Development Of Time-keeping ⁸⁷Rb Fountain Clock

The time reference system provides continuous,stable and reliable time and frequency signals for generating and maintaining the national standard time UTC（k）.The time-keeping atomic clock,as the core equipment of the time reference system,determines the performance of UTC（k）.Time-keeping ⁸⁷Rb fountain clocks,with short-term frequency stability one order of magnitude higher than that of cesium clocks and a frequency drift one order of magnitude higher than that of hydrogen clocks,are a new type of time-keeping atomic clock being developed in various countries.Around the time-keeping application requirements of time reference systems,for continuous operation and excellent frequency stability performance of time-keeping ⁸⁷Rb fountain clock,a miniaturized optical system has been developed to improve the continuous operation of the clock;to enhance the short-term frequency stability of the clock,a cooling laser collimating expenders,a detection zone and a state-selection cavity were developed as well as clock noise and Majorana transition were studied,and the corresponding suppression techniques were implemented;to achieve better daily frequency stability,the influence paths of microwave leakage from different microwave leakage sources and thermal atomic background concentration in the MOT cavity are experimentally investigated,and corresponding reduction schemes are realized.The atomic fountain with atomic temperature of the order of u K and Ramsey fingers were obtained through the intermodulation of the subsystems and parameter optimization.The clock operation was achieved and the frequency stability and daily frequency drift were measured.The specific research results and innovations are as follows:1.The development of the optical system.The design requirements are analyzed in terms of the functionality and performance of the optical system.The optical system uses a high-power fiber laser as the cooling laser source and a miniaturized spatial optical table（600 mm×500 mm×100 mm）to realize frequency and power control as the main features.Frequency control:according to the difference of the cooling laser,pushing laser and detection laser action time（one clock cycle）,the use of an AOM and microwave switch to achieve the frequency detuning of the above lasers,reducing the amount of components used.An adder was used to achieve a large detuning of the frequency at the digital frequency stabilization feedback interval of the fibre laser,with a detuning of 120 MHz compared to the conventional use of AOM,which is more conducive to sub-Doppler cooling of the atoms.In terms of power control:the optical software was used to simulate the fiber input coupler and output coupler to achieve an optimal coupler combination and improve the coupling efficiency of the fiber.The transmission of six cooling beams was achieved by using two fiber splitters,reducing the number of fiber couplers as well as the number of polarization-maintaining fiber used.2.Study of the influence of optical system on time-keeping ⁸⁷Rb fountain clock.The relationship between TOF signal drift and ambient temperature was investigated and based on the 130-day TOF signal drift of 14.9%indicates continuous operation capability of the optical system for more than 4 months.The effect of the optical system on the frequency stability of the ⁸⁷RFCC was studied,and its effect on the second frequency stability of the clock was calculated by measuring the relative noise of the detection laser as 2.03×10^-14,and that to the long-term stability of the clock was calculated by measuring the optical frequency shift as 8.98×10^-19.3.The development of physical system.In the development of the physical system,the functional units for improving the short-term frequency stability of the clock—the cooling laser collimation expender（CLCE）,detection zone and the state-selection cavity were realized.The optical and mechanical software was used to design and develop the CLCE,and its output laser characteristics（collimation<0.5 mrad,directionality<1,uniformity<2 mm）met the design requirements.To improve clock discriminating signal—TOF contrast and S/N ratio,The design,simulation,and development of the detected dual-beam optical machine device and fluorescence collection device were carried out to achieve a fluorescence collection efficiency of6.63%,an increase of 32.6%compared to the conventional design.To overcome the degradation of TOF S/N ratio by detection zone,the design of flat state-selection cavity is proposed.By experimentally studying and optimizing the sub-Doppler cooling parameters affecting the atomic temperature,an atomic fountain with an atomic temperature of 5.16 u K was obtained,which meets the operational requirements of the time-keeping ⁸⁷Rb fountain clock.Through the intermodulation of four subsystems,a Ramsey fingers with a central finger of 1.0 Hz and 95%contrast was realized.With a hydrogen clock as reference,a clock second frequency stability of 1.91×10^-13 and daily frequency stability of<7×10^-16 were obtained.4.The time-keeping ⁸⁷Rb fountain clock stability improvement.The effect of the noise on the second frequency stability of the clock is analysed and studied for the time-keeping ⁸⁷Rb fountain clock,pointing out that the main noise terms are the quantum projection noise and the phase noise of the local oscillator.The temperature of the rubidium source and the Majorana effect,which affect the short-term frequency stability of the clock,and the source of microwave leakage frequency shift,which affects the daily frequency stability of the clock,are analyzed.The solutions are given to reduce their effects.This has resulted in an increase in the short-term frequency stability of the clock a factor by 2.3,and an increase in the daily frequency stability of the clock from the order of 10^-15 to<7×10^-16.5.Implementation of a time-keeping application for the time-keeping ⁸⁷Rb fountain clock.Cooperation with the National Time Service Centre（NTSC）time-keeping team,the time-keeping ⁸⁷Rb fountain clock was added to the NTSC time-keeping system to participate in the generation of the national standard time UTC（NTSC）.Through the UTC（NTSC）,a 6-month comparison with the Bureau International des Poids et Mesures TT（BIPM）was carried out to obtain 5-month daily frequency drift of-6.983×10^-17,5.818×10^-17,2.361×10^-17,2.585×10^-18,3.511×10^-18 in that order,and to obtain weights to begin formal participation in the generation of the international standard time UTC.This device is the first one in China and also the first time-keeping ⁸⁷Rb fountain clock to participate in the generation of international standard time after the United States,which also makes China the second country in the world to achieve the continuous weighting of time-keeping ⁸⁷Rb fountain clock to participate in the generation of international standard time.