Research On The Precise Measurement Of Magnetic Field Base On Coherent Population Trapping

Atomic magnetometer, which is a kind of quantum sensor, gradually becomes core sensor in magnetic detecting system due to its high performance and easy realization in project. The requirements of magnetic sensor toward to high performance of several parameters along with the pluralistic applications of magnetic detecting system. The research on how to guarantee the high performance of several parameters of atomic magnetometer simultaneously is therefore needed.In this thesis, a kind of magnetic field detecting method based on coherent population trapping is researched. Double resonance and linearly polarized laser exciting are used to guarantee the high absolute accuracy and sensitivity of atomic magnetometer in order to precise measurement of exact value and change of magnetic field. The absolute accuracy of ±0.5nT,sensitivity of 20pT/(?) and sampling rate of 10Hz have been reached. Double resonance method uses frequency modulated laser to excite several magnetic sublevels simultaneously in order to achieving two CPT signals with opposite frequency shift which can be used to reduce the effect of frequency shift. The method of linearly polarized laser exciting eliminates the incoherent dark states of atoms which are produced by optical pumping. The utilization ratio of atoms and signal to noise ratio are improved. On this basis, a kind of differential detection method based on frequency shift is presented. The advantage of this method is the improvement of magnetometer's sensitivity by eliminating common mode noise under the circumstance of high absolute accuracy. The sensitivity can be improved to the level of 2pT / (?) . After experiments, a prototype of CPT magnetometer based on double resonance and linearly polarized laser exciting method is introduced. At last, the noise of the magnetometer is analyzed and we come to conclusion that the FM-AM noise caused by laser frequency noise is the main noise. Under this conclusion, a kind of laser frequency locking technology based on saturated absorption spectrum which uses surface reflection of cell is presented. This technology has some advantages such as simple device, high reliability and good effect of frequency locking. This technology can be used not only to engineering realization of atomic magnetometers, but also to simplify other quantum sensors' devices and improve the adaptability on moving platforms.Chapters of this thesis are as follows: The first chapter introduces the development and application requirements of magnetic field measurement as well as the types, principles,advantages, disadvantages and applications of magnetic sensors. The second chapter introduces the principle, advantages, disadvantages and development status of all kinds of atomic magnetometers, then analyzes the factors which influence the sensitivity and absolute accuracy of atomic magnetometers and put forwards the research significance. The third chapter introduces the principle of CPT magnetometer, research on double resonance method, research on linearly polarized laser exciting method and noise suppression method based on differential detection. The fourth chapter introduces the development situation of CPT magnetometer prototype including system designs and performance tests. The fifth chapter analyzes the noise of the CPT magnetometer and introduces the research on laser frequency locking. The sixth chapter makes a conclusion and prospect to this thesis.