Exploration Of Important Phase Phenomena In Quantum Frequency Standards

As application scenarios continue to expand,atomic clocks,which are benchmarks for time-frequency measurements,are required to have higher frequency stability specifications.Comparing the frequency stability characteristics of different frequency sources and analyzing from the noise angle,it is found that the frequency stability law of the atomic clock is related to the processing method of the line part.Since the active atomic clock line adopts phase locked technique,and only the phase modulation white noise is introduced during the processing.Therefore,the frequency stability is changed by???.However,due to the lack of understanding of the precise phase relationship between signals,the line portion of the conventional passive atomic clock still uses frequency locked technique.Compared with the phase locked technique,due to the introduction of FM white noise during the process of frequency locked technique,the frequency stability index becomes worse,and the transformation law of ???is presented.The purpose of this paper is to explore the important phase phenomena in passive atomic clocks and to find a way to improve the frequency stability index.In this paper,the most widely used passive rubidium atomic clock is the main research object.Through in-depth research on the phase relationship between the microwave excitation signal,the frequency discrimination curve and the light detection output signal of the passive rubidium atomic clock,it is found that the microwave excitation signal of the passive cesium atomic clock is modulated,resulting in the occurrence of the maximum probability point of the atomic energy level transition.Moreover,the microwave excitation signal carrying a small frequency difference causes the maximum probability point of the atomic level transition to shift to the left or right.that is,there is a slight time deviation?t between the actual and the ideal atomic energy level transition maximum probability point.The study found that the time difference?t has a certain conversion relationship with the frequency difference carried by the microwave excitation signal.Different from the acquisition of the traditional frequency difference of the passive cesium atomic clock,the frequency difference obtained by the time difference is the point-to-point frequency difference,and the phase change information of the signal can be obtained.In this paper,a mathematical model between frequency difference and phase difference is established by means of virtual signal reconstruction technology.And it is found that the maximum probability point of the actual atomic level transition is the lowest point of the light detection signal,and the ideal atomic level transition maximum probability point corresponds to the zero crossing of the low frequency modulated signal.Therefore,combining the mathematical model between frequency difference and phase difference,an open-loop digital phase processing scheme for passive cesium atomic clocks is designed and verified experimentally.With the help of high-precision digitization technology,the low-frequency modulation signal and the light detection signal are continuously and randomly sampled,and the low-frequency modulation zero point and the corresponding light detection output lowest point are respectively found,and the time difference?t between the two is calculated.And the instantaneous frequency difference value is obtained from the time difference?t,thereby recovering the continuous phase difference information.Finally,the phase processing of the passive rubidium atomic clock is realized.The experimental results show that the frequency stability index obtained by the digital phase processing of the passive rubidium atomic clock is improved by nearly one order of magnitude,compared with the traditional frequency-locked processing.The second-order frequency stability is4.62×10^-12,and the law of short-term frequency stability is improved compared with ???.This improvement method for frequency stability not only has important guiding significance for the improvement of passive atomic clock,but also indicates the feasible direction for the development of high-precision time-frequency measurement system in the future.