Research On Optically Pumped Helium Magnetic Measurement Technology

Magnetic field measurement technique plays an increasingly important role in thedevelopment of modern society, and among the magnetic field measurement equipments,helium optically pumped magnetometer wins a wide range of concerns due to its goodperformance, which is successfully applied in the various fields of geophysical research, oiland gas resource, mineral deposit prospecting, military affairs and geological prospecting,etc.. Helium optically pumped magnetometer is on the basis of Zeeman Effect of heliumatomic energy levels in magnetic field, measuring the magnetic field by use of opticalpumping effect for realizing optical orientation of the helium atom, and through the methodof the optical detection of magnetic resonance. In view of the slow upgrade of heliumoptically pumped magnetometer in our country and the wide gap in comparison withadvanced digitized equipments in foreign countries, this paper completes development ofdigitized helium optically pumped magnetometer funded by the major project of thenational863plans of "Airborne geophysical survey systems", and the main research workand results are as follows:1. In-depth studies of theory basis of helium optically pumped magnetometer fromseveral aspects of characteristics of helium atom, Zeeman Effect of helium atom in amagnetic field, optical pumping effect and magnetic resonance. Helium atom orientationlaw is derived by equation of atomic density changes over time on energy levels, with theinterpretation of implementation process of optical pumping effect from phenomena andtheoretical aspects. The inner mechanism and realization condition of magnetic resonance isalso studied with solutions of the equation of helium atom magnetization vector motionunder inferred in the RF field, laying a theoretical foundation for the development of heliumoptically pumped magnetometer. 2. Lighting system for helium optically pumped magnetometer is researched andconstructed. Simultaneously, the implementation mechanism of magnetic resonance opticaldetection is also studied, giving out the derived expression of the optical signal in thelighting system and its characteristic analysis. For lighting system components,achievements are made for the excitations of helium lamp and helium absorption chamber,the installation and debugging of lens and polarized components, the selection ofphotosensitive device, design of photoelectric conversion circuit, design and assembly ofRF coil and lighting system shell in order to complete the actual building of the heliumoptical pumping magnetometer lighting system.3. Development of digitized measurement and control system for helium opticalpumping magnetometer. Point at the problem of long time relocation of magnetic resonancepoint after loss of locks and the difficulties to achieve fast and continuous measurement fortraditional helium optically pumped magnetometer, studies on changing characteristics ofthe fundamental and second harmonic amplitude signal for modulated signal near themagnetic resonance region are made, with model curves set up, magnetic resonancedetection schemes proposed applying both of fundamental and second harmonicmodulated signal. Besides, digitized detection and control system for helium opticalpumping magnetometer is designed. Major accomplishments are:(1) Precise extraction of fundamental modulated amplitude signal is achieved byadopting high precision balanced modulator, which improves system accuracy. By usingdigital lock-in technology, extraction of second harmonic amplitude signal fordistinguishing signal entering the resonance region is also developed, which improvessystem integration and performance.(2) By applying digital frequency modulation technology based on direct digitalfrequency synthesis, RF frequency is modulated, which solves the insufficiency when usingthe analog FM. Digital frequency modulation(Frequency-shift keying) is also realizedthrough the use of reconfigurable high-speed high-performance FPGA chip, which meetsthe design requirements of high-sensitivity digitized helium optically pumpedmagnetometer in aspects of frequency stability, frequency resolution and modulationperformance. Ultimately, drive for RF coil is completed.(3) In view of the lock-in amplifier output for extracting fundamental modulatedamplitude signal in helium optically pumped magnetometer inspection and control system has characteristics of slow changing direct-current when tracking the magnetic field value,approximately zero near its magnetic resonance point and bipolar changing fundamentalamplitude signal within entire sweep frequency range, high precision and high resolutionΣ-Δ type ADC is adopted as the judgment basis of system locking and magnetic tracking fordata acquisition.(4) By using incremental digital PID control method, zero point tracking for magneticfield changes is realized via FPGA, and the fundamental amplitude signal is maintained tozero by using it as PID input and adjusting the center frequency of the RF coil, whichrealizes the tracking of external magnetic field changes.(5) Electromagnetic interferences are effectively suppressed by using the method ofshield in the noise source and sensitive components at the same time, in which highfrequency excitation system is set as the noise source and the lock-in detecting module aswell as other digital components are set as sensitive components, consequently, instrumentoperating reliability is guarantied. Meanwhile, power consumption, cost and portability areunder full consideration in the design process, which realizes the miniaturization of thesystem.4. Testing and analysis for the developed digitized helium optically pumpedmagnetometer are completed.(1) Critical modules and functions of system are tested. Effective optical pumpingaffect production to the system is verified through the method of absorption ratio detectionfor helium absorption chamber. For digital FM system modules, demodulation circuit isspecifically designed to perform demodulation test and verify the effectiveness of its workin the system. By using swept frequency method to sweep the measured magnetic field, theswept magnetic resonance curve prove the production of magnetic resonance effect of thesystem, moreover, the availability of RF coil design is proved.(2) The whole instrument is tested. Amplitude curves of second harmonic signal andfundamental modulated signal perfectly fit the theoretical model curves, which prove thefeasibility of this proposed options by using the second harmonic signal and fundamentalmodulated signal to lock magnetic resonance point together. Experiments of measuring thevalues of space-stable magnetic field and spatial variation magnetic field reflect the bettermagnetic measuring effect of the instrument. Actual sensitivity indicator of the instrument isgiven through analysis and inference of classical sensitivity definition of optical pumpingmagnetometer. The alternating magnetic field produced by Helmholtz coils is used to test the response speed of the instrument.Innovation points of this article:1. According to the changing characteristics of second harmonic and fundamentalmodulated amplitude signals near the area magnetic resonance, a magnetic resonancedetection scheme based on applying both second harmonic signal and fundamentalmodulated signal is proposed with model curves set up. That is large-step mode is used tosearch magnetic resonance region outside the resonance region, then resonance point withinthe tracking area is precisely positioned and tracked by the use of digital PID controltechnology and the characteristics of fundamental signal being zero at magnetic resonancepoint. Thus the quick locking and tracking measurements to the measured magnetic field arecompleted.2. Problems of low linearity and frequency stability for analog modulator, narrowoutput frequency range, fixed parameters as well as the introduced additional measuringfrequency errors when frequency counter is used to measure the output frequency of VCOare solved by applying the digital FM technology based on direct digital frequencysynthesis to the development of helium optically pumped magnetometer. Simultaneously, itimproves the magnetic measuring precision of the instrument, brings convenience toinstrument debugging and lays technical foundation for the further development of laserexcited helium optically pumped magnetometer.3. The second harmonic amplitude signal is extracted by using digital lock-intechnology, and the signal is used as the distinguishing signal of a system entered theresonance region, which improves system integration and measurement speed.Accurateextraction for fundamental modulated amplitude signal is realized by adopting dedicatedhigh precision balanced modulator, which improve precision and sensitivity of the systemand solve the problems of the poor performance of the amplifier style switchphase-sensitive detector build by separated components and operating amplifier intraditional helium optically pumped magnetometer, the same gain of both inverting andnon-inverting amplifiers and the similar dynamic characteristics.