Research On Weak Magnetic Field Detection Of CPT Magnetometer

Weak magnetic field detection is very significant to study magnets and analyze the structure and properties of materials,whether in the earth environment where humans live or in the space environment of scientific exploration,its application is universality.At present,there are many types of magnetometers used in biomedicine,geological resource detection,natural disaster warning,military antisubmarine,shipwreck salvage,material nondestructive testing,satellite geomagnetic modeling and so on.However,affected by factors such as accuracy,magnetic measurement range,dead zone and portability,their application is limited in measurement of hundreds of n T weak magnetic fields,alien magnetic field modeling and space resource detection.CPT magnetometer has the characteristics of high accuracy,small size and adjustable measurement range.Improving its weak magnetic measurement performance will hopefully fill the current state bottleneck,play an important role in space weak magnetic detection.Following are main contents of this thesis.Firstly,for the ⁸⁷Rb atom used in the CPT magnetometer,the Zeeman split and the transition selection rule of the superfine energy level in the external magnetic field are given.The method that the CPT magnetometer uses the EIT differential signal to measure the magnetic induction intensity is introduced.Secondly,the theoretical model and simulation model of EIT signal line width are established.The demodulation principle,application advantages and application limitations of phase sensitive detection are analyzed.Subsequently,the influence of the microwave FM range on the magnetometer sensitivity coefficient is analyzed.It shows that the magnetometer sensitivity coefficient is maximum,when the microwave FM range is equal to 1.1 times of the EIT signal line width.The separation condition of the EIT signal is analyzed,and the linear relationship between the line width and the magnetic field is 3.85 Hz/n T.Thirdly,the CPT magnetometer set-up including single-axis coil magnetic field,optical device and control system is constructed.The beam circular polarization,optical power stability and atomic chamber temperature stability are tested,and the results of the experiment are as follows:the ellipticity parameter is better than-44°,the optical power fluctuation in constant current mode is 1μW which produces a measurement error of 0.29n T,and the atomic chamber temperature stability is0.0045℃（1σ）.Finally,a method using Lorentz line fitting to calculate the line width of the EIT signal is proposed,and the effects of laser light power,atomic chamber temperature,and buffer gas pressure on the line width of the EIT signal are experimentally studied.The appropriate experimental conditions for narrowing the line width are obtained:optical power is less than 20μW,atomic chamber temperature is around 55℃,and buffer gas pressure is 60～80 Torr.In the magnetic shielding,it is verified that the CPT magnetometer can measure 100n T magnetic induction intensity,and the separation conditions in the EIT signal simulation model is verified.