Research On The Noise Sources And The Systematic Errors Of The High Precision Cold Atom Gravimeter

Over the last twenty years, the novel inertial sensors based on atom interferometer have a rapid development, such as atom gravimeter, atom gyroscope, atom gravity gradiometer. They have attracted worldwide attentions due to their remarkable stability, high sensitivity, high repetition rate and potentially high accuracy. They could be widely applied in the field of the fundamental science and practical research. The previous atom gravimeters could only operate in the laboratories due to their bulk volume. Lots of efforts have been made to simplify the laboratory-based experimental apparatus so as to build compact and transportable atom gravimeter. Such mobile devices are of great use for a wide range, such as geophysics, resource exploration, seismic studies, gravimeter survey, inertial navigation and so on. Recently, the mobile atomic gravimeter has become a reality as the rapid development of the new technologies; and the performances could be comparable with the best classical gravimeter. In the future, they may be installed in the mobile platforms, such as the plane, truck, warship, and even satellite.Recently, we have designed and realized a high precision atom gravimeter. This thesis focuses on the details of the experiment, and the analysis of the main noise sources and the systematic errors. The main noise sources include the vibration noise of the retro-reflection mirror, the phase noise of the Raman beams, the shot noise, the detection noise, the intensity noise of the Raman beams and so on. Among them, the vibration noise and the phase noise of the Raman beams are dominant. The systematic errors consist of the quadratic Zeeman shift, rf shift, one-photon light shift, two-photon light shift, the phase shift due to Coriolis force and the wave-front distortions of the Raman beams and so on, we will discuss them one by one in this thesis.Currently, with the interrogation time2T=120ms and the repetition rate2.2Hz, a sensitivity of1.1×10-8g@200s has been reached in our experiment. The tidal phenomenon is observed by monitoring the local gravity continuously over128h based on our atom gravimeter. Moreover, a whole seismic wave occurred in Pakistan was recorded in great detail with our atom gravimeter and the results are compared with that recorded by a traditional seismic detector, which coincide with each other very well. Finally, the absolute gravity value of our laboratory has been measured to be (9.793346±2)m/s2, the uncertainty of measurement is about1×10-7g. The current performance of our gravimeter could meet themost of the field applications. It could be a useful reference for designing the transportable atom gravimeters. The outlines of the chapter of this thesis list as follows:In the first chapter, the background of the atomic gravimeter is introduced. Firstly, we describe the history and development status of gravity measurement and give the comparison of the performance of several gravimeters. Besides, some discussions on the application prospect of the high precision gravimeter are made. The latest progress on atomic gravimeter is reviewed.In the second chapter, the basic theory of atomic gravimeter has been presented. Firstly, the theory of stimulated Raman transition is introduced so that the two-photon transition and the resonant condition could be explained. The Raman pulse used for separating and recombining the atomic wave package are introduced. Then, the atom interferometer realized by three Raman pulses is introduced. The phase accumulated along the two different interference paths is deduced. The relationship between the phase and the gravitational acceleration g is obtained. Finally, the sensitivity function of atom interferometer is introduced, which will be useful for the analysis of the noises source of atomic gravimeter.In the third chapter, the experimental realization of our atomic gravimeter is described. We first introduce the apparatus, which include the optical system, the vacuum chamber, the magnetic field system, the vibration isolation system, the computer control system and the electronic system. Then, the experimental steps are reviewed, which include the preparation of the cold atoms, states selection, the sequence of three Raman pulse, the normalized detection system. Finally, the main experimental results are given, including the sensitivity and accuracy of the gravity measurement, the long-term gravity monitoring, the measurement of the absolute gravity value, the detection of the seismic wave and so on.In the fourth chapter, the main noise sources of atomic gravimeter have been analyzed. We first introduce the evaluation method of the signal to noise, and deduce the relationship between the SNR and the resolution of gravity measurement. Then, the sensitivity at the slope of interference fringe is investigated so that the amplitude noise and the phase noise are recognized. In the end, we analyzed the main noise source of atomic gravimeter, which include the vibration noise, the phase noise of Raman beam, the shot noise, the detection noise, the intensity noise of the Raman pulses, and so on.In the fifth chapter, the systematic errors of atomic gravimeter are evaluated. At first, the evaluation method of the systematic error is presented; the different systematic phase shifts could be extracted by inversing the Raman wave vector to measure the fringes. Benefit from the different configurations of measurement, the amplitude of the phase shifts could be obtained. Finally, the variation of gravity acceleration due to the environmental effects is introduced and their impacts on gravity measurements are discussed.In the sixth chapter, the main experimental results and the existed problems have been summarized; the prospect of the future atomic gravimeter is shown.