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European Frequency and Time Forum

Characterisation of Rubidium Vapour Lamps for Atomic Clocks

Ulas Sabahattin Gokay, Nitika Gupta, Rabia Ince, Hugh A. Klein, Guilong Huang, Mohsin Haji

Published Jun 25, 2024• 0 citations

Abstract

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].

DOI: 10.1109/eftf61992.2024.10722629

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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.

The paper presents the realisation of a high-performance vapour-cell atomic frequency standard demonstrator based on laser optical pumping. The development of such devices aims to fully exploit the advantages of using a narrow-band diode laser pump light source instead of the discharge lamps commonly used in Rb gas-cell clocks. These advantages include improved control over the pump light spectrum, higher selectivity of the optical pumping, and increased signal contrast, and thus show the potential for the realisation of Rb atomic clocks with improved short-term frequency stability.

Miniature (<few cm3) vapor-cell based devices using optical pumping of alkali atoms, such as atomic clocks and magnetometers, today mostly employ vertical-cavity surface-emitting lasers as pump light sources. Here, we report on the demonstration of optical pumping in a microfabricated alkali vapor resonance cell using (1) a microfabricated Rb discharge lamp light source, as well as (2) a conventional glass-blown Rb discharge lamp. The microfabricated Rb lamp cell is a dielectric barrier discharge (DBD) light source, having the same inner cell volume of around 40 mm3 as that of the resonance cell, both filled with suitable buffer gases. A miniature (∼2 cm3 volume) test setup based on the Mz magnetometer interrogation technique was used for observation of optical-radiofrequency double-resonance signals, proving the suitability of the microfabricated discharge lamp to introduce efficient optical pumping. The pumping ability of this light source was found to be comparable to or even better than that of a conventional glass-blown lamp. The reported results indicate that the micro-fabricated DBD discharge lamp has a high potential for the development of a new class of miniature atomic clocks, magnetometers, and quantum sensors.