Characterisation of Rubidium Vapour Lamps for Atomic Clocks

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].