ULISS project: First comparison of two cryocooled sapphire oscillators at the 10−15 level

We already demonstrated a state-of-the-art Cryogenic Sapphire Oscillator (CSO), which incorporates a pulse-tube cooler instead of a bath cryostat - thus eliminating the need for regular supplies and manual transferring of liquid helium. The advent of reliable and cryocooled (CSO) open the possibility to implement such an ultra-stable reference not only in metrological laboratories with liquid helium facilities but also in remote sites like base stations for space navigation, VBLI antenna sites, ... The first prototype, i.e. ELISA, has been implemented in the ESA ground station in Malargüe (Ar) in April 2012. A second CSO was recently achieved using the same technology but specially designed to be transportable. After its complete assembly the CSO was coold down for the first time, and a preliminary frequency stability evaluation was done by direct comparison between the two CSOs. The Allan deviation was derived from data collected during more than 3 days. A short term frequency stability better than 2×10^-15 at 1s was obtained. In spite of bad environemental conditions (infrastructure works in the building: implementation of an air conditioning in our laboratory which is not still effective) the frequency stability stays better than 4×10^-15 for τ <; 1,000s and 1×10^-14 over one day.