Research On The Sensing Technology Of Magnetic Gradient Measurement With Helium Laser Pump Magnetometer

The fundamental physics of laser optically pumped magnetic measurements are the Zeeman splitting and the magnetic resonance effect.Atoms absorb laser light energy during polarization and depolarization,generating magnetic resonance signals.Magnetic values can be obtained by detecting the resonance points in resonance signals.The working substances of laser optical pump mainly include alkali metals?Ru,Cs,K,etc.?and He.Compared with alkali metals,He atoms do not need heating and insulation during magnetic measurement,and its power consumption is relatively lower.As the field expands,various environmental factors,such as magnetic diurnal variation and magnetic storm activity,can drown the measured magnetic field information in the magnetic field noise and make it difficult to obtain.In some areas where the background magnetic field is very strong,magnetic detection will often suffer from too large ratio of the background field intensity value to the measured magnetic anomaly value,and the anomaly cannot be interpreted and inferred only by relying on a single magnetic field scalar value.Therefore,the measurement of magnetic gradient is particularly important.In order to measure magnetic gradient with high quality,this paper research on the sensing technology of laser helium optical pump magnetic gradient measurement,and the main work contents are as follows:?1?Research on magnetic measurement principle of laser helium optical pump and scheme design of magnetic gradient system.The principle of magnetic measurement with helium laser pump magnetometer is analyzed,and the formula of magnetic measurement principle is deduced.The radiofrequency scanning method is studied,and the magnetic resonance signal model is established according to the Bloch equation of optical magnetism and beer-lambert law.The magnetic sensitivity calculation formula ?B=?f/??S/N? is given.The magnetic sensitivity is directly related to the light source,helium cell and circuit performance.The magnetic gradient system scheme of laser helium optical pump is designed,and the research focuses on laser source development and laser frequency stabilization technology,optical path design of magnetic sensor and photoelectric conversion technology,excitation technology of magnetic sensor absorption chamber and resonance signal detection technology based on rf modulation method.?2?Research on laser source and photoelectric technology of helium optically pumped magnetic sensor.The wavelength modulation principle of laser diode and the laser wavelength stabilization method based on the saturation absorption spectrum of helium were studied.The ECDL laser light source was designed.A dual output integrated DFB laser source for gradient measurement was designed.The test data prove that the sensitivity percentage error of the magnetic sensor caused by the power fluctuation of beams are less than 0.4%.Designed the optical path and structure of the magnetic sensor.With the help of 3D printing technology,the dual-channel sensor is manufactured.The photoelectric conversion technology was studied,and the photoelectric detection circuit of the magnetic sensor whose measured noise power density accords with the theoretical design was designed.?3?Research on excitation technology of magnetic sensor helium cell.It is determined that high frequency dielectric barrier discharge of helium occurs in the excitation process.The electric characteristic model of cell was established by analogy with gas discharge lamp.It is proved that the cell is a resistive element when the excitation circuit works stably.According to the function,the excitation energy source research is divided into high voltage pulse source research and high frequency energy source research.According to paring's law and cell parameters,the breakdown voltage of helium gas in the cell was estimated to be about 1.5kV.A high-voltage pulse source circuit composed of an oscillator,a power amplifier and a hollow booster coil was designed.Based on crystal oscillator and power amplifier,the 50 MHz high frequency energy source was designed by impedance matching technology.The total power consumption of the actually-developed excitation circuit of cell is about 6W.?4?Research on magnetic signal detection technology.The principle of signal detection technology based on rf modulation is studied by using Lorentz linear analog resonance signal form.The type of modulated signal is determined.The principle of rectangular Helmholtz coil is studied,and a rf modulation coil suitable for magnetic sensor is designed and fabricated.It is proved that there is an optimal value of magnetic sensitivity regarding the intensity of rf alternating magnetic field,when the peak-to-peak value of rf modulated magnetic field intensity is about 900 nT,the magnetic sensor designed in this paper has an optimal value of magnetic measurement sensitivity.A digital detection method based on resonance signal curve and higher harmonic amplitude characteristics is designed.The closed loop frequency controller based on incremental PID method is used to lock the magnetic resonance point.?5?Performance test of helium laser optical pump magnetic sensors and gradient measurement system.By data comparison,it is proved that using laser source can greatly reduce the power consumption of optical pump sensor.The Helmholtz-Maxwell composite coil was used to create a magnetic field environment inside the shield.By signal comparison,it is proved that the laser source can greatly improve the sensor sensitivity.After Lorentz linear fitting of the output signal of the magnetic sensor,the sensitivity of the two magnetic sensors was calculated to be 2.65pT/Hz^1/2@1Hz and 3.59pT/Hz^1/2@1Hz respectively.By the comparison with the calculated values of magnetic field theory and the dates of Cs-3 magnetometers,it is proved that the helium laser optically pumped magnetic gradient measurement system has good practicability,and the optimization direction of the system is determined.In conclusion,this paper builds a magnetic gradient measurement system based on dual-optical output integrated laser source after study the key technologies such as optics,gas discharge and circuit.The system has good practical performance.This paper can provide important guidance for the further research on the sensing technology of magnetic gradient measurement with helium laser pump magnetometer.