Research On Laser Frequency Stabilization Technology And Atomic State Selection Method Of The Cesium Fountain Clock

Among all kinds of scientific and technological measurements in the world,the most accurate one is the measurement of time frequency.Internationally,the cesium atom transition frequency of 9192631770 Hz has been officially adopted as the definition of time unit "second"(s),that is,"1s is equal to the duration of 9192631770 times of the radiation period of the ground state superfine level transition of cesium atom".Frequency of the definition of "seconds" for the above sea level 133 cesium133(Cs)atomic ground state in zero magnetic field frequency of two hyperfine energy level transition,cold cesium atomic fountain clock as can reappear the device,the definition of "seconds" actually all kinds of interference environment as well as the system factor,can make the frequency of the output signal frequency shift with the reference frequency.The frequency of controlled crystal oscillator is corrected by servo control system,and the stability of output signal frequency depends on the stability of atomic transition frequency.The basic performance index of atomic frequency standard is its frequency stability and frequency uncertainty.Frequency stability describes the fluctuation degree of output signal,which is generally represented by Allen variance.Such fluctuation is caused by frequency noise,and detecting laser noise is one of the important sources affecting frequency stability.The frequency uncertainty represents the degree of coincidence between the output frequency and the nominal frequency,and is estimated by the synthesis of the relative frequency shift uncertainty caused by various physical factors.Among them,the frequency shift of cold atom collision is a very important one,and the evaluation of its frequency shift uncertainty directly affects the performance index of accuracy of cesium atom fountain clock.This thesis focuses on the frequency stability and frequency accuracy of two performance indexes of the fountain clock.Firstly,based on the external cavity semiconductor laser with anti-vibration characteristics,the frequency stabilization technology with modulation transfer spectrum is studied to stabilize the frequency of the cesium atom fountain clock detection laser system.A quantum adiabatic transition technique for cold atomic groups with a constant ratio of 2 in selected states is studied.Then,based on the implemented technology,the cold atom collision frequency shift and its uncertainty of cesium fountain clock are measured.The research contents andresults can be summarized as follows:(1)Frequency stabilization of cesium fountain clock detection laser system.Ntsc-CsF1's 852 nm anti-vibration external cavity semiconductor laser has the function of parameter self-restoring frequency locking,good alignment and high brightness.On this basis,the circuit of modulation transfer spectrum technology and optical path frequency stabilization system were built and successfully applied in cesium atomic fountain clock device.The modulation transfer principle based on the modulation of the electro-optical modulator EOM and the modulation transfer of the beam after nearly degenerate four-wave mixing passes through the beat frequency of the detector is analyzed,and the saturation absorption spectrum and error signal characteristics of the detected laser are tested.Based on the frequency discrimination of the error signal,the detection laser frequency with higher signal-to-noise ratio and longer frequency locking time can be obtained,which improves the stability of the detection laser system.(2)Preparation of cesium atom fountain clock sample atoms.Based on the work of the cesium atom fountain clock optical system,physical system,microwave frequency synthesis system,control system and electronic system,the microwave frequency and power synthesis system satisfying the condition of quantum adiabatic transition was constructed,and the selective cold atomic groups with a constant number ratio of 2 for high and low atomic groups were successfully prepared.The selection process and principle of cesium fountain clock are analyzed,and the process and principle of quantum adiabatic transition are analyzed.On the basis of the quantum adiabatic transition,the cold atom samples with the predicted exponential relation are obtained.(3)Development of microwave power module whose frequency and power vary with any waveform.Based on the theory and experiment of quantum adiabatic transition,the required equipment and components are modularized and easy to operate.At present,domestic related products can meet the requirement,which requires the integration of parts.Finally,according to the experimental test,the power module is developed successfully.(4)Research on the evaluation technology of collision frequency shift of cold atoms.On the basis of making alternative cold atomic groups by quantum adiabatic transition technology and obtaining the constant high and low number density ratio of cold atomic groups of 2,the method of alternating high and low density up-throwoperation for 250 cycles was adopted to continuously run for 15 days,and the frequency of high and low density of cold atomic groups was measured.The principle of collision frequency shift of cold atom and atomic density is analyzed,the condition of measuring collision frequency shift of cold atom by differential method is analyzed,and the calculation method of collision frequency shift and uncertainty of cold atom is analyzed.