Stable Ammonia Laser for Cooled Single Ion Frequency Standard

Abstract

It has been demonstrated recently that an optically pumped NH₃ laser operating in the mid-infrared can provide a sub-kilohertz linewidth source when run under proper pumping conditions [1]. Although such a laser possesses an excellent spectral purity, long term stabilization must be referenced to an absolute frequency, typically provided by an atomic or molecular absorber. In the present project, it is proposed to use the NH3 laser for the required frequency source to stabilize to the 12.5 resonance of a trapped, laser cooled single ion of Barium. The trapped single ion system provides the beneficial features of an absorber system in which motional Doppler shifts and broadenings can be eliminated to very good accuracy via confinement and laser cooling [2], together with minimal perturbation by external fields. The ion’s 5d ²D_5/2 – 5d²D_3/2 magnetic dipole transition at 800.974 cm^-1 has an ultimate natural linewidth below 0.1 Hz and is situated near the NH₃ laser transitions at 12.5 μm, thus it has the interesting possibility for stabilization to an extremely high level. Such a weakly allowed transition can be detected for single ion systems by observing interruption in fluorescence from the ion cycling on a strongly allowed transition. The stability of the source may then be evaluated by comparing the ammonia laser frequency with the NRC primary cesium beam frequency standard via a frequency chain [3].