| High Precision gravity measurement is an important tool in the field of modern navigation,earth observation,resource exploitation and fundamental physics.With its outstanding development speed,gravity measurement techniques based on cold atom interferometry has demonstrated broad application prospect.Under the background of cold atom interferometry in inertial measurements,the dissertation studies the theory and key technical problems of precision gravity measurement based on atom interferometry,including the overall design and building of gravity measurement system,improvement and optimization of the subsystem,complete test of system performance,measurement noise and error modeling and analysis,as well as a new method to enhance the contrast of interference fringes.The main work of the disertation is summarized as below:1.The atom interferometry gravity measurement system is designed and builded.The principle of inertial measurement based on Raman-pulse-type Mach-Zehnder cold atom interferometry is analyzed.The vacuum,magnetic field,optical and electronics systems are designed,optimized and integrated into a whole apparatus.A laser frequency stabilization method based on acousto-optic modulation transfer spectroscopy is proposed with a high degree of freedom.The method overcomes the influence from RLC resonance circuits and high-order sidebands in eletro-optic modulation,with which a higher modulation depth,broader range of choice for modulation frequency and higher optical isolation are expected.The change in the slope of error signal with different experimental parameters is theoretically analyzed.The parameters are then optimized in experiments,which finally reduce the laser linewidth to 63kHz and provide a relative frequency stability of 1×10-12@30s.The laser frequency can be adjusted within the AOM bandwidth.A kind of optical path structure and laser frequency control method is proposed which can be easily integrated.With the help of the high-bandwidth and low-noise OPLL,frequency control can be achieved by the programming and timing of the reference signal generator.The frequency shifting work for AOMs is then reduced,thus avoiding the optical path deviation via AOM diffraction angle chang and stretching the frequency shifting range.The optical path structure and the associated drivers are then simplified.2.Atom interferometry gravimetric experiments are carried out.Based on the established system,the preparation and optimization of cold atomic ensembles are completed,with a loading rate of 4×108/s,atom number of 3.4×108,atomic temperature of 5.2?K.The speed selection and magnetic insensitive initial state preparation is achieved with reversed Raman?pulses,reducing the atomic temperature to 507nK.The two-state fluorescence detection system is designed to measure the final atomic population with a SNR of 32dB.The inteferometric fringes are achievd with co-propagating and counter-popagating Raman pulses with a free evolution time of 90ms and a fringe contrast of 17%.Gravity signal extraction experiments are carried out which measure the local gravity acceleration of?7?9.805740±0.000003?8?m s2,with a resolution of 6.5?Gal@1000s and sensitivity of 121?Gal/Hz1/2.3.The system noise and error analyses are carried out.The contribution of the main noise sources to the gravity measurement results is evaluated.Methods to improve the measurement sensitivity are discussed.An error propagation model of gravity measurements based on cold atom interferometry is established with an overall consideration of the mechanism of the main error souces,including TPLS shift,AC-stark shift,quadratic Zeeman shift,Raman beam alignment,Raman wavefront and Corilois effect.By combining the error souces with experimental parameters,such as Raman beam direction,Rabi frequency,atomic free evolution time,magnetic field distribution,atomic temperature and density distributions,the error decoupling is achieved.The systematic errors are then evaluated for compensation and correction.4.A new method of Raman optical wavefront phase shift estimation is proposed based on optimal estimatio.Combined with the atomic expansion and laser propagation in the atom interferometry experiment,the Zernike polynomials are used to decompose the Raman wavefront,and the mathematical model of the wavefront phase shift interacting with atomic density and velocity distribution,free evolution time and Zernike polynomial coefficients is established.Based on the accurate setting of experiment parameters such as the atomic cloud size and temperature,the fringe phase shifts under different parameters are measured which form a set of overdetermined linear equations with the coefficients of Zernike polynomials being unknown.The optimal estimation method is then used to solve the system of equations thus estimating the effective Raman wavefront and corresponding inteferometric phase shift.Numerical simulation results show that the accuracy of wavefront estimation is related to the number of different parameters and the experimental noise level.With the QPN noise level,the estimation uncertainty with 20 sets of parameters and 100 times of measurement can reach 6%,leading to a phase shift evaluation uncertainty of 0.2mrad.5.A fringe contrast enhancement method based on composite pulse sequences and shaped Raman pulses is explored.From the perspective of solving the contradiction between atomic temperature and interference fringe contrast,the formation principles of the composite pulse sequences and shaped pulses are discussed.Among them,the composite pulse sequences constitute a reasonable phase cycle by cascading multiple Raman pulses to compensate for the pulse detuning error in a single pulse,while the shaped pulses build an expected detuning response by properly designing the pulse waveform.The effects of different sequences and pulse shapes on the single rectangular pulse are compared by numerical simulation.Their overall effects on the whole interferometric sequence are discussed from the aspects of fringe contrast and phase shifts.The results show that with reasonable selection of the interferometric sequences,fringe contrast can be greatly enhanced for higher atomic temperature,thus improving the atomic temperature tolerance in atom interferometry,increasing the atomic flux and reducing the noise limit. |
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