Development Of Microwave Excitation Source For Ceasium Atomic Fountain Clock And The Study Of Correlation Frequency Shifts

The caesium atomic fountain clock is a benchmark device for reproducing the definition of"second".After the cold atomic group rises and falls twice through the excitation cavity,Ramsey interacts with the excitation field to realize the frequency lock of the microwave excitation source on the transition frequency of the caesium atom.The microwave excitation source affects the performance of the caesium fountain clock through the Dick effect and the microwave correlation frequency shift.This paper focuses on improving the frequency stability and frequency uncertainty of the atomic fountain clock,develops a frequency synthesis system.The frequency shifts associated with the frequency synthesis system are evaluated:microwave leakage frequency shift and spectral impurity frequency shift.In order to further improve the short-term frequency stability of the atomic fountain clock,a local oscillator with lower phase noise is developed,and a cryogenic sapphire oscillator is designed and developed,and its performance is preliminarly evaluated.The specific research results and innovations are as follows:1.Development of microwave frequency synthesis system for caesium fountain clock.Using a 100 MHz hydrogen clock as the local oscillator,the microwave signal of 9 192 631 770 Hz is obtained through the frequency synthesis link based on the combination of PLL and direct digital frequency synthesis technique.The residual phase noise of the frequency synthesizer at the frequency offset of 1 Hz is-78 d Bc/Hz,and the effect on the frequency stability of the clock second level is 4.65×10^-15.2.Study on frequency shift of spectral impurity and microwave leakage.Spectral purity has an important effect on the performance of the caesium fountain clock.If the spectrum of the microwave signal feed into the excitation cavity is impure,the transition frequency of the clock will be changed after two side bands with different powers interact with the excitation microwave.When the sideband power difference of 50 Hz is 1 d B,the resulting frequency shift is 1×10^-17.The leakage is reduced by changing the parameters of the leakage field when the atoms exit the resonator.One method is to attenuate the amplitude of the leakage field by microwave interference.This method will introduce a phase fluctuation of 5μrad,resulting in a frequency shift of 1.73×10^-16.The frequency of the leakage field is changed by the frequency detuning technique,and the phase fluctuation is nearly 1 mrad,corresponding to the resulting frequency shift of 3.5×10^-14.At the same time,passive technology is used to reduce the microwave leakage.The indium wire sealing method is used to suppress the microwave leakage of the resonator,and a method for measuring the very weak microwave field is proposed and implemented.The upper limit of microwave leakage frequency shift is evaluated to be 1.7×10^-17.3.Development of the cryogenic sapphire oscillator.At present,the short-term frequency stability of ceasium atomic fountain clocks is mainly limited by the phase noise of the local oscillator.Aiming at the fountain clock with higher performance in the future,a local oscillator is investigated.Therefore,a cryogenic sapphire oscillator is designed and studied to improve the short-term frequency stability of the fountain clock.The electromagnetic field distribution in sapphire at room temperature is studied deeply.The mode analysis of sapphire resonant cavity working in echo wall mode is realized by mode matching,finite element analysis and experiment.At room temperature,the operating mode of the sapphire resonator is WGH_15,0,0,the resonant frequency is 9.89 GHz,the coupling coefficient isβ₁=β₂=0.76,and the unloaded Q₀is 9.4×10⁴.A room temperature sapphire oscillator with low phase noise was designed and constructed.The phase noise at 1 k Hz frequency offset is-112 d Bc/Hz,and the phase noise at 10 k Hz frequency offset is-131 d Bc/Hz.Therefore,the correctness of the analysis method and design of the mode analysis,resonant frequency calculation and circuit parameters of the sapphire resonator is verified.Compared with room temperature,the Q₀ value of the cryogenic sapphire resonator can be improved by at least three orders of magnitude.Under the liquid helium temperature,the resonant frequency of the sapphire resonator WGH_15,0,0^- is 9.982 199 410 GHz,the coupling coefficientβ₁=0.77,β₂=0.55,and the unloaded Q₀ value is 1.5×10⁸.The resonant frequency of WGH_15,0,0⁺ is 9.982 175 400 GHz,the coupling coefficientβ₁=0.56,β₂=0.22,and the no-load Q₀ value is 3.0×10⁸.The Q₀ is improved by nearly four orders compared to the room temperature sapphire resonator.By correcting the phase of the loop and stabilizing the power of the loop,a cryogenic sapphire oscillator with Pound frequency-locked loop and power servo control loop is built.The phase noise at the frequency offset of 1 Hz is-90 d Bc/Hz,and the frequency stability reaches 1.24×10^-13 when the integration time is 1 s.It is of the order of 10^-13 in the integration time range of 10^-1 s-10⁴ s.