Research On High Sensitivity Micro Optical Weak Magnetic Sensing System

Precision measurement technology is a hot research field nowadays,and when the traditional research methods reach the limits,people are forced to seek more precise measurement methods.Magnetic field,as one of the basic physical quantities,is ubiquitous in human life,such as the magnetic field generated by airplanes,ships and so on.Humans themselves also generate magnetic fields,such as those in the heart and brain,and by detecting these bio-magnetic fields,diagnosis and early warning of medical conditions can be achieved.However,unlike the magnetic fields generated by other common metallic substances,bio-magnetic fields are very weak,often down to the order of p T or even f T,as a comparison,the geomagnetic field is about 50,000 n T.In order to detect such extremely weak magnetic fields,people need more sophisticated equipment,and traditional flux gates are no longer up to the requirements needed for measurement.The current commercial superconducting quantum interferometers are very sensitive magnetic field measurement devices,but they are bulky and operate in harsh environments.And the annual maintenance cost of millions of dollars is unaffordable.The atomic magnetometer based on SERF mechanism not only has higher sensitivity,but also has smaller size and lower cost,which is recognized as the next generation of ultra-high sensitivity magnetic field measurement equipment.Since its advent,the SERF-type atomic magnetometer has been designed to improve system sensitivity,reduce system size and complexity,and enhance user experience.However,due to the laser light intensity noise and the ambient magnetic field noise,the sensitivity is still far from the theoretical limit(～1 a T/Hz^1/2)until now.In order to improve the sensitivity of the system and to realize the versatility of the system,this dissertation is devoted to the optimization of the performance of the single-channel SERF-type atomic magnetometer,the optimization of the performance of the dual-channel SERF-type atomic magnetic field gradiometer and the realization of the triaxial vector SERF-type atomic magnetometer,as described below:1.In this dissertation,the atomic level structure of alkali metal rubidium atoms is introduced.In addition,the spin polarization generation process of alkali metal atoms and the interaction between the polarized atoms and the external magnetic field are revealed,and the relaxation mechanism of the alkali metal atomic system is analyzed,which provides a theoretical basis for the subsequent performance optimization.Based on the Hanle effect,the basic working principle of atomic magnetometer was studied theoretically,and the optical absorption properties of alkali metal atoms under the action of external magnetic field were established.2.A single-channel SERF-type atomic magnetometer weak magnetic sensing system was studied.And a single-beam simultaneous pump detection scheme is used to simplify the system structure and reduce the system size,and the size of the atomic vapor cell used is only 4 mm×4 mm×3 mm.An optical heating scheme is used to heat the atomic gas chamber.The laser parameters required for optical heating are theoretically simulated,and the fiber collimator is customized according to the simulation results,solving the problem that the conventional electrical heating scheme introduces magnetic field noise.The effects of atomic polarization rate,atomic vapor cell operating temperature,and modulated magnetic field amplitude on the conversion coefficient of the system are studied,and the sources of system noise are analyzed and the corresponding solutions are proposed.By optimizing the system parameters,a single-channel magnetometry sensitivity of 6 f T/Hz^1/2 was achieved at an acquisition time of 400 s,and the corresponding system response bandwidth was 154 Hz.3.A dual-channel SERF-type atomic magnetic field gradiometer weak magnetic sensing system was studied.Based on Bloch equation,the influence of system linewidth and incident light intensity on the performance of magnetic field gradiometer is studied theoretically,and the theoretical model of system common mode rejection ratio is constructed and verified experimentally.The experimental results show that the system common-mode rejection ratio and the system linewidth show a quadratic relationship,and the incident light intensity shows a one-sided relationship,which is in good agreement with the theoretical model.In the experiments,the common mode rejection ratio of 9.2 was reached.Meanwhile,the sensitivity of the single-channel system is 23f T/Hz^1/2 and 9 f T/Hz^1/2 after gradiometer differencing,which is a 2.5 times improvement of the system performance.Considering a baseline of 2 cm,the system magnetic field gradient sensitivity is 4.5 f T/cm/Hz^1/2.This study verifies that linewidth and light intensity are the key parameters that determine the performance of magnetic field gradiometers,and by optimizing these two parameters,optimization of the performance of magnetic field gradiometers can be achieved.4.The triaxial SERF-type atomic magnetometer weak magnetic sensing system was studied.To realize triaxial magnetic field measurements,two solutions are proposed:the dual atomic vapor cell structure and the magnetic moment rotation scheme.The dual atomic vapor cell structure uses two independent atomic vapor cells spaced 2 cm apart to form the sensing module,and the beam splitting method by beam splitting mirror realizes the function of pumping two atomic gas chambers simultaneously by a single light source,which solves the problem of insensitive axes from the source and simplifies the system structure.The performance of the system was tested in DC mode,single-axis modulation mode and multi-axis modulation mode,respectively.The results show that the DC mode is simple and crosstalk-free but less sensitive,the single-axis modulation mode can further improve the system sensitivity and system bandwidth,and the multi-axis modulation mode can solve the problem that the above two modes cannot achieve simultaneous measurement of three axes.As a result,three-axis magnetic field measurement sensitivity of 22 f T/Hz^1/2（x-axis）,23 f T/Hz^1/2（y-axis）and 21 f T/Hz^1/2（z-axis）was achieved,and the performance specifications in all three magnetic field directions were similar.In addition,to further reduce the size of the system,only one atomic vapor cell was used to form the sensing module,and a three-axis magnetic field measurement was achieved by a magnetic moment rotation scheme.As a result,a triaxial magnetic field measurement sensitivity of 21 f T/Hz^1/2（x-axis）,26 f T/Hz^1/2（y-axis）and29 f T/Hz^1/2（z-axis）was achieved.Finally,a triaxial demagnetization method is proposed based on a triaxial SERF-type atomic magnetometer,to ensure that the atomic magnetometer system can work in the SERF state.Compared with the conventional cross-field demagnetization scheme,this method has higher demagnetization accuracy and can be achieved without using additional equipment,relying only on the response of the magnetometer itself.