectral lamp is one of the critical parts in a rubidium atomic frequency standard (RAFS). The light emitted from the spectral lamp contains two components, one is the useful component, which contributes to the atomic transition signal, and the other is the useless component, which acts only as the light noise. In order to improve the signal-to-noise ratio of the frequency locking loop of a RAFS, and therefore to obtain a highly stable RAFS, it's very important to enhance the useful light component and suppress the useless component. In the present work the light spectra of the three commonly used rubidium spectrum lamps with bulbs separately filled with buffer gases argon, krypton and xenon were obtained by means of a monochromator. The spectra were analyzed and the methods to enhance useful light intensity and reduce useless light intensity were discussed. Results showed that the useful light intensity strongly depends on the types of buffer gases and the bulb temperature. In the whole range of bulb temperature, the Xe lamp had the highest useful light intensity among the three lamps. The Kr lamp showed a higher useful light intensity than the Ar lamp at low bulb temperature, while at high bulb temperature the Ar lamp showed a higher useful light intensity than the Kr lamp. Analysis also indicates that the light noise of a rubidium spectrum lamp can be effectively minimized by adopting suitable light filtering technique.

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This paper describes the experimental results and theoretical analysis of a newly devised compact light source for the Rubidium Frequency Standard (RFS). The light source is composed of a cylindrical Rb-87 lamp 10 mm diameter and a Rb-85 filter cell 3-7 mm long attached to the front flat face of the lamp. This composite-type device is operated in an oven at about 100-deg C. Thus a light source for Rb-87 hyperfine optical pumping less than 4 cm long by 3 cm diameter has been constructed.

•Mar 1, 1982•2 citations
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%.
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 metal-all-dielectric optical filter in the physics package of a lamp-pumped rubidium atomic clock was investigated as a compact passive filter to condition the light emitted by the rubidium lamp. The use of such a design could enable spectral conditioning of devices using natural rubidium and reduce reliance on isotope-enriched 85 Rb vapor cells. In the case of lamp-pumped atomic clocks, the broad spectrum due to the presence of different noble gases in the rubidium lamp and their distinct emission lines in the lamp radiation could cause an increase in the optical background, a decrease in double-resonance contrast, additional optical pumping, and degradation of the microwave–optical double-resonance discriminator signal. To solve the problem described above, a one-dimensional defective metal–dielectric photonic crystal filter was designed, manufactured, and used in a rubidium atomic clock physics package. The simulation results obtained via the transfer matrix method revealed that the proposed structure can generate a wavelength-selective defect-mode passband within the photonic band gap, with efficient electric-field localization and group delay in the main defect-mode region. Conversely, our experimental results with a defective photonic crystal filter and a 600 nm long-pass optical filter, indicated that the photonic crystal filter provided higher wavelength-scale spectral selectivity, suppressed a significant fraction of the unwanted broadband lamp background, and preferentially transmitted the spectral region containing the rubidium D-line emissions. This would yield a suitable double-resonance signal for error-signal extraction compared with the case where the long-pass filter was used. The photonic-crystal thin-film filters increased the peak-to-peak double-resonance amplitude by approximately 11.3 times. They increased the zero-crossing frequency-discrimination slope by approximately 11.8 compared with that of the 600 nm long-pass filter. The equivalent RMS frequency noise of the open-loop discriminator signal decreased from about 95 Hz to about 17 Hz. However, this does not imply that thin-film photonic crystal filters can fully replace conventional rubidium atomic filter cells. Rather, this aim explores a cost-reduction approach using a natural-abundance rubidium cell and reduces the need for additional enriched 85 Rb vapor filter cells. These results suggest that photonic-crystal thin-film filters can be useful passive spectral-conditioning elements for compact lamp-pumped rubidium atomic-clock physics packages.
Rubidium spectral lamp is one of the critical parts in a rubidium atomic frequency standard (RAFS). The light emitted from the spectral lamp contains two components, one is the useful component, which contributes to the atomic transition signal, and the other is the useless component, which acts only as the light noise. In order to improve the signal-to-noise ratio of the frequency locking loop of a RAFS, and therefore to obtain a highly stable RAFS, it's very important to enhance the useful light component and suppress the useless component. In the present work the light spectra of the three commonly used rubidium spectrum lamps with bulbs separately filled with buffer gases argon, krypton and xenon were obtained by means of a monochromator. The spectra were analyzed and the methods to enhance useful light intensity and reduce useless light intensity were discussed. Results showed that the useful light intensity strongly depends on the types of buffer gases and the bulb temperature. In the whole range of bulb temperature, the Xe lamp had the highest useful light intensity among the three lamps. The Kr lamp showed a higher useful light intensity than the Ar lamp at low bulb temperature, while at high bulb temperature the Ar lamp showed a higher useful light intensity than the Kr lamp. Analysis also indicates that the light noise of a rubidium spectrum lamp can be effectively minimized by adopting suitable light filtering technique.
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].
Considering the importance of start-up characteristics of the rubidium atomic clock in engineering applications, the objective of this paper is to optimize the start-up characteristics of the rubidium atomic clock by studying the theory of the rubidium atomic frequency standards, especially the light pumping process, and the effect of light intensity on frequency accuracy. Our analysis demonstrated that frequency accuracy is proportional to the light intensity, and hence, we propose a method for actively optimizing the start-up characteristics of the rubidium atomic clock by utilizing the fluctuations in light intensity. Additionally, some related experiments using the proposed method indicate that the light intensity–frequency coefficient of the rubidium atomic clock is improved from 1.84 × 10−9 to 4.21 × 10−10 V−1 within 30 min after the rubidium atomic clock is locked, and also, the lockout time is less than 5 min with a wide working temperature range (0–50 °C), indicating a significant improvement in the start-up characteristics of the rubidium atomic clock.