Atomic fountain clock with very high frequency stability employing a pulse-tube-cryocooled sapphire oscillator

Abstract

The frequency stability of an atomic fountain clock was significantly improved by employing an ultra-stable local oscillator and increasing the number of atoms detected after the Ramsey interrogation, resulting in a measured Allan deviation of 8.3 × 10(-14)τ(-1/2)). A cryogenic sapphire oscillator using an ultra-low-vibration pulse-tube cryocooler and cryostat, without the need for refilling with liquid helium, was applied as a local oscillator and a frequency reference. High atom number was achieved by the high power of the cooling laser beams and optical pumping to the Zeeman sublevel m(F) = 0 employed for a frequency measurement, although vapor-loaded optical molasses with the simple (001) configuration was used for the atomic fountain clock. The resulting stability is not limited by the Dick effect as it is when a BVA quartz oscillator is used as the local oscillator. The stability reached the quantum projection noise limit to within 11%. Using a combination of a cryocooled sapphire oscillator and techniques to enhance the atom number, the frequency stability of any atomic fountain clock, already established as primary frequency standard, may be improved without opening its vacuum chamber.

Autonomous cryogenic sapphire oscillators employing low vibration pulse-tube cryocoolers at NMIJ

“…The first cryoCSO is now used with our Cs atomic fountain frequency standard, NMIJ-F2, which has achieved a nearly quantum-projection-noise-limited frequency stability of 8×10 -14 at 1 second [3]. The cryostat design achieves heat exchange between the cryocoolers cold stage and the cold finger by use of helium gas.…”

“…In order to avoid the influence of the Dick effect (the degradation of frequency stability due to dead time), a more stable local oscillator than obtainable in commercially available devices is required. Thus CSOs have been successfully implemented as local oscillators for such frequency standards [2], [3].On the other hand, the development of optical atomic clocks is rapid and their frequency stability and uncertainty approaches 10 -18 . In near future, the redefinition of the unit of time, the "second," is expected [4].…”

Operation of NIM5 fountain with 1.5x10-15uncertainty and design of new NIM6 in NIM

“…A new cesium fountain clock NIM6 is under construction in the National Institute of Metrology China, and aiming to operate in 2016. Besides some improvements on the design of the Ramsey cavity to reduce the distributed cavity phase shift and microwave leakage, NIM6 is also aiming to collect more atoms from a MOT loading optical molasses (OM) and optical pumping [11,12], leading to a better signal to noise ratio at the detection. A new cryogenic sapphire oscillator (CSO) based frequency synthesizer [13] and ultra-stable microwave generated from ultra-stable laser are also under developing to reduce the microwave phase noise in order to reach the quantum projection noise, thus leading to a lower Type A uncertainty of the new fountain.…”

Preliminary Evaluation of the Cesium Fountain Primary Frequency Standard NMIJ-F2

“…No liquid helium refills are needed for a cryoCSO. In NMIJ, one of our CSOs was modified with the addition of a low-vibration pulse-tube cryocooler and has now been employed as the local oscillator for NMIJ-F2 [20].…”

“…One of the advantages of NMIJ-F2 over NMIJ-F1 and some other fountains is the high frequency stability below 1 ×10 −13 at averaging times of 1 s [20]. A cryogenic sapphire oscillator (CSO) using a pulse-tube cryocooler [21], [22] is employed as the local oscillator to avoid the degradation of frequency stability due to the Dick effect [23], [24].…”