Performance Evaluation And Improvement Of The Cold Atom Interferometry Gravimeter

Atom interferometers are commonly recognized to have inherent advantages and technological potentials in the area of precision measurement,where it can greatly enhance the measurement sensitivity and measurement precision of the gyroscope,the gravimeter,and the gravity gradiometer so as to be widely applied in the areas of earth gravity field mapping,gravity matching navigation and inertial navigation,industrial production,resource exploration,earthquake prediction,basic science and geophysics,etc..Though the measurement sensitivities of the Raman pulse-based cold atom interferometers have surpassed the classical devices of the same category,but they are still far lower than the theoretical limits.Moreover,substantial efforts are required in order to transfer these instruments from laboratory demonstration to field and industrial applications.Therefore,improving the measurement performance while minimizing the size of the apparatus has become one of the central subjects in the research field of cold atom interferometry gravimeters.This thesis is devoted to the mission of evaluating and enhancing the performance of the current cold atom interferometry gravimeter and has carried out a detailed and thorough theoretical analysis and experimental study on the cold atom interferometry gravimeter:a measurement model has been established for the classical Mach-Zehnder type cold atom interferometry gravimeter;the atomic fountain has been optimized to a temperature as low as 2?K with atom number as large as 10⁹;high precision absolute gravity measurement has been experimentally realized via the cold atom interferometer,where the resolution of 5.1?Gal(1?Gal=10^-9g=10^-8m/s²)is obtained within 100s;the measurement noise of this interferometer is evaluated,where the limits on the gravity measurement resolution imposed by the detection noise,the Raman laser phase noise and the reflection mirror vibration noise are 0.75?Gal,0.24?Gal and 1.96?Gal,respectively;the magnetic field distribution in the region of interferometry has been measured using a method of short Raman spectrum,and the measurement error induced by the non-uniform distribution of the magnetic field has been estimated to 2.04?Gal;last but not least,three technological proposals to improve the performance of the atom interferometry gravimeters have been studied,including the method of laser intensity modulation,the method of Raman sideband cooling?RSC?combined with adiabatic rapid passage?ARP?,and the method of nth order Bragg diffraction.The major novelties of this thesis are summarized as below:1.A fast and accurate method based on short pulse Raman spectrum has been proposed to measure the magnetic field inside the vacuum chamber.The previous long pulse Raman spectrum-based magnetic field measurement method suffers from several drawbacks such as requiring long interrogation time and human intervention during measurement process.Through studying the dependence of the magnetic field measurement uncertainties on the Raman pulse duration and scanning step size,we propose to carry out the magnetic field measurement process with the parameters of1ms pulse duration and 400Hz scanning step size.Therefore,the total measurement time for an interferometry chamber of 68cm has been compressed from 125 hours to 14hours,and an automatical measurement process can be realized.The obtained experimental results show that the uncertainty of the measured magnetic field is 0.72nT,the Allan deviation of 2000s is 0.4nT,and the spatial resolution is better than 12mm.With the obtained information,the gravity measurement error induced by the non-uniform distribution of the magnetic field in the interferometry region is evaluated as 2.04?Gal.2.A method has been proposed to cancel the errors induced by the ac Stark shift in the Raman spectrum-based magnetic field measurement method.A viewpoint has been commonly held that the light shift has no influence on the Raman spectrum-based magnetic field measurement result,whose validity are carefully examined and concluded here.Through theoretically analyzing the generation mechanism of the light shift and the measurement principle of the Raman spectrum-based magnetic field measurement method,we find that the vector and tensor light shift will induce measurement errors.And we verified this conclusion experimentally:when the Raman pulse duration is 1ms,the fictitious magnetic field induced by the vector and tensor light shift is 26.8nT and 2.2nT respectively.We have also introduced a method of taking the average value of the measured magnetic field with the left-and right-circularly polarized lights in order to get rid of the negative effects caused by the vector light shift,and taking the difference of the measured magnetic field using the two magnetically-sensitive states_Fm?28??10?1 and_Fm?28?-1 in order to get rid of the negative effects caused by the tensor light shift.3.A method of using laser intensity modulation has been proposed to improve the contrast of the interference fringe.Due to the Gaussian distribution of the Raman laser intensity profile,as the volume expansion of the cold atom cloud during the interference process,a large number of atoms travelling away from the center of the laser profile will experience lower laser intensities,and henceforth a supposed?pulse for the atom-laser interaction is no longer?pulse for those atoms.The degeneration of?pulse fidelity unavoidably reduces the contrast of the interference fringes.We theoretically analyzed the principles of improving the interference fringe contrast via laser intensity modulation,that is,modulating the Raman laser intensity according to the volume of the atoms after expansion to keep the?pulse condition exactly.We have constructed numerical simulations to examine the improvement effect and the results show that this method can increase the fringe contrast by more than ten percent.A feasible experimental strategy of this laser intensity modulation method as simple as two steps has been designed.4.A method of efficiently preparing the atom states combining Raman sideband cooling and adiabatic rapid passage technologies has been proposed.The traditional way for atomic velocity selection and state preparation,relying upon Raman process and microwave transitions,only has an efficiency of about 2%.We proposed to apply the RSC technique after the MOT stage to cool the atoms further to about 500nK followed by the ARP process to transfer the atoms to the magnetic insensitive state F?28?1,m_F?28?0.We have introduced the experimental principles of RSC and ARP,analyzed the requirement for experimental conditions and discussed the feasible experimental scheme in detail.Studies show that this method has a state preparation efficiency as high as 40%,which is 20-30 times of the original state preparation efficiency.Besides,this method enjoys good robustness,could suppress the atom number fluctuations and thus enhance experimental stability.5.An nth order Bragg diffraction-based large momentum transfer?LMT?method has been proposed.By describing the basic principles behind the atom interferometry gravimeter based on nth order Bragg diffraction,we have modeled and analyzed the key conditions to realize a time-domain Bragg diffraction atom gravimeter when the atomic incident direction parallels to the wave vector of the Bragg lasers.Studies show that the nth order Bragg diffraction technique can increase the atomic splitting momentum and thus the gravity measurement sensitivity by n times compared with the Raman atom gravimeter.And the unique superiority of Bragg diffraction can effectively reduce the common mode errors and environmental disturbances at the same time.