Oscillators or clocks are widely used in number of large diffusion and scientific applications. The security and information transfer capability of modern telecommunication systems are conditioned by the degree of time synchronisation at the web nodes. Accuracy of vehicle positioning with a satellite navigation system (GPS, Galileo…) is limited by the performances of clocks and oscillators placed at every stage of the system. Metrology and Experimental Physics are demanding of ever-increasing resolution and thus of improved frequency references.

vg-stab-ref-comp - Relative frequency stability of various frequency standards. In red, new requirements for quartz X-tal oscillator industry, space navigation and Very Long Base Interferometry.

The blue area in the figure 1 represents the typical performance of microwave oscillators incorporating a dielectric sapphire resonator cooled in a liquid helium bath. The Cryogenic Sapphire Oscillator (CSO) can exhibit the highest short-term frequency stability (<1x10-15 for 10s<<100s). CSO is the only available technology able to meet the requirements expressed by various users (red lines). Few liquid helium cryogenic sapphire oscillators have been built in a small number of worldwide research laboratories. Some of these instruments have been used as local oscillator for atomic fountain clocks and for fundamental physical experiments. Nevertheless they are still experimental prototypes that are difficult to implement outside a well-equipped laboratory. Moreover the liquid helium cryostat has to be refilled periodically adding a heavy maintenance constraint. Yet, various applications could benefit of the performances offered by the cryogenic sapphire oscillator:
 - Space. The interplanetary navigation is based on microwave signal exchanges between the space vehicle and a ground station equipped with a high stability timing reference. Ranging and Doppler tracking resolutions are limited by the clock instabilities. To be able to met the stability requirements for the future space missions, the European Space Agency (ESA) funded a research program with the objective to get better than 3x10-15 frequency stability over 1 s to 1,000 s while insuring at least one year autonomy. This project (ELISA) coordinated by the Femto-ST Institute was finalised in April 2010: the main objective being realized.

- Metrology: primary frequency standards. In an atomic clock, a local oscillator (LO) is frequency locked on the resonance frequency of an atomic transition. The best current atomic clocks using cold atoms operate sequentially. Indeed time is needed to cool the atoms before sensing the atomic transition. During this period of time no information is available for the frequency of the LO which is then free-running. Consequently the overall clock frequency stability is degraded by the short-term instability of the LO. Better than 1x10-14 over 1-100s is required.

- Metrology: frequency stability certification. National Metrological Institutes have in charge to maintain the standards and to establish certifications for industrial products and laboratory instruments. The frequency stability is one of the major characteristics of the signal source that could impact on the security and performances of number of systems. It is then a metrological quantity that has been to be measured with the best resolution available. A standard presenting frequency stability better than 1x10-14 over 1-1000s will notably increase the current calibration capability making it compatible with all current industrial requirements.

- Very Long Baseline Interferometry. VLBI allows data from widely separated radio telescopes to be combined to get exceptional astronomical measurement resolution providing each radio telescope is equipped with an ultra-stable clock. VLBI is most well known for imaging distant cosmic radio sources, spacecraft tracking, and for applications in astrometry. However, since the VLBI technique measures the time differences between the arrival of radio waves at separate antennas, it can also be used "in reverse" to perform earth rotation studies, map movements of tectonic plates very precisely (within millimetres), and perform other types of geodesy.

- Industry of high performance clocks and oscillators. To estimate, without ambiguity the frequency performances of state-of-the-art oscillators and clocks requires an ultra-stable reference clock. Some industrial companies making high performance quartz oscillators are interested by a versatile frequency reference of less than 1x10-14 frequency instability.

- Fundamental Physics. Special Relativity is one basic pillar of modern physics. Each basic theory has to be tested as its best, because it is so fundamentally important. The best tests of Special Relativity are based on ultra-stable oscillators for which we are locking for a frequency variation as a function of the speed and orientation of the laboratory. The frequency fluctuations of the oscillator limit the measurement resolution.

For all these applications, the autonomy and reliability of the frequency reference are major requirements. In all cases, liquid helium bath has to be rejected.