Cryogenic sapphire oscillators

Cryogenic sapphire oscillators (Contact: Vincent Giordano)

The Cryogenic Sapphire Oscillator (CSO) is currently the most stable source of microwave signal at short integration time, exhibiting a stability of parts in 10-16 from 1 s to 5x103 s, and a drift of 5x10-15 at 1 day. These unprecedented frequency stability performances come from the exceptional regularity of the beat of its heart: a high-purity sapphire crystal placed at low temperature in a well-controlled environment.

 10 GHz Sapphire ResonatorThe sapphire crystal has the shape of a cylinder: approximately 5 cm diameter, 3 mm high. It constitutes a Whispering Gallery Microwave Resonator in which a 10 GHz signal can propagate around the cylinder making 1 billion of cycles before undergoing noticeable attenuation. Beside low dielectric losses, the cryogenic sapphire resonator presents a low sensitivity to temperature fluctuations and to mechanical vibrations. It constitutes an ultra-stable frequency reference that does not show appreciable drift before one day of integration.

Two CSO operating at FEMTO-STThe sapphire resonator is maintained at 6 Kelvin into a Closed Cycle Cryocooler specially designed to limit mechanical vibrations and thermal fluctuations. The autonomy of the whole system is thus the lifetime of the cryocooler (2 years between maintenance). The cooled sapphire resonator is the frequency-determining element of an oscillator loop whom electrical length and circulating power are stabilized thanks to specially designed electronic controls. Our CSO are complemented with a low noise frequency synthesis generating useful ultra-stable signals at 10 GHz, 100 MHz and 10 MHz. The output frequencies can be adjusted by acting on the internal Direct Digital Synthesizer enabling a relative frequency resolution of 1x10-16. A Phase Comparator can be provided to lock the CSO output signals to an external 100 MHz reference.

Allan Deviation measured by beating to quasi-equivalent optimized CSOs

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