| The high-precision measurement of magnetic fields is widely used in various fields,such as geoscience,national defense,military,biomedicine,chemical and material science,and fundamental physics.It is one of the core technologies related to natural science,national economy,people’s livelihood,and national security.In recent years,a new mechanism called Spin-Exchange-Relaxation-Free(SERF)has been used in atomic magnetometers to eliminate the magnetic resonance broadening caused by spin-exchange collisions between alkali metal atoms,thereby achieving a higher sensitivity than superconducting quantum interference magnetometers and becoming the world’s most sensitive magnetic field sensor.However,the SERF atomic magnetometer can only operate in a magnetic shielding environment of 10 n T or less due to the limitations of its principles,and is not suitable for applications in the earth’s magnetic field environment.With the advantages of the ultra-high sensitivity,relatively low cost,and easy integration of the SERF atomic magnetometer,once been freed from this limitation and applied in an unshielded environment,it will be an important breakthrough in magnetic measurement technology.Replacing the shielding device with a magnetic field compensation method to create an extremely weak magnetic field environment for the atoms is a feasible approach to design unshielded SERF magnetometers.However,the sensitivity achieved by several experimental studies at home and abroad is much lower than expected,and cannot exploit the high sensitivity advantage of SERF magnetic field measurement.To develop a more complete unshielded SERF magnetometer system with high sensitivity and release the potential of SERF atomic magnetometers,this thesis focuses on the two problems faced by unshielded SERF magnetometer: the difficulty of quickly decoupling the three-axis magnetic field signal with low crosstalk and the large noise of the magnetic compensation coil driving source.Based on an in-depth understanding of the high-sensitivity magnetic measurement principle of SERF,this thesis establishes a vector magnetic field decoupling model for the SERF atomic magnetometer,developes a low-noise compensation coil driving source,and finally forms a relatively complete unshielded SERF high-sensitivity magnetic field measurement method,enables unshielded SERF atomic magnetometers to achieve a sensitivity level equivalent to that in a shielded environment.The main works of this thesis are as follows:(1)The traditional three-axis magnetic field decoupling method based on cross-field modulation for the SERF magnetometer was optimized.A time-dependent solution of the Bloch equation was proposed to give a theoretical explanation for the response of the traditional three-axis magnetic field decoupling scheme based on cross-field modulation,which deviates from the steady-state solution under highfrequency modulation.An experimental platform for SERF atomic magnetometry was constructed to verify the theory.With the modulation frequency selected based on the theory,the suppression of the system to the 50 Hz power frequency exceeded three times,demonstrating the potential for shield-free measurement and also verifying that the dual-frequency modulation method used in this solution imposes certain limitations on the system bandwidth.(2)The vector field decoupling method based on rotation field modulation was optimized.To obtain a faster response speed for vector decoupling,a single-beam vector field measurement configuration with single modulation frequency was introduced into the unshielded SERF magnetometer.The single-beam configuration can perform single-axis or dual-axis vector decoupling by single-directional sinusoidal or planar rotation field modulation.The demodulation principle of singleaxis and dual-axis vector magnetic field signals was analyzed.A two-input-twooutput transfer function model of the dual-axis vector magnetic fields decoupling system was established.A compact single-beam magnetometer probe was used to verify the accuracy of the model,and the cross-talk between the channels of the probe was optimized based on the model,reducing the cross-talk between the two axes from 25 % to 10 % and almost eliminating the delay in the system response.(3)A low-noise,high-dynamic-range magnetic field compensation coil driving source was developed.A two-stage parallel current source structure with ultra-high dynamic range and ultra-low noise was proposed to meet the requirements of unshielded SERF magnetometers for response speed,driving capacity,and noise level of the compensation system.The current output capability reached ±202 m A,and a resolution of at least 4 n A was achieved,with a current noise as low as 28.3 p A/Hz1/2.The problem of the fast response of low-noise current sources was also solved,and a rapid response of 1 k Hz could be achieved in a range of 4 m A.This solution overcomes the limitations of the noise and other performances of the compensation magnetic field on the sensitivity of unshielded SERF atomic magnetometers.(4)The structural design and an ignition method of a new type of unshielded SERF probe were studied.To meet the demand for suppressing power frequency and other noise fluctuations in unshielded SERF magnetometers,the single-beam dual-axis magnetic field vector decoupling method was chosen,and a scheme of two independent single-beam probes placed perpendicular to each other was proposed to obtain three-axis magnetic field vector signals.To solve the magnetic gradient problem between the two single-beam probes,a probe structure with high uniformity magnetic compensation coils and a corresponding ignition method were designed,and a matching data acquisition and feedback control system was developed.The final prototype of the unshielded SERF magnetometer achieved a sensitivity level of 30 f T/Hz1/2@6 Hz under an open magnetic environment in a standard laboratory,and the magnetic field noise gradient in the environment was measured as4 p T/Hz1/2/cm approximately.The crosstalk among the three axes was better than0.6 %,and the suppression ratio of background noise for each axis was higher than50 d B at 50 Hz.Based on the research above,a complete unshielded SERF high-sensitivity magnetic measurement method has been developed,and the prototype achieved a sensitivity level equivalent to that in a shielded environment.The high suppression ratio of noise fluctuations in the prototype allows it to operate in a completely open magnetic environment without heavy aluminum shielding layers used in previous foreign research.The true ”unshielded” performance was achieved and greater applicability and robustness were demonstrated.This lays a solid foundation for the engineering and practical application of unshielded SERF atomic magnetometers in the future. |
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