The Development And Frequency Stability Evaluation Of Cesium Fountain Clock

Time is the physical quantity with the highest measurement accuracy at present.With the development of science and technology,together with the needs of production and life,the demand for accuracy of time measurement is getting higher and higher.The basic unit of time is“second”.The definition of“second”evolves from the ephemeris time based on the period of the earth’s revolution in a particular epoch to the atomic time based on the oscillation period of the hyperfine energy level transition of ¹³³Cs atom,and the corresponding measurement devices also evolve from sundials to atomic clocks.With the development of atomic clock technology,the cesium atomic fountain clock improves the time measurement accuracy to the order of 10^-16 and is chosen as the primary frequency standard due to its advantages of high accuracy and long-term stability.As a high-precision time measuring equipment,cesium atomic fountain clocks play important roles not only in basic physics research fields such as the measurements of fundamental constants and tests of general relativity,but also in application fields auch as navigation positioning,deep space exploration,and atomic time-keeping.Development of a cesium atomic fountain clock as the local time and frequency reference is one of the important research goals of the National Precise Gravity Measurement Facility（PGMF）at Huazhong University of Science and Technology（HUST）.Cesium atomic fountain clock can provide a local time-frequency standard for gravity datum tracing,in order that precision gravity measurement results can be traced back to the international standards.The system of a cesium atomic fountain clock is complex,consists of four parts:vacuum physical system,optical system,microwave synthesizer,and electronic control system.This dissertation mainly focuses on the development of the cesium atomic fountain clock,which is also my main research work during the doctoral research period,and the main contents can be summarized as follows:1.The design and construction of the optical system of the cesium atomic fountain clock is realized,participated in the development of the vacuum physics system and the electronic control system,and successfully realized the atomic fountain and related physical experiment.The prototype of the cesium atomic fountain clock is successfully developed,and Ramsey fringe with a contrast of 90.8%is obtained,which is close to the international advanced level.2.The factors affecting the signal-to-noise ratio of the returning TOF signals are optimized experimentally.First,the frequency stabilization scheme is improved from the standard saturation absorption frequency stabilization technique by introducing further frequency voltage converter（FVC）frequency stabilization technique.Second,the integrated tunable fiber beam expander is designed by using optical simulation software.Based on the beam expanders,the cooling light is expanded to a collimated beam with a diameter of about 19 mm with a divergence angle less than 0.5 mrad,and the loading rate of cesium atoms is measured about 6×10⁸/s.The number of atoms is ultimately increased by nearly an order of magnitude,and the initial temperature of the cesium atoms before launching is successfully reduced from 6.9（2）μK to 2.6（1）μK.3.A method based on Monte Carlo simulation is proposed to analyze the factors affecting the ratio of the falling atoms,and a simulation model is established to simulate the evolution process of atoms,and the variation trend of the ratio of the returning atoms under different variables is given in this work.The simulation results are used to guide the experiment and to optimize the ratio of the returning atoms,and ultimately the experimental results are in good agreement with the simulation results,which solve the problem that the number of falling atoms is insufficient.4.A closed-loop locking of the cesium atomic fountain clock is achieved,and its frequency stability is preliminarily evaluated as 2.5×10^-13τ^-1/2,and the main factors affecting the frequency stability is analyzed,including quantum projection noise,local oscillator noise,detection noise,and photon shot noise.It turns out that the frequency stability is limited by detection noise and local oscillator noise at the moment,points out the direction for further improvement.