Long term operation, performance and applications of cryogenic sapphire oscillators

Cryogenic sapphire oscillators (CSO) developed at UWA have now been in operation around the world continuously for many years. Such oscillators are essential for interrogating atomic frequency standards at the quantum limit of projection noise; otherwise aliasing effects dominate due to the periodic sampling between successive interrogations of the atomic transition. For this reason, UWA oscillators are now operational at NMI (Sydney), LNE-SYRTE (Paris), the French Space Agency (CNES, Toulouse) and at UWA (Perth). Other applications, which have attracted attention in recent years, include tests on fundamental principles of physics, such as tests of Lorentz invariance. This paper reports on the long-term operation, performance of such oscillators. We compare the long-term drift of some different CSO's. The drift rates turn out to be linear over many years and in the same direction. However, the magnitude seems to vary by more than one order of magnitude between the oscillators, ranging from a few parts in 1014 per day to a few parts in 1013 per day