Academic Symposium on Optoelectronics and Microelectronics Technology and Chinese-Russian Symposium on Laser Physics and Laser TechnologyOptoelectronics Technology
The study on the angular dependence of the resonance signal for optical pumping of the rubidium magnetometer
Caiping Zheng, Chong Yang, Zhang Qingtao, Lei Wang, Cheng Yuefang, Yang Jiajie Liu
In this paper, the resonance signal of the rubidium optical pumping magnetometer was studied. The theory is based on the quantum physics and atomic physics. Through establishing the relation between the magnetic sub-level rate equations and the light intensity, the angular dependence of the resonance signal was discussed. When θ=0° the resonance signal is maximum, when θ=90° the signal strength becomes zero, and the equipment won't work. Through the simulated curves, the relationship between the resonance signal and the relaxation time as well as the density of rubidium vapor has also been done. This research provides a theoretical basis to solve the key technology of posture correction. As the rubidium vapour magnetometer measures the underwater geomagnetic field, it also plays a vital role for improving the accuracy of the instrument.
In this paper, the resonance signal of the rubidium optical pumping magnetometer was studied. The theory is based on the quantum physics and atomic physics. Through establishing the relation between the magnetic sub-level rate equations and the light intensity, the angular dependence of the resonance signal was discussed. When θ=0° the resonance signal is maximum, when θ=90° the signal strength becomes zero, and the equipment won't work. Through the simulated curves, the relationship between the resonance signal and the relaxation time as well as the density of rubidium vapor has also been done. This research provides a theoretical basis to solve the key technology of posture correction. As the rubidium vapour magnetometer measures the underwater geomagnetic field, it also plays a vital role for improving the accuracy of the instrument.
As a weak magnetic sensor, optical pump magnetometer is widely used in resource exploration, space exploration, and other fields. According to the principle of measuring weak magnetic anomaly signal under geomagnetic field, a rubidium optical pump magnetometer experimental system is built based on the methods of laser pumping and RF(radio frequency) field depolarization, The influence of RF field intensity on the resonance spectrum of rubidium optical pump magnetometer is studied. The results show that the intensity and linewidth of the resonance spectrum increase with the increase of RF field intensity, but the ratio of linewidth to intensity has a minimum. This paper provides a technical basis for the sensitivity optimization of rubidium optically pumped magnetometer.
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.
Atomics magnetometers achieve remarkable accuracy, applying to production and scientific research. However, their size and bulk components make it difficult to achieve higher accuracy. We investigate the influence of the skew angle of the pump beam on the optical pumping rate in an atomic magnetometer. An analysis based on the Bloch equation is proposed to decrease evaluate optical errors in the process of production and assembly. When the incident angle is non-zero, the pumping rate has a projection in the direction of a static magnetic field. By establishing the pumping rate equation, the pumping rate of each position in the vapor cell in the direction of static magnetic field at different pump light skew angles is calculated in our study. The sensitivity was measured experimentally to demonstrate the simulation results. The results indicate that the optical pumping rate decreases as the amplitude of skew angle and propagation distance increasing which can be evaluated by one-dimensional distribution while the decay rate increases with the rise of the angle. The simulation values of the rubidium pumping rate, obtained with an incident angle of 0.5° , in the center of the vapor cell are reduced by 46%. The sensitivity decreases with the increasing skew angle similar to the attenuation trend of the optical pumping rate but not the same. Our work provides a reference for evaluating the optical error of atomic magnetometer which is useful for miniaturization design.
A rubidium vapour magnetometer has been made which measures the total geomagnetic field and records its variations continuously over the frequency band 0-1 c/s with sensitivity of 5 × 10 -6 Oe. Optical pumping technique is used to observe the energy separation between the Zeeman sub-levels in the ground state of 85 Rb atom, which is the measure of the ambient magnetic field and is equal to 220 kc/s for the local field of 472 × 10 -3 Oe, with the line width of 200 c/s. Resonance is detected when an applied r.f. field at the Larmor frequency causes redistribution of the population of the energy levels thus increasing absorption in the gas cell of the pumping radiation at 7948 Å. A voltage controlled oscillator supplying the r.f. field is locked on to the Larmor frequency. Resonance radiation is obtained from a rubidium lamp excited from a 150 Mc/s, 2 w power supply. The gas cell contains natural rubidium. This is kept at a temperature of 35-45° C thus maintaining vapour pressure of about 10 -6 mmHg. The design of the lamp and the gas cell is given. Further work to improve the light source and the gas cell should improve the sensitivity of the instrument. Redesign of the electronics to make the system oscillate at the Larmor frequency may also improve sensitivity and increase the frequency response of the instrument. The instrument described was demonstrated at The Institute of Physics and The Physical Society Exhibition in January 1961.
Rubidium laser optically pumped magnetometers (OPMs) are widely used magnetic sensors based on the Zeeman effect, laser pumping, and magnetic resonance principles. They measure the magnetic field by measuring the magnetic resonance signal passing through a rubidium atomic gas cell. The quality of the magnetic resonance signal is a necessary condition for a magnetometer to achieve high sensitivity. In this research, to obtain the best magnetic resonance signal of rubidium laser OPMs in the Earth’s magnetic field intensity, the experiment system of rubidium laser OPMs is built with a rubidium atomic gas cell as the core component. The linewidth and amplitude ratio (LAR) of magnetic resonance signals is utilized as the optimization objective function. The magnetic resonance signals of the magnetometer experiment system are experimentally measured for different laser frequencies, radio frequency (RF) intensities, laser powers, and atomic gas cell temperatures in a background magnetic field of 50,765 nT. The experimental results indicate that optimizing these parameters can reduce the LAR by one order of magnitude. This shows that the optimal parameter combination can effectively improve the sensitivity of the magnetometer. The sensitivity defined using the noise spectral density measured under optimal experimental parameters is 1.5 pT/Hz1/2@1 Hz. This work will provide key technical support for rubidium laser OPMs’ product development.
For the experimental implementation of an optically pumped atomic magnetometer, the magnetic resonance signal with a narrow linewidth and a high signal-to-noise ratio (SNR) is required for achieving a high sensitivity. Using 795-nm laser as both the pumping and the probe laser, we compare the magnetic resonance signals from different rubidium atomic vapor cells and investigate the variations of magnetic resonance signals with temperature. Optimized magnetic resonance signal is achieved with a paraffin-coated rubidium atomic vapor cell. Then the 780-nm laser at rubidium D2 line is introduced as a repumping laser, and we explore the changes of linewidth and SNR of the magnetic resonance signal under different power of the pumping laser and the repumping laser. Owing to the 780-nm repumping laser beam, the signal amplitude of rubidium-85 magnetic resonance signal is improved remarkably because more rubidium-85 atoms are spin- polarized by the 795-nm pumping laser beam. At the same time, the linewidth of rubidium-85 magnetic resonance signal is roughly not broadened anymore. We realize a closed-loop optically pumped rubidium-85 atomic magnetometer with a bandwidth of ~1.2 kHz, and the sensitivity is calibrated to be ~245.5 pT/Hz1/2 only with the 795-nm pumping laser beam. Owing to the employment of the 780-nm repumping laser beam, the sensitivity is improved to be ~26.4 pT/Hz1/2 which is improved roughly by one order of magnitude. We also calibrate the measurement accuracy and deviation of a commercial fluxgate magnetometer by using the enhanced rubidium magnetic resonance signal.