Atomic magnetometers use the coherent precession of polarized atomic spins to detect and measure magnetic fields. When an off-resonant, linearly polarized probe beam propagates through the atomic vapor cell, the precession will cause a rotation of its polarization plane. Measuring the angle of the rotation, the magnetic field can be calculated. In this paper, we introduce the basic principles of atomic magnetometers and describe the method to measure the angle. Then a dual-phase lock-in amplifier (LIA) is proposed to detect the weak signal of the atomic magnetometer. And we design a simulation for the LIA based on Matlab/Simulink. Our theoretical analysis and simulation results show that the detecting system is well working and it is possible to detect weak signal of magnetic fields in an accurate way.
Atomic magnetometers use the coherent precession of polarized atomic spins to detect and measure magnetic fields. When an off-resonant, linearly polarized probe beam propagates through the atomic vapor cell, the precession will cause a rotation of its polarization plane. Measuring the angle of the rotation, the magnetic field can be calculated. In this paper, we introduce the basic principles of atomic magnetometers and describe the method to measure the angle. Then a dual-phase lock-in amplifier (LIA) is proposed to detect the weak signal of the atomic magnetometer. And we design a simulation for the LIA based on Matlab/Simulink. Our theoretical analysis and simulation results show that the detecting system is well working and it is possible to detect weak signal of magnetic fields in an accurate way.
The measurement of ultra-weak magnetic fields relies on the conversion of magnetic field information to atom spins using alkali metals. In this context, the detection of magnetic fields is accomplished through rotation angle measurement of linearly polarized light. This paper proposes a novel method to suppress mechanical errors between polarizers in the rotation angle measurement, taking advantage of the optical setup characteristics in atomic magnetometers. The method involves applying two separate frequency modulations to the pump beam and probe beam, effectively eliminating mechanical errors between the polarizers as a direct current component using a double-channel lock-in amplifier. Additionally, the double modulation method offers a solution to suppress shot noise caused by incident beam fluctuations or transverse spin relaxation, as well as mechanical errors among optical elements in the light path, enabling high-precision measurements.
We report a rubidium atomic magnetometer based on pump-probe nonlinear magneto-optical rotation. The rubidium vapor cell is placed in a five-layer magnetic shield with inner coils that can generate uniform magnetic fields along the direction of pump beam, and the cell is also placed in the center of a Helmholtz coil that can generate an oscillating magnetic field perpendicular to the direction of pump beam. The atoms are optically pumped by circularly polarized pump beam along a constant magnetic field in a period of time, then the pump beam is turned off and a /2 pulse of oscillating magnetic field for 87Rb atoms is applied. After the above process, the individual atomic magnetic moments become phase coherent, resulting in a transverse magnetization vector precessing at the Larmor frequency in the magnetic field. The linearly polarized probing beam is perpendicular to the direction of magnetic field, and can be seen as a superposition of the left and right circularly polarized light. Because of the different absorptions and dispersions of the left and right circularly polarized light by rubidium atoms, the polarization direction of probing beam rotates when probing beam passes through rubidium vapor cell. The rotation of the polarization is subsequently converted into an electric signal through a polarizing beam splitter. Finally, the decay signal related to the transverse magnetization vector is measured. The Larmor frequency proportional to magnetic field is obtained by the Fourier transform of the decay signal. The value of magnetic field is calculated from the formula:B=(2/) f, where and f are the gyromagnetic ratio and Larmor frequency, respectively. In order to measure the magnetic field in a wide range, the tracking lock mode is proposed and tested. The atomic magnetometer can track the magnetic field jump of 1000 nT or 10000 nT, indicating that the atomic magnetometer has strong locking ability and can be easily locked after start-up. The main performances in different magnetic fields are tested. The results show that the measurement range of the atomic magnetometer is from 100 nT to 100000 nT, the extreme sensitivity is 0.2 pT/Hz1/2, and the magnetic field resolution is 0.1 pT. The transverse relaxation times of the transverse magnetization vector in different magnetic fields are obtained, and the relaxation time decreases with the increase of the magnetic field. When the measurement range is from 5000 nT to 100000 nT, the magnetic field sampling rate of the atomic magnetometer can be adjusted in a range from 1 Hz to 1000 Hz. The atomic magnetometer in high sampling rate can measure weak alternating magnetic field at low frequency. This paper provides an important reference for developing the atomic magnetometer with large measurement range, high sensitivity and high sampling rate.
Optically pumped magnetometers are important instruments for imaging biological magnetic signals without the need for cryogenic cooling. These magnetometers are presently available in the commercial market and utilize the principles of atomic alignment or orientation, enabling remarkable sensitivity and precision in the measurement of magnetic fields. This research focuses on utilizing a spin-aligned atomic ensemble for magnetometry at zero field. An approach is introduced which involves evaluating how the linear polarization of light rotates as it passes through the atomic vapor to null the magnetic field. Analytical expressions are derived for the resulting spin alignment and photodetection signals. Experimental results are provided, demonstrating good agreement with the theoretical predictions. The sensitivity and bandwidth of the magnetometer are characterized based on the detected polarization rotation signal. Lastly, the practical utility of the magnetometer for medical applications is demonstrated by successfully detecting a synthetic cardiac signal.
Modulation of laser's left-handed and right-handed polarization was used in the paper. This method has realized all-optical detection of magnetic field and avoided cross talk existing in conventional atomic magnetometer. The detection principle of precession frequency of atomic magnetic moment in magnetic field was demonstrated based on the method, and the signal was analyzed, then the detection and processing system was designed based on the analysis. The in-phase signal was displayed as Lorenz lineshape. The quadrature signal was displayed as dispersion lineshape, and the magnitude was 0 at resonance frequency. The phase signal was displayed as monotonically decreasing function with frequency, and the magnitude was 0 at resonance frequency. Both the quadrature and phase signals can be used to lock resonance frequency and realize measurement of magnetic field.
In order to measure a weak alternating magnetic field, an optically-pumped Rb magnetometer based on pump-probe structure is investigated and demonstrated. The pumping light and probing light propagate along the z axis and x axis, respectively. A constant polarization magnetic field along the pumping light is applied, which not only stabilizes the polarization of Rb atoms but also tunes resonance frequency of Rb atoms. When a weak alternating magnetic field is applied perpendicularly to the constant magnetic field, the magnetic moment will tip off the z axis and rotate around the z axis. And then the polarization plane of probing light is modulated correspondingly. The x component of the magnetic moment can be obtained with a balanced detector. As a result, a signal proportional to weak alternating magnetic field can be obtained.In order to obtain the magnetic response of the magnetometer, we analyze the signal amplitude as a function of polarization magnetic field strength B0 and transverse relaxation time 2 with numerical simulation. The amplitude-frequency response of the magnetometer is determined mainly by two parameters, namely cutoff frequency c=1/2 and resonance frequency 0= B0, where is the gyromagnetic ratio of Rb atom. At low frequency, that is a0 and a 0c2, the magnetometer has a flat response, here a is the frequency of the weak alternating magnetic field. If 0c, the signal amplitude will be large for large 0 or small c. For a given c, the peak response appears at 0=c. In the vicinity of resonance frequency, if c0, a peak will appear and if c 0, no peak occurs. At high frequency, the amplitude will decrease with the increase of a.We verify the above analyses in experiment. A vapor cell with a short transverse relaxation time is used to obtain large frequency response bandwidth. Through optimizing the powers and frequencies of pumping laser and probing laser, high polarization and detection sensitivity of atomic spin can be obtained. Moreover, through choosing an appropriate polarization magnetic field, the magnetometer can be maximally sensitive to the magnetic field to be measured. The experimental results show that the magnetometer has a sensitivity of about m 0.2; pT/HzHz and bandwidth of about 3.5 kHz. It can be used to detect low field magnetic resonance and high frequency abnormal physical phenomena.
Magnetic phenomenon is one of the earliest physical phenomena realized by human. The technology of high sensitivity magnetic field measurement has been used in many applications such as geophysical prospection, aerospace engineering, earth-magnetism navigation, biomedicine, fundamental physics, and so on. With the development of laser technology, the study of atomic magnetometer makes great progresses for the past few decades. This paper reports a magnetic field sensor with optical resonant measurement. Experiments have demonstrated the scalar component of the magnetic field with the resonant precession frequency of the coherent atoms and the vector component with the Lock-in Amplifier (LIA) phase shift on the field angle.