Properties of sapphire crystals elaborated with different grow technics for microwave ultra-stable oscillator applications

State-of-the-art microwave ultra-stable oscillators are currently based on sapphire resonator operated in whispering gallery modes in the range 5 – 12 GHz. Indeed the best near carrier phase noise is achieved with commercial systems incorporating a room temperature sapphire reference associated with a sophisticated electronics degenerating the noise of the sustaining oscillator stage [1]. On the other part, relative frequency instabilities better than few 1 × 10−15 are achieved with laboratory Cryogenic Sapphire Oscillator (CSO) in which the sapphire crystal is cooled into a large liquid Helium dewar and maintained at its turnover temperature (between 5–8 K) [2]. More reccenlty, we demonstrated an original and reliable technology incorporating a pulse-tube cooler instead of a bath cryostat thus eliminating the need for regular supplies and manual transferring of liquid helium [3]. 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, … This technology is today available through a newly created business unit: ULISS{su®) [4].