| Time is a basic parameter to measure the movement of all objects,and it has the highest accuracy."Second" is the unit of the time.The thirteenth International Metrology Conference which held in 1967 adopted the definition of "atomic second" based on cesium atom: The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.The equipment that reproduces the mentioned "second" is called a cesium atomic fountain clock.The frequency which defined by the "second" is transition frequency between two hyperfine levels of cesium 133 atom at the sea level and without fluctuations.The resonant frequency of the atom in the actual environment is not equal to the frequency that defined by "second",because of the influence of external environment and the internal atomic interaction.These effects are called "frequency shift",and the uncertainty of these frequency shifts is the most important parameter that decides the performance of the cesium atomic fountain clock.The National Time Service Center of Chinese Academy of Sciences generates and maintains the national standard time UTC（NTSC）,while the accuracy of local atomic time TA（NTSC）depends on the Time and Frequency Bulletin of the BIPM that late for one month.Developing the cesium fountain clock independently can calibrate the TA（NTSC）real time and improve the performance of national standard time UTC（NTSC）.In this paper,the research work is focused on improving the performance of cesium fountain clock（NTSC-F1）especially its frequency stability and frequency uncertainty.We analyzed the origin of the noise which affects the cesium fountain clock,developed a technology to improve the signal-to-noise ratio of flight time signals and improved the frequency stability of NTSC-F1.At the same time,we evaluated the second-order Zeeman frequency shift of the fountain which is one of the main frequency shifts of NTSC-F1.The C field of cesium fountain clock was measured accurately by two methods,rigid analysis shows that the uncertainty of the second-order Zeeman shift is 1.08 × 10-16.The research contents can be summarized as following:1.Research on the technology of improved the signal-to-noise ratio of time-of-flight（TOF）signal.The signal-to-noise ratio is improved by enhancing the intensity of TOF signal and reducing the noise of TOF signal.（1）Improving the vacuum system of two-dimensional magneto-optical trap to reduce the fluctuation of dropped atomic number.The original way of cohering the glass and the metal cavity with epoxide-resin glue is replaced by the way of pressing indium wire.The improved two-dimensional magneto-optical trap can provide cold atomic beam continuously,which enhanced the intensity of TOF signal.The fluctuation of the vacuum degree of cesium fountain clock has reduced by two orders of magnitude.The experimental results showed that the variation of vacuum degree in fountain tube is confined in 1×10-7Pa,and the fluctuation of dropped atomic number caused by the vacuum changes was less than 10%.（2）Reducing the temperature of the molasses in order to enhancing the intensity of TOF signal.The fluctuation of magnetic field is better than 30 nT in the range of 1 cm near the center of the 3D-MOT by improving the technology of compensation.The atomic temperature is reduced from 10μK to 3μK,which reduces the loss during the flight process and improves the intensity of the TOF signal.（3）Improving the dual-level detection system to reduce the relative intensity noise of probe light.Optimized the optical path of the dual-level detection system and added a power-stabilized loop for the probe light to reduce its power fluctuation.Different from the traditional power-stabilized loop,a new scheme has benn used in the stabilization system by using a piece of uncoated optical glass plate as a beam splitter in the control-loop.The power stabilization loop developed in this way can not only prevent the direct change of the laser power,but also suppress the indirect change in laser power caused by polarization.In this way,the power-stabilization loop realized the simultaneous stability of the dual-level detection lights.Experiments showed that the relative intensity noise of probe light is less than10-7/f(Hz-1),thus the fountain stability limitation is less than 1×10-14（τ）-1/2（τ=140000s）due to this noise only.2.Improving of the optical system of NTSC-F1 to avoid long-time power change of the laser.The stability of the optical system is the main factor that affecting the continuous operation of the cesium fountain clock.We used a commercial semiconductor laser as the light source providing cooling and the probe light of the fountain clock.All-metal and miniaturized optical components were used in the optical system to improve the stability and enhance the coupling efficiency of the system.In order to reduce the interaction effect of the cooling power and the frequency during the polarization gradient cooling phase,we design a twice traverse path of cat’s eye structure for the acousto-optic modulator.The laser beam is physically turned off using a silent mechanical switch to reduce the effect of mechanical.The power monitoring of optical system shows that,the optical system can be locked for more than one month.The power of laser has declined 3% during the operation time,which is in full compliance with the requirements of the fountain clock’s operation.3.Study of second-order Zeeman frequency shift.A low frequency transition method is applied to measure the magnetic field distribution of the atomic excitation region and free flight region.The AC demagnetization method is employed to demagnetize the magnetic shielding,for the purpose of optimizing the inhomogeneity of C field.A high stable current supply is developed to reduce the variation of C field.The second-order Zeeman frequency shift is measured using the central fringe of（1,1）Ramsey transition.The uncertainty of second-order Zeeman frequency shift is caused by inhomogeneity of C field and the temporal instability of C field.The measurement uncertainty of second-order Zeeman frequency of NTSC-F1 is 1.08×10-16.4.Study of the clock transition frequency shift caused by mean magnetic field in the excitation region unequal to its mean value in the drift space.Actually,the interaction between atom and microwave includes two Rabi processes and a free evolution process,both of which are related to the magnetic field.However,the principle of the frequency shift induced by the magnetic field is not the same in two kinds of processes.The traditional studies of second-order Zeeman shift just focus on the frequency shift caused by the magnetic field,without distinguishing two kinds of processes.The mean magnetic field in the excitation region unequal to its mean value in the drift space will affect the clock transition and produce frequency shift.In this paper,we studied the shift caused by the inhomogeneity of the magnetic theoretically,measured the shift based on NTSC-F1 for the first time,and the relative frequency shift is in 10-17 level.5.Study of high performance magnetic shield.In the cesium fountain clock,the second-order Zeeman frequency shift,Rabi and Ramsey pulling frequency shift,and the Majorana transition frequency shift are related to the performance of the magnetic shield.The uniform region of the NTSC-F1 magnetic shield is only 6cm,and the uniformity of the C field is realized by many compensation coils,which increases the complexity and instability of the fountain clock.We study the magnetic shield from material selection,installation structure,and the way of demagnetization.The magnetic field uniform region is 51 cm obtained experimentally. |
PDF Full Text Request